Enhancement of helium exhaust by resonant magnetic perturbation fields at LHD and TEXTOR

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1 PAPER Enhancement of helium exhaust by resonant magnetic perturbation fields at LHD and TEXTOR To cite this article: O. Schmitz et al 0 Nucl. Fusion 00 Manuscript version: Accepted Manuscript Accepted Manuscript is the version of the article accepted for publication including all changes made as a result of the peer review process, and which may also include the addition to the article by IOP Publishing of a header, an article ID, a cover sheet and/or an Accepted Manuscript watermark, but excluding any other editing, typesetting or other changes made by IOP Publishing and/or its licensors This Accepted Manuscript is 0 IAEA, Vienna. During the embargo period (the month period from the publication of the Version of Record of this article), the Accepted Manuscript is fully protected by copyright and cannot be reused or reposted elsewhere. As the Version of Record of this article is going to be / has been published on a subscription basis, this Accepted Manuscript is available for reuse under a CC BY-NC-ND.0 licence after the month embargo period. After the embargo period, everyone is permitted to use copy and redistribute this article for non-commercial purposes only, provided that they adhere to all the terms of the licence Although reasonable endeavours have been taken to obtain all necessary permissions from third parties to include their copyrighted content within this article, their full citation and copyright line may not be present in this Accepted Manuscript version. Before using any content from this article, please refer to the Version of Record on IOPscience once published for full citation and copyright details, as permissions will likely be required. All third party content is fully copyright protected, unless specifically stated otherwise in the figure caption in the Version of Record. View the article online for updates and enhancements. This content was downloaded from IP address... on 0/0/0 at :

2 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Enhancement of helium exhaust by resonant magnetic perturbation fields at LHD and TEXTOR. Introduction O. Schmitz, K. Ida, M. Kobayashi, A. Bader, S. Brezinsek, T.E. Evans, H. Funaba, M. Goto, O. Mitarai, T. Morisaki, G. Motojima, Y. Nakamura, Y. Narushima, D. Nicolai, U. Samm, H. Tanaka, H. Yamada, M. Yoshinuma, Y. Xu and the TEXTOR and LHD experiment groups University of Wisconsin - Madison, Department of Engineering Physics, 0 Madison, Wisconsin, USA National Institute for Fusion Science, Toki, Gifu 0-, Japan Institut für Energieforschung - Plasmaphysik, Forschungszentrum Jülich GmbH, Juelich, Germany General Atomics, P.O. Box 0, San Diego, California -0, USA Liberal Arts Education Center, Kumamoto Campus, Tokai University, -- Toroku, Kumamoto -, Japan Southwestern Institute of Physics, Chendgu Sichuan, 00, China oschmitz@wisc.edu Abstract. The ability to exhaust helium as the fusion born plasma impurity is a critical requirement for burning plasmas. We demonstrate in this paper that resonant magnetic perturbation (RMP) fields can be used to actively manipulate helium exhaust characteristics. We present results from puff/pump studies at TEXTOR as example of a tokamak with a pumped limiter and from the Large Helical Device LHD with the closed helical divertor as example for a heliotron/stellarator device. For LHD, the effective helium confinement time τp,he is a factor of - higher in the low and high density regimes explored when compared to TEXTOR discharges. This is attributed to ion root impurity transport which is one particular impurity transport regime assessed experimentally at LHD and which facilitates helium penetration to the plasma core. However, when an edge magnetic island is induced by externally applied RMP fields, τp,he is decreased by up to 0% and hence τ p,he values closer to those of TEXTOR can be established. The combination of TEXTOR and LHD results suggest that a magnetic island induced by the RMP field in the plasma source region is an important ingredient for improving helium exhaust. The reduction in τp,he seen is caused by a combination of improved helium exhaust due to an enhanced coupling to the pumping systems, increased outward transport and a reduced fueling efficiency for the helium injected and recycling from the wall elements. One critical issue for fusion energy production is the reliable and stable exhaust of the fusion born helium atoms []. They can dilute the thermonuclear plasma, iradiate

3 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of INTRODUCTION confined energy and decrease the fusion energy rate. Also, control of the helium ion density at the plasma facing components and at the material surfaces in direct contact with the plasma is mandatory to achieve stable plasma edge conditions and hence establish a stable and well-defined plasma wall interface. The helium exhaust has to be controlled such that the confinement time for the fusion born helium atoms is smaller then the energy confinement time. A minimum ratio of τ phe /τ E 0 has been deduced for instance from considerations for the burn criterion in []. At TEXTOR it was demonstrated that the helium exhaust is effective using the Advanced Limiter Test II (ALT-II) toroidal belt limiter. However, the actual helium exhaust efficiency from the system accomplished did not satisfy a feasible ratio of helium and energy confinement time [, ]. Effective helium exhaust capabilities in poloidal divertor geometry have been demonstrated at Asdex-Upgrade [] and DIII-D []. Here, the ratio of τ phe /τ E was at the margin [] or in excess of the feasible ratio of 0. Therefore, methods to improve helium exhaust from the fusion systems in general are of great interest. This is in particular relevant for heliotron/stellarator device, where - among other beneficial impurity transport regimes - one regime exists in which a negative radial electric field produces an inward directed neoclassical impurity transport - the so-called ion-root transport regime [,, 0]. Therefore, it is of great importance to selectively enhance the outward transport of helium. Because typical exhaust efficiencies for helium ɛ He are well below 0%, the capacity to retain helium in the scrape off layer is an important issue to avoid helium transport back to the confined plasma where it is prone to accumulation. In this paper we report results from optimization of helium exhaust and fueling characteristics by means of resonant magnetic perturbation (RMP fields). RMP fields have been introduced in tokamaks and later also in helical devices as a tool to control plasma edge transport and stabilize adverse edge instabilities - so-called edge localized modes (ELMs) - which cause cyclic heat and particle fluxes to the wall elements [,,,,, ]. Based on these findings, ELM control by RMP fields is being prepared as one of the ELM control techniques for ITER [, ]. It has been shown that during application of RMP fields in a tokamak, the axisymmetry of the tokamak plasma boundary is broken and a three-dimensional plasma boundary is introduced [, 0,,,,,,,, ]. Recent modeling of RMP scenarios at ITER suggest that such D boundary structure can also be expected at ITER []. It has been shown that the actual shape and radial extension of the D boundary is directly linked to particle [0,,,,,, ] and impurity transport [,,, ] and consequently, it is appealing to explore in how far it can be used as a tool to control and optimize helium exhaust. At LHD, five different impurity transport regimes haven been experimentally identified in previous research as referenced below. First, in the case of positive radial electric field E r, an outward-directed neoclassical flow of impurities is seen. At higher ion temperatures, E r is dominated by increased ion losses resulting in a negative E r which

4 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R INTRODUCTION is capable of driving an inward-directed neoclassical impurity flow []. This regime is called the ion-root transport regime and is prone to impurity accumulation. We focus our investigations on this regime to test if this adverse generic impurity transport regime in heliotrons/stellarators can be improved by application of RMP fields and deliberately seeding of a magnetic island in the plasma edge. However, even in ion-root, a decontamination of the plasma core from impurities driven by large ion temperature gradients T i at mid radius was seen experimentally at LHD - the so-called impurity hole [, ]. In addition to these three regimes resulting from the specific E r situation, the impurity transport in general was also improved at LHD by manipulating the intrinsic stochastic layer and by seeding magnetic islands deeper inside of the plasma. Both regimes yield decontamination of impurities in the plasma by geometrical effects due to the collisionality gradient in the plasma edge through frictional forces. In the experiments presented in this paper, we focused on exploiting this knowledge of impurity transport with magnetic islands by seeding a magnetic island into the intrinsic edge stochastic layer at LHD and to improve the ion-root impurity transport. Recently it was shown, that an m/n = / RMP field can be used to screen the impurities from penetration into the confined plasma domain and hence retain them in the plasma edge outside of the magnetic island on the m/n = / resonance surface typically at located r eff = 0. [0]. RMP fields were also used to fine tune edge transport characteristics and stabilize detachment by trapping of the carbon radiation front at the island X-point []. Based on this breadth of previous results on the impact of RMP fields on impurity transport and fueling obtained on both, tokamak and stellarator type devices, the study presented in this paper aims to explore if these features can be used to improve helium exhaust and reduce the helium core fueling efficiency for recycled helium atoms. In order to approach this, we report on comparative experiments at TEXTOR and LHD. TEXTOR is a medium size limiter tokamak with a circular plasma shape featuring the Advanced Limiter Test II device (ALT-II)[, ]. Dedicated studies of its exhaust capacities have shown that it is efficient to exhaust hydrogen and also helium from both circular L-mode as well as radiative improved mode (RI mode) TEXTOR plasmas [, ]. TEXTOR is equipped with the Dynamic Ergodic Divertor (DED) [] as flexible RMP coil set and in the experiments discussed in this paper, we applied an RMP field with dominant toroidal/poloidal mode number m/n = /. LHD [] is a heliotron type, superconducting device with a 0 fold symmetry and a intrinsic stochastic layer at the plasma edge. Ten window framed coils on top of the device can be used to generate RMP fields with m/n = / and m/n = / base mode numbers. The magnetic axis of LHD can be shifted inward and outward such that the actual rotational transform, magnetic shear and location of the last closed flux surface (LCFS) can be varied. In this experiment, we target on the outward shifted configuration where the magnetic TEXTOR has been shut down in March 0, but we will refer to as still existing because data analysis is still being conducted on a variety of topics

5 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of TEXTOR RESULTS axis is located at R =.m. The magnetic shear profile ι(r) in this configuration enables to generate an m/n = / island inside of the intrinsic stochastic edge layer at the very plasma edge. This makes the LHD setup comparable to the TEXTOR experiments concerning the island location with respect to the distance to the last closed flux surface. We will discuss in this paper typical signatures of plasma edge transport to characterize the edge magnetic island situation and demonstrate how the magnetic island was identified for both devices and configurations. Then the helium transport and exhaust features will be discussed for both devices separately. The results from TEXTOR will be shown in section and the results from LHD will be shown in section. A comparison, conclusion and outlook to further studies will be presented in section.. TEXTOR results At TEXTOR two plasma boundary situations with RMP field applied can be distinguished. First, there is the formation of a so-called laminar zone where interleaved short and long connection length magnetic field lines in the stochastic layer form a set of correlated magnetic flux bundles of different wall to wall connection lengths [,,,, ]. In regard of particle transport, this configuration features a reduced particle confinement when the dominant resonance layer is located inside of the ionization source [0] but can also yield improved particle confinement when the resonant surface is moderately perturbed [, 0, ]. In the second perturbed boundary condition, particle transport is largely enhanced and particle fueling is reduced when a magnetic island is induced in the plasma edge. This configuration also features screening of carbon impurities released from the wall []. We report in the following of the influence of this edge magnetic island on helium exhaust. We discuss a class of TEXTOR discharges with the following typical plasma parameters: toroidal magnetic field B t =.T, plasma current I P = 0kA, ohmic heating power P OH = 0kW, neutral beam heating power P NBI =.MW, DED current I DED =.ka with m/n = / base mode configuration, edge safety factor q a =., central plasma density n c =. 0 m... Identification and role of a magnetic island in the plasma edge The presence of the magnetic island is identified by a variety of signatures in the plasma parameters. First, once the island is established we detect a flux surface structure in the island domain as seen in figure,a. In this figure, the emission of doubly ionized carbon C III at a wavelength of λ =.nm is shown. The location and size of the m/n = / contours of an island located on the q = surface at r/a = 0. are mapped out along isothermal flux surfaces in the island boundary. They agree with the topology obtained from field line tracing using the vacuum approach, i.e. without considering a plasma response. The vacuum magnetic topology is shown in figure and

6 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Identification and role of a magnetic island in the plasma edge TEXTOR RESULTS we see a clear correlation of the poloidal location of the modeled island location with the measured carbon emission pattern. This means that there is no poloidal or toroidal phase between the island which agrees with ideal MHD resulting in a resonant field penetration []. Also, the size of the island is largely comparable with the vacuum field line tracing result. Measurements of the m/n = / plasma response fields at TEXTOR are not available at the time, so we have to restrict ourselves to this visual verification of the existence of the island. In addition to the spectroscopic signal, we also measured for the first time at TEXTOR the plasma potential in the vicinity of the island. To achieve this measurement we used the capacity of the DED to shift the current maximum from one coil quartett to the adjacent one which was also used to map out plasma parameters in the laminar zone of TEXTOR-DED []. This yields a movement of the island O-point at the low field side of TEXTOR by θ = deg poloidally. Accordingly we can sample the radial profiles of the plasma potential with the reciprocating Langmuir system at the LFS of TEXTOR and obtain a poloidial map of the plasma potential from the X- to the O-point of the island. This procedure is sketched in figure,b. The map of the plasma potential is shown in figure,c. A clear increase of the plasma potential from 0V in the very edge of the plasma to 0V in the island O-point is measured. This equals to a radial electric field of negative E isl = 0V/cm across the island domain pointing from the island s O-point outward with strongest E r towards the island X-point. Recently, the evolution of such electric field structures in the vicinity of magnetic islands has been studied with the Hamiltonian drift code ORBIT []. Non-ambipolar drifts of ions and electrons inside of and around the magnetic island yield the measured space potential distribution and this potential structure can cause outward-directed radial transport. Similar results have also been found in quasi-linear MHD modeling for poloidal divertor tokamaks with RMP with the presence of magnetic island in the outer plasma boundary []. The potential structure formed by this non-ambipolar flow around the island and along the stochastic field lines departing from the island X-point towards the wall is aided by the local transport features in the island in establishing the local electric field feature which impacts the outward transport. At LHD is has been shown that inside of the island, a reduced cross-field transport is seen [, ]. Together with a large ratio of the parallel to perpendicular heat transport levels χ /χ >>, this yields a flattening of the temperature and density gradients across the island due to the direct connection of radially distant plasma regions along the island separatrix. However, the specific radial location of the island at TEXTOR causes connection of the invariant manifolds of the island separatrix to the DED target surface which induces a fast electrons loss channel. The ions seek to balance this fast electron loss by perpendicular drifts because of their larger gyro-orbits and hence it depends on the T e /T i ratio if electron of ion loss dominates [, ]. Because of this topological

7 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of τp,he with edge magnetic island TEXTOR RESULTS difference between LHD and TEXTOR, a poloidally oscillating plasma potential was measured at TEXTOR in agreement with the basic particle drift approach in ORBIT, which then can affect the outward-directed particle flux by localized ExB drifts (see for instance [] with results from the MHD code Jorek and [, ]). The measurements of the radial electric field transferring the plasma edge into a more positive plasma potential due to electron loss along open field lines was also discussed for TEXTOR in []. However, as we expect good flux surfaces inside of the island, it was shown that the island actually acts as a sub-domain for transport and interaction across the island separatrix depends on the level of viscous energy and momentum transfer [0]. At TEXTOR we measure a reduction of the particle confinement time τ P by 0 % with an edge island present. Moreover, it is seen that at the same time a re-fueling of the density loss caused by this confinement degradation is not possible. Therefore, the edge island situation shows the strongest reduction of particle confinement, i.e. the most severe level of particle pump out [, 0]. We hence expect the island having a strong impact on the exhaust and fueling characteristics of helium as well... Assessment of He fueling and transport and measurement of τp,he with edge magnetic island The overall particle balance for helium as a trace impurity in an H plasma is explored conducting puff/pump studies. A small amount of helium gas (flux density Γ He =.0 0 s for t puff = 0.s) is injected into a discharge with stable plasma density at t =.s. This results in a rapid raise of the line averaged density in the plasma as shown in figure, a. Here, time traces of the line averaged density are shown for a case without RMP (normp) and for three different phases of the DED current distribution representing three different positions of the island O-point poloidally. Three island positions were assessed and each configuration is denoted by the central poloidal angles for the island O-point of θ = 0deg, θ = deg and θ = deg relative to the start current distribution for θ. An offset in density is superimposed with the time traces shown in figure (a) to make the raise and decay features visible. The actual start point in density is the same for all four discharges. Two characteristic features are visible. First, the RMP field inducing the m/n = / island results in a reduction of the helium fueling efficiency seen by an overall reduction in the density rise for a constant additional gas puff compared to the normp case. Obviously, the edge magnetic island hinders the helium from penetrating into the plasma. Second, a faster decay time is seen for the RMP cases and the decay time seems to depend on the actual phase of the DED induced RMP field. This is more clear in figure, b. Here, the exponential decay times of the density rise induced by the short helium pulse are seen. The decay time of the line averaged density is reduced from about τ normp = s down to τ RMP = s, which is close to a factor of three. This observation confirms the reduced overall particle confinement with the edge island present but at the same time shows a reduced fueling efficiency for helium. The helium injected from a gas source in the outer periphery of

8 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Neutral pressure distribution with island TEXTOR RESULTS the TEXTOR vessel (approximately cm away from the last closed flux surface) is retained in the plasma edge and does not penetrate as efficiently the plasma core as in the axisymmetric configuration without RMP field applied. To investigate helium transport and exhaust further, the intensity of an ionized helium emission line (He II line at λ He II =.nm) as line integrated signal in the SOL was measured. The time behavior just after the helium injection is shown in figure, a. A clear accumulation of helium in the SOL with RMP applied is seen (color code is the same as before). The He II intensity increase is strongest for the intermediate phase (green curve) and also for the start phase for the DED currents (red curve). For the strongest phase shift of the magnetic island (purple curve) we see an emission signal comparable to the normp case which, however, features a faster decay as seen in figure, b. Here, the exponential decay times of the He II emission is shown as a measure for the effective helium confinement time τp,he. This measure for τ p,he represents a local quantity in the plasma edge, but as we in particular will discuss the helium confinement at the transition between plasma core and edge, this is used as appropriate measure for the helium confinement following [,,, ]. A clear overall reduction of τp,he with RMP is seen with a clear dependence of the actual level of reduction on the phasing of the magnetic islands in space. For the highest phase we find a reduction of τp,he from 000ms without RMP to 0ms, i.e. a reduction by %. This represents a significant enhancement of the helium exhaust efficiency also when compared to the first assessments using the ALT-II limiter [, ]. The edge magnetic island located in the close vicinity to the plasma source is shown to be a feasible control tool for helium exhaust characteristics... Impact of the magnetic island on the neutral pressure distribution These results provide substantial experimental evidence that magnetic island enhance the outward transport as well as reducing the fueling efficiency of helium. At the same time, the D magnetic topology in the plasma edge a mixture of closed field lines inside of the island separatrix and open field lines of various magnetic connection length connecting form the island X-point region to the wall [,, ]) is likely to alter the coupling to the ALT-II limiter blades as exhaust device. For details of the ALT-II layout and the pumping geometry, please see figure in []. This is assessed using the total neutral pressure measured in the scoop of the ALT-II limiter blades. This neutral pressure represents qualitatively the collection efficiency of the ALT-II scoop entrance in the given magnetic topology. The result is shown in figure. From left to right, the three selected phases of the DED current are shown. The red time trace in each figure is the neutral flux measured in the ALT-II blade at φ =.deg toroidally and the green time trace is from the blade at φ = deg. The black time trace is the averaged neutral pressure from all eight pressure measurements. Hot cathode ion gauges are em-

9 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of Neutral pressure distribution with island TEXTOR RESULTS ployed and all data are plotted as values normalized to the start time before the DED is switched on. For the left and right DED phase samples (θ = 0deg and θ = deg), a clear, i.e. 0% increase of the toroidally averaged neutral pressure in the ALT-II pump ducts is measured. A modulation of the neutral pressure is seen before and after the helium injection when inspecting the averaged neutral pressure. The first roll-over at about. s is a consequence of the RMP application which is known to enhance the outward particle transport, called particle pump out [0]. This yields increase of the neutral pressure in the pumping duct in order to exhaust this increased outward particle flux. Then the time trace goes down and a further increase is seen again when the He is injected. The neutral pressure signal equilibrates afterwards to a 0% increase. The same trends in the time traces of the averaged neutral pressure (black time traces in all three plots of figure ) are seen, however, for the intermediate phase at θ = deg, no constantly increased neutral flux in the pumping device was measured, but only a short time spike right after the helium injection. This shows that the relative phase of the magnetic island induced to the pumping devices, i.e. ALT-II pumping duct openings defines the coupling efficiency to the significantly increased neutral helium population in the SOL. This D effect is also seen for each individual DED phasing situation comparing two neighboring measurements. This is shown as green and red time trace in figure. They show the time traces from two neighboring ALT-II blades positioned within a toroidal angle of φ =.deg. A significant variation of the neutral pressure is measured when comparing both adjacent ALT-II pump duct neutral pressures. For instance, for phase (figure,a), an % reduction of the neutral pressure is seen in the green time trace obtained in the pump duct at deg toroidally while a 0 0% increase is seen on just the next ALT-II limiter blade pumping ducts at.deg toroidally (red time trace). The relative ration of neutral pressures between the two adjacent ALT-II pumping positions is comparable for all three phases, but the relative amplitude changes such that a clear increase of the overall neutral pressure in the toroidal average value is seen for phase one and three as discussed before. These measurements suggests that the connection of the magnetic island defining the shape of the SOL under RMP conditions in the scenario considered defines the pumping efficiency for the helium accumulating in the plasma edge due to enhanced transport and reduced fueling with edge magnetic islands present. In this regard it is important to note that the averaged neutral pressure after the DED is switched off is different for phase two when compared to phase one and three. While for phase one and three, the total neutral pressure returns to values very close to the time before RMP was energized for the minimal (figure, a) and maximal phase figure, c), a clear increase of the averaged neutral pressure is seen after the RMP field was terminated (t >.s) for the intermediate phase two in figure, b). Together with the fact that the He II intensity increased most strongly for this DED phasing (see figure,

10 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R LHD RESULTS b), this supports helium accumulation in the plasma edge and release to the pump once the magnetic island has vanished. Most likely the helium injected is accumulating in the island itself or the long connection length domain around the island X-point without a direct connection to the pump entrance for this DED phase setup. For the other two DED phase settings, the results suggest that open field line domain is connected in an optimized way to the ALT-II pump duct openings such that the actual pumping is enhanced. Detailed assessment of the fine structured perturbed magnetic field topology is ongoing and will be subject of a future publication. This experimental assessment provides substantial evidence that the RMP field in general has a direct effect on helium exhaust and fueling and at the same time suggests RMP amplitude and phase can be new control parameters to define the actual mechanism of the enhanced helium exhaust characteristics.. LHD results At the Large helical Device (LHD) [], the magnetic topology is altered by a resonant magnetic perturbation field supplied from a row of 0 window frame coils on top of the device []. Two different RMP configurations are addressed in this study. First, we examine an m/n = / dominated RMP field. The discharge assessed is in the outward shifted configuration with central plasma radius at R 0 =.m supplies a resonant surface for this mode at about half radius. The outward shifted configuration was used because it is a representative setup for ion-root dominated confinement and the magnetic shear profile is such that a m/n = / island can be generated in the very plasma edge. This is used in the second RMP configuration, in which a m/n = / dominated RMP field was applied and a resonant m/n = / island was opened in the very edge of the plasma at about r eff /a = 0.. We applied an m/n = / field with a nominal current of I RMP =.ka and the m/n = / field with a current of I RMP =.ka and a second current value of I RMP =.ka. This overall RMP setup is focused on comparing the effect of core and edge magnetic islands on helium transport and exhaust. Other typical plasma parameter for the discharge series considered are as follows: Central plasma density is.0 0 m at a heating power supplied by neutral beam heating P NBI = 0 MW. The thermal energy in the plasma for these discharges was typically W P = 0kJ at a magnetic field of B t =.T. The diamagnetic pressure value normalized to the magnetic field is β dia = 0. for the discharges considered making the plasma prone to island healing [] at a level below typically I RMP < ka in the m/n = / configuration [,, ]. We will discuss evidence for island healing and penetration in our actual experiment in the following... Perturbed magnetic field topology The most direct comparison to the TEXTOR results will be possible for the m/n = / configuration. To discuss the magnetic field topology changes induced by this RMP

11 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page 0 of Perturbed magnetic field topology LHD RESULTS field configuration, we analyze the magnetic field line connection length pattern for the normp case and the case with the m/n = / base mode at highest current amplitude of I RMP =.ka. The results from vacuum field line tracing for these two configurations are shown in figure. The magnetic topology at the horizontally elongated cross section of LHD is shown for the normp case in figure,a and for the m/n = / case at I RMP =.ka in figure, b. The edge surface layer as a stochastic mixture of magnetic field lines with different connection length L c values is seen already in the normp case []. When the RMP field is applied, a clear broadening of this stochastic edge layer is seen including the development of an m/n = / island seen as a long L c magnetic field line domain in the lower part of the stochastic layer. This results from the fact that, inside of the island field lines connect with infinite length onto themselves as the island is formed on a resonant surface. Towards the X-point, bending and stretching of the long L c domain is seen because of onset of the perturbation of the regular island field topology when approaching the X-point. The flux expansion in the mid-plane at the elongated section and the impact on the actual magnetic shear causes this deformation (see [0,, ] for more details about the edge stochastic layer and plasma transport therein at LHD). In addition, the X-point as a hyperbolic fixed point is prone to the decomposition of its invariant manifolds by the radial magnetic field of higher order resonant field components which yield formation tangles as an envelope of magnetic field lines which can mix with the stochastic exterior. This was discussed for poloidal divertor tokamaks [, ] as well as for helical stochastic edge systems in tokamaks like the one induced at TEX- TOR by the DED field [,, ]. Because this effect is generic to the Hamiltonian nature of the magnetic field line motion in a conservative system without energy dissipation, we expect such a process to be relevant for the X-point of magnetic islands in stellarators as well. The comparison between the normp and the RMP case considered for LHD show a clear increase of the stochastic layer width including formation of a magnetic island. The plasma response to the RMP fields applied is discussed in comparison with the normp situation in figure. The three RMP configurations considered are shown from left to the right with the m/n = / configuration at I RMP =.ka first in figure,a. The normp reference profiles for each case are shown with black data points and the RMP profiles are depicted colored. The normp profiles were measured just before the RMP field coils were energized. In the lower part of the plot, the Poincare maps of the magnetic field configuration are shown and in the upper plot the electron temperature profile T e (r) obtained from Thomson Scattering is depicted. No difference is seen between the normp T e (r) profile plotted in black and the m/n = / RMP profile suggesting island healing, i.e. no field penetration to form the magnetic island. This is supported by Fourier mode decomposition of the radial magnetic field from the plasma measured by magnetic sensors at the vessel wall as described for instance in []. These 0

12 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Helium transport and contamination of the core plasma LHD RESULTS measurements show a radial m/n = / field amplitude of B r,/ = G at a phase relative to the RMP field applied of ξ 0deg. which is a signature for generation of a internal response current preventing the external field from penetrating and hence the island from forming [, ]. The expected flattening of the temperature profile is missing in agreement with this signature for island healing. A similar conclusion can be drawn for the m/n = / RMP case at lower current of I RMP =.ka as shown in figure,b. A magnetic island is expected to be located in the plasma edge. However, again a significant m/n = / plasma response is measured with B r,/ = G and a phase of ξ 0deg. The T e (r) profile only shows a very moderate flattening on the right side of the profile. Both observations together support that the island penetration is marginal and a partial healing of the m/n = / island at this low RMP current level is seen. We therefore consider this as a case where the plasma shields the RMP field but the discharge might just be at the margin of dissipating the shielding current and allowing the island to penetrate. The island which starts to open has a width of less than 0% of the island established at I RMP =.ka. At this current level, a clear signature for island penetration is seen as shown in figure,c. Here, the T e (r) profile undergoes a clear flattening in the outer region of the plasma with a flat spot radial extension of r cm on both the inside and on the outside. This is in agreement with the vacuum calculated magnetic island width at these positions. Moreover, the amplitude of the m/n = / mode radial magnetic field B r,/ is nearly vanishing as well as the relative phase ξ which is a clear sign for dissipation of the local shielding current and reconnection of the local field lines into a magnetic island topology. In conclusion, we address two configurations with significant radial perturbation amplitude where island healing is seen (m/n = / case at I RMP =.ka and m/n = / case at I RMP =.ka) and one case with clear island penetration (m/n = / case at I RMP =.ka). The impact of these three different RMP situations on helium transport and exhaust is assessed again with puff/pump studies... Helium transport and contamination of the core plasma The penetration of the helium injected to the SOL and confined plasma region is assessed by the measurement of He I intensities at λ He =.nm [] and by CXRS measurement of He II emission at the diagnostic neutral beams []. The He I intensities are also used to calculate a He/H and He/(He+H) ratio in the plasma edge using an advanced collisional radiative model for generalization of measurement of the two single transitions for helium and hydrogen []. The active He II signal is also used for calculation of the He/H and He/(He + H) ratio using a cross calibration with the edge system using the He I emission as both systems have overlapping lines of sight []. Time traces of the He I intensities in the plasma edge for all four plasma

13 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of Helium transport and contamination of the core plasma LHD RESULTS configurations considered are shown in figure, a. Time traces of the He II emission at r =.m, which is inside of the m/n = / resonant surface at r r =.m, are depicted in figure,b. In the plasma edge, about an 0% reduction of the He I emission is seen only for the case with penetrated edge island. In contrast a much stronger effect is seen when assessing the level of ionized helium emission from inside of the resonant surface. Here, the configuration with penetrated island shows a strong reduction of the emissivities of ionized helium from CXRS which provides strong evidence for a significant reduction of the helium level in the plasma inside of the magnetic island. This finding is supported by analysis of the He/H ratio from both spectroscopic diagnostics. The time traces of He/H for the edge system employing He I emission for all four configurations is shown in figure, a and the correspondent analysis for the CXRS system is shown in figure, b. Here, we show He/H(t) at two different positions. The time traces at r =.m, i.e. just outside of the magnetic island location, are shown as circle markers and the time traces at r =.m, i.e. just inside of the island position, are shown as diamond markers. The color code again depicts the different plasma configurations addressed. The edge channels show only a marginal difference between the configurations but a noticeable reduction of 0% in the He/H ratio for the configuration with edge island present. This setup also features a faster decay of the He/H ratio pointing towards a faster - selective - exhaust of helium from the SOL domain. The strongest impact of the magnetic island is seen when comparing the He/H ratio inside and outside of the island in figure. Here, we find a clear reduction by more then 0% of the helium concentration relative to H at the radial position just inside of the magnetic island position. This is most obvious when comparing the radial profiles of He/H(r eff /a ) as shown in figure 0. In this figure, the helium concentration relative to hydrogen are shown as radial profiles just before the helium pulse which is used as transient source in the puff/pump studies employed. Both discharges feature initially a small level (0%) of helium concentration. This level is increased to a peak value of 0. for the case without RMP field and reduced again to 0. when the magnetic island is present in the plasma edge. This measurement shows a substantial level of helium contamination for the standard stochastic magnetic topology. Once the magnetic island is penetrated in the plasma edge, the helium contamination of the plasma core is strongly mitigated along the entire profile range. The time dependence, as well as the radial profile behavior of He/H ratio discussed previously provide substantial evidence for screening of the helium from penetration to the confined plasma domain due to the magnetic island. It s important to note that the He/H ratio inside of the m/n = / resonance layer shows a high He concentration of up to 0%. This emphasizes for the normp case that in this particular (unfavorable) impurity transport regime, light impurities like helium undergo an effective inward transport

14 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Helium confinement time LHD RESULTS and a much smaller effective outward decontaminating force is present to prevent the helium injected from effective plasma fueling. This is in agreement with previous findings on the inward directed impurity transport for high Z impurities like iron but also for lighter impurities like neon [,, 0]. In all discharges considered in this study, a negative radial electric field of kv/m in the region inside of the resonance layer was measured with the CXRS system, suggesting also for this study that the branch of ion-root transport regimes in LHD facilitate neoclassical inward transport of impurities. However, the study presented here points out that helium with a source at the plasma edge can be retained in the SOL domain when a magnetic island which prevents the helium from penetrating is generated. For recycling helium sources, the magnetic island can clearly prevent this contribution to the helium household from penetration. While in a burning plasma the helium source will be located inside of the plasma core, the actual helium concentration will be dominated by the wall recycling since at moderate pumping efficiencies of helium from the plasma periphery, each helium atom has many attempts to enter the plasma before it is exhausted. The results presented here therefore show evidence for strongly reduced helium fueling efficiencies and support that an edge magnetic island can achieve this goal of retaining the recycled helium in the plasma periphery... Effective helium confinement time τ p,he and helium decontamination To quantify the confinement behavior of helium in the plasma and in the vacuum chamber we treat the combined system in a single reservoir model [0, ]. This means that we assume typical decay rates of helium intensities and He/H ratio as being representative for the entire system. Since we deal with a D system this has obviously some caveats but this analysis can help to quantify the overall system performance with respect to helium exhaust efficiency as also discussed previously for example in [, ]. Therefore we extract an effective helium confinement time τp,he in the same way as done for the TEXTOR analysis discussed before by an exponential fit to the decay of the He I intensity in the plasma edge. The result is seen in figure, a. First, we note that τp,he is a factor of four higher than for the tokamak example based on TEX- TOR discussed here when comparing the normp cases on both devices. This reflects the tendency of the particular ion-root impurity transport regime at LHD which we have chosen to address in this study to accumulate impurities once they can enter the neoclassical transport domain, i.e. inside of good flux surfaces. It is of particular importance for heliotron/stellarator devices that at high density and temperature (both will operate in the ion-root regime), appropriate methods for enhancement of the outward and reduction of the inward radial transport of helium are developed. The values of τp,he extracted for the four configurations addressed here show that the m/n = / magnetic island induced by RMP fields into the edge stochastic layer at LHD can yield

15 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of Particle exhaust with magnetic island LHD RESULTS a reduction to τp,he =.s, i.e. coming closer to the tokamak value from TEXTOR. However, the two RMP cases without field penetration result in an increased effective confinement behavior. It is also important to note that a strong reduction of the helium dilution time τ dil is measured. This is defined as the time scale on which the relative concentration of He measured by the He/(H+He) radio decreases as shown in figure, b. We define this additional time scale because it is well known that RMP field increase the radial particle transport and reduce the global particle confinement time for the main plasma species (see e.g. [0, 0, ] for discussions about particle pump out in tokamak plasmas with RMP fields). This feature is in general not desirable as for ITER such a reduction of τ P for hydrogen would result in a reduction of density and hence in a reduction of the fusion gain. Recent modeling of the particle balance during RMP Elm control discharges at ITER have shown that a reduction of the particle confinement is expected []. With the assessment of τ dil we want to discriminate the helium exhaust from the general exhaust of H which might also be affected by the RMP field. This time scale is extracted from the He/(He+H) measurement as exponential decay time. A strong and very promising decrease of τ dil =.s without RMP down to τ dil =.s with magnetic edge island present is found. The reduction of τ dil (%) is by a factor of stronger then the reduction in τp,he (%). This measurement provides evidence that the enhancement of helium exhaust is selectively stronger than a possible overall increase of the radial flux or decrease of the overall fueling efficiencies... Particle exhaust with magnetic island In order to assess the exhaust characteristics and neutral gas dynamics in the LHD plasma periphery, measurements of the total neutral pressure utilizing Asdex-type hot cathode gauges [] at two characteristic positions in the LHD geometry are presented in figure. We pick one position inside of the inner helical divertor private flux region in order to characterize the divertor neutral pressure labeled with I (inner position in module ). Additionally we characterize the neutral pressure in the outer divertor private flux region in module which is labeled by O. The time traces of the neutral pressures at these two locations are shown in figure,a and c. A 0 0% increase in the absolute neutral pressure is measured at these two locations when the magnetic island is present. Under the assumption of a comparable overall neutral source (wall recycling being comparable and the gas puff was held constant), this suggests that the fueling efficiency is reduced and hence helium neutral gas injected is repelled from penetrating the plasma. The magnetic island acts as a fueling barrier and enables the recycled helium in the far SOL domain to be retained. In order to study the toroidal dependency of this effect, the averaged neutral pressure between t =..s was measured at three toroidal positions assuming a nearly

16 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Particle exhaust with magnetic island LHD RESULTS stationary behavior during this time window. The absolute neutral pressures at the three toroidal locations are shown in figure, b and d. A clear increase of the total neutral pressure on all toroidal locations is seen for the configuration with a penetrated m/n = / magnetic island located in the plasma edge. The total neutral pressure is found to be a function of the toroidal position at both poloidal locations, but the relative increase with magnetic island relative to the normp value is almost identical for all three toroidal positions assessed. This shows that the successful retention of H and He in the divertor modules is toroidally isotropic for LHD which is in stark contrast to the characteristic seen at TEXTOR. Here, the fact that the helical alignment of the edge magnetic island is not matched by the ALT-II limiter pumping geometry resulted in a strong sensitivity of the actual collection efficiency in the ALT-II pump ducts on the relative position of the magnetic island. At LHD, the helical divertor is intrinsically aligned with the pitch of the magnetic island which strongly mitigates this geometric sensitivity and consequentially a very comparable improvement of the neutral compression in all inner and outer divertor positions assessed is seen. This is a very promising result for divertor design in stellarators. To investigate the fractional helium neutral pressure contribution to this overall increase of the neutral pressure in the plasma periphery, we use spectroscopy on two Penning gauges. One is mounted at I and another one in the main chamber in port. U - i.e. an upper port in half-module.. This port is just one large vertical port on the LHD device equipped by a spectroscopically assisted Penning gauge. The line emission of helium and hydrogen from the Penning discharge inside of the gauge are measured, calibrated and then the total neutral pressure signal is attributed in relative fractions based on the emissivities yielding a fractional H (p n,h ) and He (p n,he ) neutral pressure measurement (see citedenner for an explanation of the general concept for example). The results for p n,h (t) and p n,he (t) are shown in figure and we find that both the hydrogen as well as helium fractional pressure increase substantially by 0% at the divertor location. This is in agreement with the previously discussed total neutral pressure increment obtained with hot-cathode gauges [] (the 0 0% deviation in the absolute values measured by both types of neutral pressure gauges is matter of relative calibration of the gauges for H and He gas species). This is clear evidence that the hydrogen fueling, as well s the helium fueling, is affected by the magnetic island. Even more noticeably, the hydrogen and helium neutral pressures are increased by > 00% in the main chamber as seen in figure c and d. At this position, the level of increase is comparable for both gas species. One additional feature is seen in these measurements. The fractional neutral pressures increase immediately after the discharge is terminated for the cases without islands present. In particular the helium fractional pressure at the main chamber position (see

17 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of CONCLUSION AND FINAL DISCUSSION figure d) rises immediately after the discharge ends and nearly reaches the intershot level of helium neutral pressure with the magnetic island present. This shows substantial accumulation inside of the plasma without the island which was also seen in the He/H ratio of up to 0%. Once the discharge ends, this amount of helium gas is released, neutralized at the walls and seen as neutral pressure in the plasma periphery. These results suggests that the magnetic island yields a reduction of the fueling efficiency and hence results in an accumulation of hydrogen and helium neutrals in the plasma periphery. However, it needs to be noted, that the source levels for helium and hydrogen are different. For hydrogen, a substantial reservoir exists from H retention in the ferritic walls of LHD and H is further supplied from the gas feed system. For He, the short pulse gas injection is the only source and we only inject about Φ H e 0 particle during the short He gas pulse. These considerations on the particle source for both species are important because they are necessary to gauge the relevance of the two different levels of neutral pressure increase. In general, the ratio of both fractional neutral pressures reached just after the He puff is about p n,h /p n,he 0 for the main chamber position (. I) and p n,h /p n,he 0 0 for the divertor position ( I). These ratios are slightly reduced to p n,h /p n,he at. I and p n,h /p n,he 0 at I. The rapid increase of p n,h after He injection points towards enhanced wall recycling and H release from the wall reservoir. Both sources together then fuel the plasma in the given magnetic topology and result in the neutral pressure dynamics and the density response discussed.. Conclusion and Final Discussion The comparative study on the impact of RMP fields on helium transport and exhaust features between TEXTOR as example for a tokamak and LHD as example for a heliotron/stellarator device has demonstrated that magnetic islands in the plasma edge have the potential to be an effective control tool to improve helium exhaust characteristics. On both devices a significant reduction of the helium concentration in the plasma has been measured exhibiting a reduced effective confinement time for helium τp,he and increased total as well as fractional He and H neutral pressures in the pump devices as well as in the main chamber. The reduction of τp,he is very promising because this overall system confinement time shows that the helium extraction from the two devices considered can be enhanced with RMP fields when they induce a magnetic island in the plasma edge. However, understanding the underlying mechanism is a challenge because of the complexity of the physical processes which determine the particle balance for hydrogen and helium. The experimental results presented show evidence for an enhanced outward transport because the decay times of the helium concentration in the plasma core are shorter when

18 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R CONCLUSION AND FINAL DISCUSSION the magnetic island is present. We see reductions of τp,he of % at TEXTOR and % at LHD. In addition, the actual density rise in the plasma core in response to a brief He gas pulse from the outside is seen to be substantially lower with the edge magnetic island present on both devices. The density rise is 0% smaller compared to the no-rmp situation at TEXTOR and 0 % at LHD. This is a strong indication for a reduced fueling efficiency from the SOL into the core plasma domain. This in turn impacts τp,he as this effective number is the result of the outward directed transport term as well as the re-fueling of the plasma from recycled gas before it can be pumped away. This re-fueling is defined by the inward directed transport and hence, the reduction of τp,he can not determine if the inward or outward directed flux is most strongly affected by the magnetic island. The strong accumulation of neutrals in the divertor and even stronger accumulation in the plasma periphery indicates a substantially reduced fueling efficiency and hence a strong role of reduced inward transport. However, at the same time there is evidence of a poloidal potential structure around the edge island at TEXTOR which enhances the outward directed transport for the particular T e /T i ratio resulting in an electron-root like outward flux in the TEXTOR experiment []. At LHD, the specific scenario addressed here is in ion-root and hence the inward directed neoclassical flow of impurities makes the plasma in this configuration prone to impurity accumulation. However, on both devices, the magnetic islands are shown to reduce the He contamination of the plasma significantly. In addition to the local impact of the magnetic island on plasma transport, it yields local flattening of radial and by this also parallel temperature gradients which reduce the inward directed thermo-force on impurities [, ]. Last but not least, the magnetic island is located in the very plasma edge and hence it can increase the radial extension of SOL like transport due to short-circuiting the radial region around the island to the wall elements. This would increase the effective radial domain which includes sonic plasma flow towards the wall elements and hence it will increase the net outward-directed plasma flux [,,, 0]. All transport elements together will actually define how much neutral gas will be retained in the plasma periphery where it can be pumped away. The neutral gas measurements presented support reduced fueling efficiencies since a constant increase of the neutral pressure of both helium and and hydrogen in the plasma periphery has been measured on both devices. If the fueling efficiency would be comparable, the increased neutral gas pressure would yield increased plasma density. This neutral pressure increase is only measured with a magnetic island in the plasma edge and consequently all other configurations considered feature a release of He and H neutral gas after the discharge is terminated. The geometrical alignment of the mechanical exhaust device is significantly better in the helical divertor at LHD compared to TEXTOR as here the divertor baffle and housing are intrinsically aligned with the edge resonance

19 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of CONCLUSION AND FINAL DISCUSSION on which the island is generated. This points out that if a -D boundary is used in an axisymmetric device - as currently planned for ITER using RMP ELM control - the impact on the neutral and helium exhaust features has to be addressed. Another critical question is also if He is preferentially exhausted from the plasma and also, if it can be preferentially retained in the plasma periphery until it is pumped away. If the beneficial RMP impact on τp,he is dirtecty related to a strong degradation of the overall particle confinement, the method is not feasible to improve helium exhaust. However, the results shown support that a reduced re-fueling efficiency is a strong contributor to the effect seen and that the actual outward transport level might not be very significant. This is supported by comparison of the characteristics of the τp,he reduction and the reduction of the He dilution τ dill time presented in the paper. The reduction of τ dill is a factor of two larger then the already significant % reduction of τp,he at LHD. This shows that a selective impact on the helium decontamination on the plasma is found. These results show for the first time that RMP field and in particular magnetic islands located in the very edge of the plasma can be used to increase the effective helium exhaust from plasmas in toroidal magnetic confinement systems. Further studies to investigate the impact on the helium transport through the stochastic field domain surrounding the islands and into the plasma core [] show that the effect of the RMP fields on the edge helium dynamics dominate the helium transport and exhaust features. To elaborate exactly on the relative contribution of inward and outward transport changes vs. improved coupling to pumping systems is matter of future analysis. While at TEX- TOR a complete particle balance was obtained [0, ] and discussed as reference in this paper, this is extremely complicated for LHD. In particular the closed helical divertor [, ] makes it almost impossible to measure the total recycling source of hydrogen and helium with a useful accuracy. Hence, separation of helium and hydrogen confinement based on absolute confinement times for instance is rather difficult. Our goal is to deploy the EMC-Eirene D fluid plasma and kinetic neutral transport code [] aided by as much as possible information from experiment, including new D imaging spectroscopy and bolometer measurements. An initial step in this direction has been made [] showing that indeed the helium recycling and the interaction with the plasma gradient region defines the actual plasma fueling levels. The interpretation of the measurement results on the level of a global, single reservoir particle balance as employed in this paper, however, clearly demonstrates that RMP fields are a powerful control mechanism for helium exhaust. The combination with exhaust schemes like the island divertor at Wendelstein -X [0, ] is promising as there the magnetic island, which is used in the present study to manipulate the helium household, actually defines the plasma wall interface itself.

20 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R CONCLUSION AND FINAL DISCUSSION Acknowledgements: This work was supported by JSPS KAKENHI Grant Numbers 0, by start up funds of the Department of Engineering Physics and of the College of Engineering at the University of Wisconsin - Madison, USA and under grant DE- SC000 and DE-SC0000 of the U.S. Department of Energy. The authors thank all members of device engineering and experimental groups for their operational support of LHD and TEXTOR. Generous travel support by the LHD management and support for research at LHD by the entire LHD experiment group for O. Schmitz is acknowledged and highly appreciated. The efforts of T.Akiyama, M.Emoto, T.Kobayashi, C.Moon in development, maintenance and user support of the MyView data viewer are highly appreciated.

21 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page 0 of Figures a.) m/n=/ island Radius R HFS CIII b.) Sketch of island at LFS position O-Point X-Point CONCLUSION AND FINAL DISCUSSION c.) Poloidal Direction (DED steps) ~ 0 degree poloidally Plasma Potential between Island X- and O-point Distance from LCFS (cm) Figure : Identification of the edge magnetic island structure induced by RMP. In figure a.) the double ionized carbon emission at high field side is shown as direct image of an m/n=/ island. In figure b.) the magnetic topology for this scenario at the outer mid plane is depicted and the plasma potential in the vicinity of the magnetic island at the outer mid plane in figure c Plasma Potential (V) 0

22 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R DED target ALT-II limiter CONCLUSION AND FINAL DISCUSSION Figure : Overlay of a Poincare plot and the magnetic field line connection length (color coded) showing the perturbed magnetic topology for exhaust studies at TEXTOR Magnetic field line connection length [m]

23 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of CONCLUSION AND FINAL DISCUSSION Figure : Time trace of the line averaged density around the he gas puff (left figure) and density decay times (right figure) for the normp phase of the discharges and the three DED current phases. A strong enhancement of the density decay as well as a reduced fueling efficiency of the helium gas is detected.

24 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R CONCLUSION AND FINAL DISCUSSION Figure : Time trace of He-II emission (λ =.nm) (left figure) and τ p,he extracted from the He-II emission (right figure).

25 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of CONCLUSION AND FINAL DISCUSSION Figure : Time trace of neutral pressure in selected ALT-II pump ducts (for details see figure in []) (red - blade at.deg toroidally), green - blade at deg toroidally) and of the neutral pressure averaged over all eight ALT-II blades (black curve). The time traces on bottom shows the DED current indicating the time with RMP field and edge island present.

26 Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R CONCLUSION AND FINAL DISCUSSION Figure : Magnetic topology of unperturbed LHD boundary (normp case, figure a) and of the case with edge m/n = / RMP field where island penetration is detected (figure b) from magnetic signal analysis represented in terms as the wall to wall magnetic field line connection lengthh. The edge surface layer as mixed stochastic and SOL length scale domain is seen in the RMP case with magnetic island embedded as long connection length domain.

27 AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of CONCLUSION AND FINAL DISCUSSION Figure : Electron temperature profiles T e (R) for all three LHD RMP cases considered. Only for the m/n = / base mode number RMP field at high current amplitude, a flattening of T e in the plasma edge is seen indicating penetration of the m/n = / island. The normp reference profiles are shown with black data points and the RMP profiles are depicted colored. The lower plots show the magnetic field geometry as superposition of a calculation of the magnetic field line connection length and a Poincare plot.