Plasma Science and Fusion Center Massachusetts Institute of Technology Cambridge, MA USA

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1 PSFC/JA-7-53 The effect of electron cyclotron heating on density fluctuations at ion and electron scales in ITER baseline scenario discharges on the DIII-D tokamak A. Marinoni, R.I. Pinsker, M. Porkolab, J.C. Rost, E.M. Davis, K.H. Burrell, J. Candy, G.M. Staebler, B.A. Grierson, G.R. McKee 3, T.L. Rhodes 4 and the DIII-D Team July 7 Plasma Science and Fusion Center Massachusetts Institute of Technology Cambridge, MA 39 USA General Atomics, P.O. Box 8568 San Diego, CA , USA Princeton Plasma Physics Laboratory, PO Box 45, Princeton NJ 854 USA 3 Univ. Wisconsin, Madison WI 5376 USA 4 Univ. California, Los Angeles, 45 Hilgard Ave, Los Angeles CA 995 USA This work was supported by the U.S. Department of Energy under DE-FG-94ER5435. Reproduction, translation, publication, use and disposal, in whole or in part, by or for the United States government is permitted.

2 The effect of Electron Cyclotron Heating on density fluctuations at ion and electron scales in ITER Baseline Scenario discharges on the DIII-D tokamak Experiments simulating the ITER Baseline Scenario on the DIII-D tokamak show that torque-free pure electron heating, when coupled to plasmas subject to a net co-current beam torque, affects density fluctuations at electron scales on a sub-confinement time scale, whereas fluctuations at ion scales change only after profiles have evolved to a new stationary state. Modifications to the density fluctuations measured by the Phase Contrast Imaging diagnostic (PCI) are assessed by analyzing the time evolution following the switch-off of Electron Cyclotron Heating (ECH), thus going from mixed beam/ech to pure neutral beam heating at fixed β N. Within ms after turning off ECH, the intensity of fluctuations is observed to increase at frequencies higher than khz; in contrast, fluctuations at lower frequency are seen to decrease in intensity on a longer time scale, after other equilibrium quantities have evolved. Non-linear gyro-kinetic modeling at ion and electron scales scales suggest that, while the low frequency response of the diagnostic is consistent with the dominant ITG modes being weakened by the slow-time increase in flow shear, the high frequency response is due to prompt changes to the electron temperature profile that enhance electron modes and generate a larger heat flux and an inward particle pinch. These results suggest that electron heated regimes in ITER will feature multi-scale fluctuations that might affect fusion performance via modifications to profiles. PACS numbers: 5.5.Xz, 5.3.Gz, 5.55.Fa, 5.65.Tt, 5.3.Gz, 5.7.Kz Keywords: ITER, ETG, Phase Contrast, turbulence, ECH

3 Introduction The characterization of the ITER Baseline Scenario (IBS) regime is being pursued on the DIII-D tokamak due to its importance for predicting fusion performance in ITER. Such scenario has been recently extended to more reactor relevant regimes by carrying out experiments at low external torque and electron temperature close to that of ions, as it is projected for ITER, utilizing Neutral Beams (NB) oriented in co- and counter-current directions thus providing near balanced torque, and torque-free direct electron heating via the Electron Cyclotron Heating (ECH). The advantage of using ECH is twofold: first, it simulates electron heating from alpha particles generated in a burning D-T plasma, such as in ITER; second, being a direct electron heating whose deposition location can be finely controlled, it enables detailed studies on electron transport which, as opposed to ion transport, is still poorly understood. The experiment considered in this work is aimed at investigating how transport depends on E B shear and on the type of auxiliary power used. In particular, whether fusion power provided by alpha particles, mimicked by ECH, is likely to be a heating scheme as effective as the mixed ion-electron heating provided by neutral beams. Although the main actuator used to probe the plasmas considered in this experiment is a torque-free source of direct electron heating, it is usually observed that other quantities, such as density and flow, are greatly affected and provide a feedback effect on fluctuations and confinement. This motivates the need for comprehensive transport studies in ITER relevant discharges, which is the topic of the present work. This paper is organized as follows. Section briefly describes how the experiment was performed and physical quantities were measured; the effect of heating scheme on ion scale and electron scale fluctuations is reported, respectively, in Sections 3 and 4; Section 5 is devoted to the interpretation of experimental measurements through linear gyro-fluid and non-linear gyrokinetic modeling; conclusions are offered at the end. Experimental set-up A typical equilibrium is depicted in figure, where the contour lines of the normalized poloidal flux are superimposed on the viewing chord of the Phase

4 Contrast Imaging (PCI) diagnostic, which is the main system used to monitor density fluctuations in this work. The electron temperature was measured by Thomson Scattering (TS) [] and Electron Cyclotron Emission (ECE) []; density was gauged by TS, CO Interferometer [3] and microwave Reflectometer [4]; Charge Exchange Recombination (CER) [5] was used to obtain the ion temperature, along with the toroidal and poloidal components of its velocity ; density fluctuations were monitored by Beam Emission Spectroscopy (BES) [6] in the long wavelength range, by the Doppler Back Scattering (DBS) [7] in an intermediate wavelength region, and by the Phase Contrast Imaging (PCI) [8, 9] that measures line averaged density fluctuations in a broad intermediate-short wavelength regime and a wide frequency bandwidth. In this paper turbulence measurements will mainly be provided by PCI measurements, and results interpreted with synthetic diagnostic predictions based on the non-linear gyrokinetic GYRO code []. The scenario in the experiments reported in this paper aims at mimicking the ITER baseline scenario operation with as many elements relevant to the ITER conditions as can be reasonably achieved in DIII-D, with the main goals of maintaing a scaled ITER shape, normalized plasma current I p /(ab T ).45, normalized pressure β N and T e T i. Specifically, it is characterized by plasma current I p =.3 MA, line averaged density n e m 3, elongation κ =.9, top triangularity δ t =.4, bottom triangularity δ b =.8, normalized beta β N, confinement quality factor H 98y, ; the toroidal field was fixed at.8 T, resulting in q In this paper the radial coordinate, ρ, is defined as the squared root of the normalized toroidal flux. Plasmas are heated with the GHz ECH configured to deposit power near the ρ =.4 surface, as well as with neutral beams whose applied torque could be modified from co-current to balanced; the beam power was varied in real time to keep β N constant when ECH power was varied. The time evolution of a typical plasma discharge is illustrated in figure, which provides an overview of how parameters vary when ECH replaces part of the beam power. The primary control tool in this experiment was to turn on and off ECH to assess the impact of torque-free direct electron heating on confinement and fluctuations. In particular, it can be seen how the line averaged density, electron temperature, plasma flow as well as ELM frequency and size are affected by the heating scheme. Even though sawteeth The recently installed CER main ion system was not yet available for the data presented here

5 are generally active, their impact on the analysis presented in this paper was eliminated by choosing appropriate discharges and time windows. Due to MHD activity usually present in IBS plasmas, the analysis presented in the following is limited to time windows free of tearing modes. 3 Effect of heating and torque on confinement and ion-scale fluctuations One of the most important goals of the experiment, and the main subject covered in this report, is to compare the impact of torque-free direct electron heating, i.e. ECH as a fusion alpha power mock-up, to finite torque mixed electron-ion beam heating on density fluctuations. The most important result of the experiment is a significant reduction in confinement when ECH is applied to replace beam power while maintaining a constant value of β N ; in particular, after ECH is switched-off the NB system needed to supply only a fraction ( 3%) of the power provided by the gyrotrons. In more quantitative terms, the effect of ECH on confinement in a sample case can be seen in figure 3, which shows the time evolution of the energy confinement time and enhanced confinement quality factor H 98y,, in time windows free of tearing modes, as ECH power is turned off at 4.5 s. The confinement degradation with ECH, which on DIII-D has been observed at both low and high levels of beam torque [], is consistent with increased fluctuation levels at ion scales measured by the afore-mentioned three turbulence diagnostics on DIII-D; an example of which can be seen in figure 4, where the time evolution of the rms signal (unnormalized to the line averaged density along the line of sight) measured by the PCI diagnostic across the ECH switch-off time in discharge #5483 is observed to monotonically decrease in time towards a new stationary state. While the initial experimental result was reported in [], here we will describe in much greater detail the data interpretation of the PCI diagnostic. The PCI measures fluctuations with a broad range of horizontally propagating wave numbers, k R from to 5 cm ; such wave numbers, along the vertical line integral, are formed by a combination of k ρ and k θ due to the projection of the horizontal direction on the flux surface intersected by the laser beam at any given vertical position. In figure 5 we display the frequency power spectrum of density fluctuations computed in two time windows dur- 3

6 ing which the main plasma parameters are stationary: the first one refers to a phase for which the only auxiliary heating power is provided by the NBI system, while the second one also includes the ECH system, at constant β N. It can be seen that fluctuations with the largest intensity, which are below khz, are strongly enhanced by ECH, in agreement with what is observed by the BES and DBS systems and reported in []. However, at higher frequencies, this behavior reverses with stronger fluctuations being excited in the beam-only heated part of the discharge. In order to obtain a better understanding of the low frequency response of the PCI, it is useful to examine a comparison of two discharges for which the main operational difference was beam torque. Figure 6 shows that the discharge at higher torque is characterized by fluctuations with larger bandwidth as well as by lower intensity at low frequency. Such behavior may be understood by considering that beams configured to provide high torque cause the plasma to spin faster toroidally which, in turn, has two effects: an increased E B shearing rate which suppresses long scale fluctuations and a larger Doppler shift that widens the apparent bandwidth of the fluctuations to higher frequencies. This hypothesis can be validated by examining synthetic PCI data [3] reconstructed from non-linear gyrokinetic modeling of a discharge of this experimental campaign in which, while maintaining fixed kinetic profiles, plasma flow and flow shear are independently varied in the simulations. The simulations were performed with the GYRO code with electrons, deuterons and carbon as kinetic species, perturbations to the electrostatic and to the parallel vector potentials, as well as collisions; the fluctuations are evolved in a box of normalized dimensions (L x /ρ s, L y /ρ s ) = (, 3) whose spatial grids are such that GYRO evolves modes in the spectral range k y ρ s and k x ρ s.8. In Figure 7 we compare synthetic spectra from the base case, corresponding to experimental parameters, to those computed in simulations where the E B flow was increased by a constant value, and the E B radial shear was increased while maintaining constant plasma flow in the center of the simulation box. While the total intensity of fluctuations is, as expected, significantly reduced by a larger E B shear, a larger flow at constant shear widens the frequency spectrum due to the increased Doppler shift. In reality, when varying the plasma rotation by altering beam torque, the plasma rotation and its radial shear are simultaneously modified, resulting in a PCI spectrum that is altered in both magnitude and bandwidth. Unfortunately, we are not able to quantitatively compare measured PCI spectra to synthetic ones because, 4

7 as will be explained in Section 5, the discharges studied in this experiment exhibit strong fluctuations at electron scales. Indeed, as can be seen by comparing figure 7 to 5, the intensity of fluctuations decays much more rapidly with increasing frequency in the synthetic spectrum than in the measured one, corresponding to high-frequency, or short scale, fluctuations not being captured in the simulations. To carry out such a comparison one would require multi-scale non-linear simulations which, however, are beyond the computational resources available for this work. 4 The effect of direct electron heating on electron scale fluctuations In the previous section we reported how confinement and ion scale fluctuations are affected by the heating scheme, and hinted at flow shear as the main contributor to the observed confinement degradation. However, as can be seen in figure and pointed out in Section, most of the equilibrium quantities are also significantly altered: some, like the electron temperature, respond on a sub-energy confinement time scale while others, like density or flow, undergo considerably slower transitions. This reflects the fact that the heating scheme is an actuator on the intensity and the characteristics of fluctuations that are, however, strongly influenced by the evolution of all the profiles. In order to understand the plasma dynamics in response to the heat flux variation, in this section we restrict our analysis to the rapid time evolution of profiles and fluctuations immediately after the ECH switch-off time, therefore separating quantities that respond to the heat flux variation itself from those that are affected by subsequent modifications to other quantities. We thus focus our attention to the instants immediately following the time at which the heating scheme is modified by examining plasma discharge 5483, in which the ECH power was switched off at 4.5 seconds and replaced by beam power at fixed β N. In figure 8 we display the evolution of main plasma profiles on a short timescale and show that the electron temperature responds almost instantly, while density and flow need a few energy confinement times to evolve towards a new stationary state, and the ion temperature profile responds on different time scales at the edge and in the core; a few snapshots of profiles in the same time window are shown in figure 9. The PCI diagnostic does not need a long integration time to achieve good 5

8 signal, and is therefore well suited to the study of transient phenomena, such as the one discussed above. In figure we report the time evolution of the intensity of fluctuations by comparing the frequency power spectrum computed in time windows immediately before and after the ECH switch-off. While the former time window can be chosen at will as long as the discharge is stationary, the latter has to be short enough for equilibrium quantities other than the electron temperature to stay close to their values before the ECH switch-off. The PCI diagnostic detects a significant variation in the intensity of fluctuations in as little as ms from the ECH power switch-off; such time window is short enough that most of the profiles are still frozen to pre-existing values, with the exception of the electron temperature profile which responds almost immediately to the heat flux variation. It is apparent that fluctuations at the low-end of the frequency spectrum are unaffected by the switch-off, while those at higher frequencies increase in magnitude. The PCI diagnostic images the line integrated density fluctuations onto a linear array of detectors, thus allowing the extraction of two dimensional spectra. Figure shows the ratio of the two-dimensional, frequency-wavenumber, power spectrum measured immediately after the switch-off to that measured before, and show that most of the increased fluctuations are located in the range f khz and cm k R cm ; this wave number range would roughly correspond to.8 k θ ρ s 5 for fluctuations localized in the region around ρ =.65. Two-dimensional spectra reconstructed from the PCI diagnostic also enable computation of the line-averaged phase velocity of the fluctuations; this quantity is deduced from a linear fit to the phase velocities corresponding to a number of contour lines in the two-dimensional frequency spectrum. Figure shows the time evolution of the phase velocity across the ECH switch-off time, from which we conclude that the larger intensity of fluctuations at high frequencies shown in figure is not due to an increased Doppler shift. Indeed, the variation in Doppler shift correlates very well with the time evolution of the toroidal velocity which, as shown in figure 8, varies on a much longer time scale and reaches a new stationary state around 4.7 s, in excellent agreement with the time evolution of the phase velocity reported in figure. The larger amplitude of the spectrum at high frequency is, therefore, an actual enhancement of fluctuations and is likely due to the response of the electron temperature profile to the heat flux variation. 6

9 5 Gyrokinetic Modeling Simulations were performed with the non-linear gyrokinetic (GK) code GYRO [], that solves the GK Vlasov-Maxwell system of equations as an initial value problem, and the gyrofluid code TGLF [4]. An initial equilibrium reconstruction was performed by the EFIT code [5] constrained by measurements from magnetic loops and the Motional Stark Effect diagnostic [6]; the.5 dimensional ONETWO transport code [7] was used to derive the stationary current profile which, taking into account pressure and current constraints from the bootstrap current and neutral beams, was then provided as input to the EFIT code during the calculation of the complete kinetic equilibrium, which is subsequently read directly by GYRO and TGLF. This method produces the most accurate reconstruction of the magnetic geometry. One of the most important validation tests for transport models, also in view of the need to accurately predict fusion performances in future devices, is the reproduction of profiles. While it is feasible to perform linear stability analysis with GYRO on selected radii and wavenumbers, it is computationally prohibitive to evolve profiles on transport timescales by computing non-linear fluxes at every time step. Therefore, we have used the reduced fluid model TGLF, which uses saturation rules calibrated against a database of non-linear GYRO simulations to evolve profiles on transport timescales. The TGYRO [8] transport solver uses the TGLF and the NEO [9] codes to predict anomalous and neoclassical transport, respectively, by simultaneously evolving density, electron and ion temperature radial profiles, on a fixed equilibrium, to match the heat and particle fluxes from power balance analysis. In Fig.3 we display a typical comparison between outputs from TGYRO, in time windows with and without ECH, and measured profiles. Data consistency was evaluated with the TRANSP code which, used in interpretative mode, computed neutron yield and stored energy to be within 7% of the experimentally measured values. Even though the solver overpredicts the profiles on axis, it reproduces the measurements to a high degree of accuracy outside the flux surface at ρ =.3, which is the region of interest for this work. The radial electric field used by TGYRO was not evolved, instead it was kept fixed to an ad-hoc value, within the uncertainty of the experimental value, that optimized the agreement in each case. Indeed, the optimal rescaling factor is expected to fluctuate with time around unity because a fixed value would indicate some sort of bias in either the measurement of the 7

10 model. In the NBI+ECH case, the overall agreement was greatly improved by reducing the radial electric field to about 9% of the experimental value, suggesting that the measured flow shear rate is rather close to the instability threshold. In the NBI-only case, instead, the flow shear is significantly larger than the threshold, resulting in little sensitivity of the overall agreement to the rescaling factor. Even though a 9% reduction still produces a very similar agreement to that shown in Figure 3, though slightly worse, we decided to show the optimal result, which was achieved without rescaling. A study on the Monte-Carlo propagation of the experimental profile uncertainties through the power balance fluxes and TGYRO solution in similar plasmas found that uncertainties in inverse scale lengths reconstructed by TGYRO are within 5% across the entire profile []. The long time-scale variation in confinement following the ECH switch-off, shown in figure 3, and that of the measured intensity of fluctuations, can be explained by changes in the radial profiles of the Hahm-Burrell shearing rate [] in the outer half of the minor radius. While the observed shearing rate in the ECH-heated phase of the discharge is well below the linear growth-rate of the most unstable mode in the ion scale region, which is where flow shear is expected to be effective, the two become quantitatively comparable in the ECH-free phase of the discharge due to the toroidal spin-up. Turbulence quenching is expected to take place when the shearing rate γ E and the maximum linear growth rate in the absence of flow shear γ max satisfy the relation αγ E γ max [], where α is a numerical coefficient of order unity for the case considered. Therefore, the observed reduction of the intensity of fluctuations and the amelioration of confinement are likely due to flow shear. This qualitative reasoning is confirmed by figure 4 where we display the Hahm-Burrell shearing rate along with the ion heat flux computed by the GYRO code in global non-linear simulations carried out in the region.5 ρ.8 and in the spectral region k y ρ s, demonstrating that the inclusion of flow shear lowered the non-linear fluxes to values close to those from the experiment. Focusing on the transient behavior of fluctuations at the ECH switch-off, the interpretation of a line-integrated measurement, such as the PCI, can be greatly simplified by detailed modeling of the radial profiles of the dominant instabilities at play in the plasma. In particular, a first order understanding of a transient phenomenon can be obtained by computing the linear growth rates in response to what caused the transient behavior. In order to interpret the spectra in figure, we used the TGLF code to compute the radial profile of the time evolution of the growth rate of the most unstable modes at any 8

11 wavelength in the region. k θ ρ s. For this broad analysis, the TGLF code was preferred to a more fundamental kinetic model as predictor of the intensity of fluctuations because, despite being much faster, it is able to compute growth-rates with reasonable accuracy; however, more detailed analysis in this section required the use of the GYRO code. As displayed in figure 5, the ECH turn-off is predicted to enhance electron scale fluctuations in the radial region ρ.6 and for wavenumbers such that k y ρ s 5. The fact that the ECH turn-off enhances electron scale fluctuations, which is at first counter-intuitive, can be understood from figure 8, where it is shown that, as a consequence of the ECH switch-off, the electron temperature profile drops by an almost uniform value across more than half of the minor radius. The time constant of such temperature drop is approximately 3 ms, which is comparable to the time scale over which the PCI detects an increase in the high-frequency fluctuations (fig. ). As it is derived in Eq., a uniform temperature drop, δt, results in an increased inverse scale length, which is one of the main driving parameters for electron scale fluctuations; in particular, the fractional increase becomes larger with decreasing temperature, i.e. approaching the plasma edge [ ] Te (ρ) T e (ρ) [T e(ρ) δt ] T e(ρ) δt Te(ρ) T e(ρ) T e(ρ) T e(ρ) T e(ρ) T e(ρ) [ + δt T e(ρ) T e(ρ) T e(ρ) ] T e(ρ) T e(ρ) = δt T e (ρ). () This reasoning is not valid near the pedestal top, where the electron temperature decrease is smaller in absolute units, causing a reduced radial temperature gradient that translates into a lower inverse scale length. This result suggests that the increase in the intensity of fluctuations observed by the PCI diagnostic at frequencies larger than khz, as shown in figure, is due to electron modes originating in plasma regions where the modification of the electron temperature gradient scale length in response to the heat flux variation is largest. Comprehensive modeling of such plasmas would require the use of multi-scale simulations in order to capture the combined dynamics of ion and electron scale fluctuations [3] and is beyond the scope of this work. However, in order to validate the results from TGLF and therefore obtain a reasonably accurate response of electron scale fluctuations to the ECH 9

12 turn-off, non-linear local gyro-kinetic simulations were performed at electron scales at the radial location ρ =.7. This choice of radial location was dictated by results from TGLF, which indicated that the effect of ECH on the electron heat flux is clearly visible but limited to wave-vectors not exceeding /ρ s. In view of these observations it was felt that nonlinear simulations at wavelengths approaching the electron gyro-radius are not needed. The non-linear GYRO simulations were run in the flux tube approximation, with two kinetic species, no impurities and in the collisionless and electrostatic limits. As shown in figure 6, the modification to the electron temperature profile after the ECH switch-off is predicted to promptly enhance electron heat flux at electron scales; both the ion heat flux and its response to the ECH switch-off are negligible at the wavelengths computed in the simulations and are not shown. Such enhanced outward energy flux is consistent with figure, which shows a larger intensity of fluctuations at high frequency immediately after the ECH switch-off. It reasonable to assume that the EC removal causes the electron temperature to drop which increases the outward heat flux at electron scales via a modification to the temperature scale length; the larger flux would then cause the electron temperature to drop even further. This reasoning is supported by time dependent TRANSP simulations that used TGLF and NEO to predict the time evolution of the electron temperature profile alone, with all other quantities kept fixed to the experimental values: simulations evolving only fluctuations at ion scales predict a shallower temperature drop than those evolving the entire spectrum. The ECH switch-off also causes an increased inward particle pinch at electron scales, as displayed in Figure 6, whose wavelength integral accounts for about 5% of the outward particle flux generated by ion scale fluctuations. Such pinch might be connected to the well known ECH density pump-out effect [4], whose experimental characterization is the subject of active research [5, 6] due to its importance in determining the density radial profile in future machines. Previous work identified the cause of the pump-out in the switch of the direction of propagation of the dominant ion-scale instability, which would result in a modification of density peaking via a reversal of the direction of the thermo-diffusive pinch [7]; this work was based on detailed gyrokinetic analysis of plasmas in two different stationary phases: before and after the modification of the density peaking was observed. Recent work challenged that picture by focusing on prompt changes, rather than analyzing plasmas in stationary phases, and pointed, instead, to a prompt enhancement of the intensity of the underlying ITG modes due to a modification of

13 the ion temperature profile, with the main ion-scale instability maintaining its direction of propagation [8]. In this work we also focus on transient phenomena, rather than stationary phases, but do not see a significant effect played by the ion temperature profile; moreover, as opposed to [7], we investigate a change in the line averaged density without detailing the effect on the peaking factor. We identify two different effects caused by the ECH switch-off: a prompt enhancement of fluctuations at electron scales which generates an inward particle pinch on a sub-energy confinement time, and an increased flow shear which, over about three energy confinement times, reduces the outward particle flux by quenching instabilities at ion scales. Time dependent TRANSP simulations were performed to quantify the relative importance of such effects on the density rise: while maintaining T e, T i and flow to the experimental values, the electron density predicted by TGLF and NEO is seen to roughly reproduce the density rise observed in figure 8, with the reduction in the outward flux at ion scales to be the dominant contributor (even though the increased pinch at electron scales might be responsible for prompt, though smaller, modification of profiles). According to this picture, the ECH pump-out effect is caused by an inward particle pinch at electron scales and, subsequently, enhanced by a reduced outward flux generated at larger scales by fluctuations weakened by a larger flow-shear; the mechanism responsible for the large variation in the toroidal rotation after the removal of ECH at fixed beam torque is still to be identified. However, as pointed out at the end of Section, since the size and the frequency of ELMs are also affected by the ECH switch-off, thus modifying the particle flux boundary condition, detailed modeling of the time evolution of the density and rotation radial profiles in response to the ECH switch-off requires dedicated experiments and is deferred to future work. 6 Summary and conclusions We reported on the effect of torque-free direct electron heating on DIII- D discharges in the IBS regime; in particular, we studied the behavior of density fluctuations in response to turning-off ECH power. Although such study employed multiple diagnostics for validation purposes, this work focuses on measurements from the Phase Contrast Imaging diagnostic which, thanks to its large bandwidth, measures broadband fluctuations without cross-calibration issues. The variations in the characteristics of fluctuations

14 appear to be qualitatively consistent with linear and non-linear gyrokinetic modeling. The TGLF transport model captures the observed profile and confinement changes with ECH when used in the TGYRO transport solver, and shows the importance of capturing fluctuations at ion and electron spatial scales. Confinement and the intensity of fluctuations at low frequencies are shown to significantly increase a few energy confinement times after the removal of ECH, following the progressive increase in the E B shearing rate. On a sub-energy confinement time scale, after turning off ECH, the PCI detects a sudden increase in the intensity of fluctuations at higher frequencies, corresponding to spatial scales between and cm or.8 k θ ρ s 5. Non-linear gyro-kinetic simulations, carried out at electron gyro-radius scales, suggest that the high wavenumber response of the diagnostic is due to electron modes enhanced by changes to the electron temperature profile that happen over less than half the energy confinement time. Such modes generate a larger heat flux and an inward particle pinch that might be related to the ECH density pump-out effect and, based on linear scans, are localized in the outer third of the minor radius, where the variation of the electron temperature scale length is the largest. Changes to other quantities, such as the density or the ion temperature profiles, are minimal and, based on linear gyro-kinetic simulations, predicted to be of secondary importance. Nonlinear modeling at ion scales indicates that the low wavenumber response is dictated by the slower time evolution of other equilibrium quantities such as density and flow shear; in particular it is consistent with the dominant ITG modes being weakened by the increased flow shear in the new stationary state. The diagnosis of multi-scale fluctuations and induced profile modifications in ITER relevant regimes, such as the electron heated IBS, is of paramount importance to envision proper strategies to improve confinement in future fusion devices. Acknowledgements Work supported by the US Department of Energy under DE-FG-94ER5435 and DE-FC-4ER The numerical simulations were executed on the NERSC cluster. Part of the data analysis was performed using the OMFIT [9] framework.

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17 Figure : (colors on-line) Poloidal cross section of the DIII-D vacuum vessel superimposed to a contourplot of the normalized poloidal flux of a typical equilibrium in the experiment (red) along with the vertical beampath of the PCI diagnostic (purple). 5

18 .4. H 98,y I p [MA] 3 P ECH [MW] P NBI [MW] V 3 tor [km/s*.] T NBI [Nm] 3 T e (ρ=.9) [kev] T e (ρ=.35) [kev] 3 T i (ρ=.9) [kev] T i (ρ=.35) [kev] 6 4 D α [e6 ph/s] <n e > [m 3 ] Time [s] Figure : (colors on-line) Time evolution of a typical IBS plasma discharge. From top to bottom: Plasma current and confinement factor H 98y,, NBI and ECH power, beam torque and core toroidal velocity, edge and core electron temperature, edge and core ion temperatures, line averaged density and D α signal. 6

19 E [s] H 98,y [] P ECH [MW] a) b) c) Time [s] Figure 3: Time evolution of the electron energy confinement time (a), confinement enhancement factor H 98y, (b) and coupled ECH power (c) for DIII-D discharge #5483. PCI rms [a.u.] P ECH [MW] Time [s] Figure 4: (Colors on-line) Time evolution of the rms PCI signal (blue) and ECH power (red) across the ECH switch-off time in discharge #5483. Errorbars are computed as population uncertainties in time windows 5 ms long. Data appear to be unevenly distributed due to an ELM rejection algorithm that discards data affected by ELMs..5 7

20 Power Spectral Density [a.u.] 6 8 #5598 NB only NBI+ECH Frequency [Hz] Figure 5: (colors on-line) Comparison of frequency power spectra from the PCI diagnostic for IBS plasma discharge #5598, measured during phases without and with ECH replacing part of the beam power at fixed β N. Power Spectral Density [a.u.] 6 8 #548 high torque #54 low torque Frequency [Hz] Figure 6: (colors on-line) Comparison of frequency power spectra from the PCI diagnostic for IBS plasma discharges #548 and #54, heated by beams configured to apply and.3 Nm torque, respectively, at fixed power. 8

21 Figure 7: (colors on-line) Synthetic PCI intensity spectrum predicted by nonlinear GYRO simulations at ion scale. The effects of varying the E B drift by adding a constant value at fixed gradient and by increasing the gradient at fixed value are shown in red and yellow, respectively P ECH [MW] V tor ( Te( Ti(.3) [km/s].3) [kev].3) [kev] P [MW].5 NBI <ne> [ 9 m ] 4.65 Te(.6) [kev] Ti(.8) [kev] Time [s] Figure 8: (colors on-line) Time evolution of, from top to bottom, left and (right): ECH and (beam) power, line averaged density and (core toroidal velocity), edge and (core) electron temperature, edge and (core) ion temperature for plasma discharge #5483 in a short time window around the ECH power switch-off. 9

22 n e [ 9 m -3 ] T i [kev] t=4.4 s 3 t=4.53 s t=4.68 s t=4.8 s a) t=4.4 s t=4.53 s t=4.68 s t=4.8 s.5 c) T e [kev] [krad/s] 4 3 t=4.4 s t=4.53 s t=4.68 s t=4.8 s b) d) t=4.4 s t=4.53 s t=4.68 s t=4.8 s Figure 9: (colors on-line) Radial profiles of electron density and temperature (top), carbon temperature and angular velocity (bottom) across the ECH switch-off shown in figure 8. The shaded region around the experimental curves describes the -σ Monte Carlo uncertainty resulting from the numerical radial fits.

23 Power Spectral Density [a.u.] 6 8 #5483 NBI only NBI + ECH Frequency [Hz] Figure : (colors on-line) Comparison of frequency power spectra measured by the PCI diagnostic in a time window during which the plasma was heated by beams and ECH (blue) and by beams only (red) ms after having switched off ECH power. Figure : (colors on-line) Ratio of the two dimensional, frequencywavenumber, power spectral density computed immediately after the ECH power switch-off to that measured when the plasma is heated by beams and ECH.

24 ω/k R [km/s] Time [s] Figure : Time evolution of the line averaged phase velocity of fluctuations as computed by the PCI diagnostic accross the ECH power switch-off.

25 8 8 n e [ 9 m -3 ] 6 4 n e [ 9 m -3 ] 6 4 T e kev] - P ECH [MW/m 3 ] ρ 4 3 Exp TGLF+NEO PECH ρ 4 T e [kev] ρ 4 3 Exp TGLF+NEO ρ T i [kev] T i [kev] ρ ρ Figure 3: (colors on-line) Density (top), electron (middle), and ion (bottom) temperature radial profiles measured (blue) and computed by the TGYRO transport solver using the TGLF and NEO codes in discharge #5483 at 4. ±. s in the NBI-ECH phase (left) and at 4.8 ±. s in the NBIonly (right) heated case. The shaded region around the experimental curves describes the -σ Monte Carlo uncertainty resulting from the numerical radial fits. The boundary condition set in the solver was located at ρ =.8. 3

26 4 γ Hahm Burrel [krad/s] ρ Q i /Q i,exp γ ExB/γ ExB,exp Figure 4: (colors on-line) Left: comparison of the experimentally reconstructed Hahm-Burrell shearing rate for plasma discharge #5483 in time windows heated by ECH and beams (blue) and beams only (green) and main plasma parameters are stationary. Errorbars resulting from experimental uncertainties and fitting procedures are estimated of about krad/s. Right: Ion heat flux normalized to its experimental value as a function of flow shear as computed by global non-linear gyrokinetic simulations. The fluctuations were evolved in the spectral region k θ ρ s and in the radial box.5 < ρ <.8; profiles were linearized around their experimental values in the center of the box..5 k θ ρ s ρ Figure 5: (colors on-line) Ratio of linear growth-rates computed by TGLF in the time window immediately after the ECH power switch-off to that during which the plasma is heated by beams and ECH. 4

27 Q e /Q GB NBI+ECH prompt (sub τ E ) Γ i /Γ GB k ρ θ s. 5 5 k ρ θ s Figure 6: (colors on-line) Non-linear electron heat (left) and particle (right) fluxes in gyrobohm units computed accross the ECH switch-off for discharge #5483 by the GYRO code at electron scales. 5