Anomalous transport in the H-mode pedestal of Alcator C-Mod discharges

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PAPER Anomalous transport in the H-mode pedestal of Alcator C-Mod discharges To cite this article: A.Y. Pankin et al 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 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 https://creativecommons.org/licences/by-nc-nd/.0 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// at :0

Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R 0 Anomalous Transport in the H-mode Pedestal of Alcator C-Mod Discharges A.Y. Pankin, J.W. Hughes, M.J. Greenwald, A.H. Kritz, and T. Rafiq Tech-X Corporation, Arapahoe Ave., Boulder, CO 0, USA MIT Plasma Science and Fusion Center, Albany St., Cambridge, MA 0, USA Lehigh University, Memorial Drive East, Bethlehem, PA 0, USA E-mail: E-mail: pankin@txcorp.com Abstract. Anomalous transport in the H-mode pedestal region of five Alcator C- Mod discharges, representing a plasma density scan is analyzed. The understanding of anomalous transport in the pedestal region is important for the development of a comprehensive model for the H-mode pedestal slope. In this research, a possible role of the drift resistive inertial ballooning modes [T. Rafiq et al. Phys. Plasmas (0) 0] in the edge of Alcator C-Mod discharges is analyzed. The stability analysis, carried out using the TRANSP code, indicates that the DRIBM modes are strongly unstable in Alcator C-Mod discharges with large electron collisionality. An improved interpretive analysis of H-mode pedestal experimental data is carried out utilizing the additive flux minimization technique [A.Y. Pankin et al. Phys. Plasmas () 00] together with the guiding-center neoclassical kinetic XGC0 code. The neoclassical and neutral physics are simulated in the XGC0 code and the anomalous fluxes are computed using the additive flux minimization technique. The anomalous fluxes are reconstructed and compared with each other for the density scan Alcator C- Mod discharges. It is found that the electron thermal anomalous diffusivities at the pedestal top increase with the electron collisionality. This dependence can also point to the drift resistive inertial ballooning modes as the modes that drive the anomalous transport in the plasma edge of highly collisional discharges. Keywords: H-mode pedestal, anomalous transport, drift resistive ballooning modes, kinetic ballooning modes, validation, particle transport, thermal transport Submitted to: Nucl. Fusion PACS numbers:..fi,..kt,..ra,..fa,..tn,..kj

AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of 0 Anomalous transport in the H-Mode Pedestal of Alcator C-Mod Discharges. Introduction Tokamak edge physics plays a key role in controlling the global plasma confinement of tokamak discharges since the edge region sets the boundary conditions for the whole discharge. In particular, the transition from the low plasma confinement mode (L-mode) to enhanced plasma confinement mode (H-mode) is associated with the formation of an edge transport barrier or H-mode pedestal. It is commonly accepted that the H- mode pedestal height is defined by the stability conditions of peeling and ballooning modes, which limit the accumulation of free energy in the pedestal. It is also believed that the H-mode pedestal slope is determined by the level of thermal, particle and momentum transport in the pedestal. While the nature of neoclassical transport is well established, the driving mechanisms of turbulence which result in anomalous transport in the plasma edge region are still being discussed. Understanding edge anomalous transport and the mechanisms that result in suppression is important for the development of a comprehensive model for the H-mode pedestal. A large number of models for anomalous transport has been developed during the last several decades. For the model described in Ref. [], the anomalous transport in the pedestal is driven by drift-wave turbulence which is significantly suppressed by the E B flow shear. The idea of pedestal anomalous transport driven by ideal ballooning modes suppressed by the diamagnetic effects is explored in Ref. []. An alternative approach to the pedestal transport driven through the paleoclassical mechanism is investigated in Refs. [,, ]. Several models of anomalous transport that define the H-mode pedestal slope are compared in Ref. []. Many drift-wave instabilities drive the anomalous transport which is very stiff. As a result of the transport stiffness, the plasma gradients in the H-mode region remain close to the linear stability boundary of the most unstable modes that drive the anomalous transport. The pedestal slope in these models can be obtained from the analysis of the stability conditions of the most unstable modes. This approach is used in the EPED model[] that postulates that the main modes that drive the pedestal transport are kinetic ballooning modes (KBMs) and that the pedestal slope is defined by the linear stability of these modes. The KBMs are electromagnetic modes that are destabilized above critical value of β crit. Similar to the ITG modes, the stability of KBM modes are dependent on the parameter η i that is defined as a ratio of temperature and density gradients. The EPED model has been verified for a number of tokamaks and good agreement between experimental data and model prediction has been reported for Alcator C-Mod, DIII-D, and NSTX discharges. []. In spite the success of the EPED model, the questions remain whether the KBMs are the dominant pedestal modes in all plasma regimes and what is the confidence level for using this model for future tokamaks and for new plasma regimes. For example, if is not clear what modes drive the anomalous transport in low β plasmas where the KBMs are expected to be stable and in highly collisional plasmas where the most driftwave instabilities are expected to significantly damped by collisions. In this study, drift resistive ballooning inertial modes (DRIBMs) [] are examined to determine if they can also play a role in the setting of the H-mode pedestal slope. The DRIBMs

Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R 0 Anomalous transport in the H-Mode Pedestal of Alcator C-Mod Discharges can result in the turbulence that drives the anomalous transport in the edge region of tokamaks[, 0]. These modes can explain some experimental observation such as a strong poloidal asymmetry in poloidal particle transport []. It has also been demonstrated previously that the DRIBMs can play a significant role at the plasma edge of Ohmic and L-mode discharges []. The differences in the destabilization mechanisms of KBMs and DRIBMs can be used to distinguish these modes in the analysis of experimental data. In this paper, the integrated modeling TRANSP code is used to investigate the possible role of the DRIBMs in the plasma edge region of five Alcator C-Mod discharges that represent a plasma density scan. The DRIBM model, based on the reduced Braginski equations, is implemented as a component of the Multi-Mode Model for anomalous transport [] in the TRANSP code and is used in the analysis presented in this paper. In addition, the kinetic neoclassical XGC0 code [] is used to investigate the scaling of anomalous transport in the H-mode pedestal region with respect to plasma collisionality. In order to investigate the dependence of anomalous transport on plasma collisionality, the additive flux minimization technique [], based on uncertainty quantification analysis, is used. This paper is organized as follows. Section describes the Alcator C-Mod discharges that are analyzed in this paper. In Sec., the linear stability analysis of experimental plasma profiles with respect to drift resistive inertial ballooning modes is presented. The interpretive analysis of the collisionality scaling of anomalous transport using the additive flux minimization technique in given in Sec.. Finally, implications and limitations of the modeling carried out are discussed and possible future research directions are described.. Alcator C-Mod Plasma Density Scan Five Alcator C-Mod discharges that represent the collisionality scan [] are analyzed in this paper. The discharge numbers and their some basic parameters are given in the table below: The Alcator C-Mod collisionality scan is completed on August,. The only Discharge Analysis time frame (sec) n e (0 m ) T e (0) (kev) P aux (MW) 0 0.-0....0 0.-0...0. 0.-0.... 0.0-.0... 0.-.0... source of auxiliary heating during the analysis time frames given in the second column of the table was the ICRF heating. The third column gives the line average electron densities and the forth column indicates the central electron temperatures averaged over the analysis time frames. Analytic fits to the measured electron density and temperature profiles are given in Fig.. A modified hyperbolic tangent fit for the pedestal region [] and a polynomial form for the core are used to fit the Thomson scattering data. Thomson scattering is performed with high spatial resolution in the pedestal region, allowing for accurate determination of the pedestal height, width and gradient. Measurements are

AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of 0 Anomalous transport in the H-Mode Pedestal of Alcator C-Mod Discharges Figure. [Color online] The electron density (left) and electron temperature (right) profiles versus square root of normalized toroidal flux for five Alcator C-Mod discharges studied. mapped onto flux surfaces prior to the fitting being performed. The discharges analyzed in the paper have exhibited clear L-H transitions, and formation of density and temperature pedestals, as shown in Fig.. Comparing the H-mode pedestal regions, one can notice that the Alcator C-Mod discharges 0 and have largest plasma and pedestal densities, the Alcator C-Mod discharge has an intermediate plasma and pedestal density, and the Alcator C-Mod discharges and have the lowest plasma and pedestal densities and highest plasma temperatures at the diagnostic times chosen. The five Alcator C-Mod discharges studied have similar plasma current I p 0.MA during the discharge period considered and similar current profiles. As result of similar current profiles, the discharges have also similar q-profiles which are found during EFIT equilibrium reconstruction. The q-profiles have the almost the same q, but they somewhat diverge in the outer region of the H-mode pedestals as seen in Fig.. The drift resistive ballooning modes are sensitive to the q-profiles and the differences outside of ρ = 0. can affect the comparison. In our linear stability analysis studies described in Sec., we limit our comparison to the region from ρ = 0.0 to ρ = 0., which corresponds to the region where the top of the pedestal is located in these discharges. The plasma collisionality and neoclassical resistivity depend on the temperatures and densities and vary significantly in the Alcator C-Mod discharges studied. The resistivity and collisionality are important factors affecting the DRIBMs and both have a destabilizing effect on the modes. The neoclassical resistivity profiles for the plasma edge regions are given in Fig. and the electron and ion collisionalities are given in Fig... Linear Stability Analysis of DRIBMs The anomalous transport driven by the DRIBMs can be described using the DRIBM model [] which is a part of the Multi-Mode transport model [] and is implemented in the integrated modeling TRANSP code. The DRIBM model is based on fluid description

Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R 0 Anomalous transport in the H-Mode Pedestal of Alcator C-Mod Discharges Figure. [Color online] The q-profiles versus square root of normalized toroidal flux for five Alcator C-Mod discharges studied. Figure. [Color online] The neoclassical resistivity profiles versus square root of normalized toroidal flux for five Alcator C-Mod discharges studied. Figure. [Color online] Ion (left) and electron (right) collisionality versus square root of normalized toroidal flux for five Alcator C-Mod discharges studied.

AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of 0 Anomalous transport in the H-Mode Pedestal of Alcator C-Mod Discharges and utilizes the reduced Braginski equations. The model consists of six linearized equations including the ion continuity equation, the equations for vorticity, electron and ion energies, and total momentum, as well as the generalized Ohm s law. The equations take into account the diamagnetic effects, parallel electron and ion dynamics, electron inertia, magnetic perturbations, and gyro-viscous stress terms. The electron trapping, displacement currents and effects of impurity ions are neglected in the model. The DRIBM model solves the differential equations which are integrated as an initial value problem with guesses for unknown initial values [] in order to compute the growth rates and frequencies. The DRIBMs are destabilized by the current density, density gradients, collisionality and resistivity. The modes are stabilized by the temperature gradients. Typical dependence of the DRIBM effective diffusivities on collision frequency are shown in Fig.. The DRIBMs differ from the KBMs which can be destabilized by density and temperature gradients, stabilized by collisionality, and which are independent of resistivity. The growth rates and frequencies computed using the DRIBM model are plotted in Fig. as a function of the normalized radius. The DRIBMs are found to be unstable in the region near the top of the pedestal for all five Alcator C-Mod discharges. However, the growth rates for the Alcator C-Mod discharge remains marginally small through the whole plasma edge region analyzed in this study. The DRIBMs are found to be the most unstable for the Alcator C-Mod discharges 0 and which have the largest pedestal densities and are among the discharges with the smallest pedestal temperatures. These two discharges also have the largest electron collisionalities. The ion collisionalities for these two discharges in the middle range and the neoclassical resistivities are below average. While all collisionalities as well as neoclassical resistivity remain the destabilizing factors for the DRIBMs, the electron collisionality is found to be the strongest destabilizing factor for the DRIBMs in the Alcator C-Mod discharges. The dependence on the plasma collisionality distinguishes the DRIBMs from many drift-wave instabilities that are damped by the plasma collisions. While the stability analysis of other modes that can drive the anomalous transport in the H-mode pedestal is outside the scope of this paper, one can easily envision the situation when the DRIBMs are strongly destabilized and all other potential contributors to the H-mode pedestal structure are stabilized in highly collisional plasmas. It is likely that the DRIBMs need to be considered along with other modes when developing a model for the H-mode pedestal slope in highly collisional plasmas with relatively low pedestal temperatures.. Collisionality Scaling of Anomalous Transport in the H-Mode Pedestal An alternative approach for testing the role of the DRIBMs in the plasma edge region is to verify that the anomalous transport follows a particular scaling characteristic associated with these modes. Such a test is not linked to any particular model for the DRIBMs. The dependence on the plasma collisionality distinguishes the DRIBMs from many other modes that can drive the anomalous transport in the plasma edge. If the dominant instabilities that drive the edge anomalous transport are the DRIBMs, the dependence of the effective diffusivity on the collisionality will look similar to the dependence shown in

Page of AUTHOR SUBMITTED MANUSCRIPT - NF-00.R 0 Anomalous transport in the H-Mode Pedestal of Alcator C-Mod Discharges Figure. [Color online] The effective particle and thermal diffusivities as a function of plasma collision frequency computed using the DRIBM model for the Alcator C-Mod geometry. Figure. [Color online] The growth rates (left) and frequencies (right) versus square root of normalized toroidal flux for five Alcator C-Mod discharges studied. Fig.. For the analysis of the anomalous transport dependence on the collisionality, the additive flux minimization technique (AFMT) [] is used together with the kinetic neoclassical XGC0 code []. The XGC0 code computes the neoclassical transport at high fidelity, but utilizes a simplified model for anomalous transport []. The electron thermal, ion thermal, and particle effective diffusivities are free parameters in the code that can be specified in the XGC0 input files. Using AFMT, one can use these effective diffusivities as parameters in the optimization task on minimizing the differences between plasma profiles predicted with the XGC0 code and the corresponding experimental plasma profiles. For stiff-transport models which are sensitive to fluxes, the key AFMT concept is to add an additional time-varying flux that then becomes an unknown to be optimized so as to maintain the profiles near the experimental profiles. Specifically, for our analysis,

AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page of 0 Anomalous transport in the H-Mode Pedestal of Alcator C-Mod Discharges the most general unknown vector is x = (n s,e s ), where s ranges over all of the species within the plasma; n s and E s are the density and energy of plasma specie s. Defining x predict and x exp as the state variables that come from the simulation and experiment, respectively, one can separate the fluxes for the system into those that are predictive, based on high-fidelity validated simulations, and those that are interpretive: Γ x = Γ interp (ρ, x)+γ predict (ρ, x). () Here, the predictive flux is the neoclassical flux computed using the XGC0 code and the interpretive flux is the anomalous flux. With this definition, one can choose an optimization metric and cast the problem as an optimization problem to minimize the integrateddifferencebetweenthepredictedandexperimentalvalues,m = x exp x predict (L norm) []. In this research, the DAKOTA toolkit for uncertainty quantification, sensitivity analysis and optimization [] is used to minimize the rms deviations between the computed and experimental temperature and density profiles. The gradient-based unconstrained optimization in DAKOTA is used. The computation workflow used in this analysis is shown in Fig.. The anomalous transport coefficients are adjusted in the way that the plasma profiles predicted with the XGC0 code do not significantly differ from the experimental values. The left panel in Fig. shows the time dependence of the plasma density profile for Alcator C-Mod discharge 0 as a function of normalized square root of poloidal flux using the XGC0 code. The profiles nearly do not evolve from the initial profile which is set equal to the experimental profile. A similar approach has been applied to ion and electron temperature profiles. In this paper, the particle diffusivities at the top of the H-mode pedestal are compared. The combined results for five Alcator C-Mod discharges are shown in the right panel of Fig.. While there is a significant scatter in the particle diffusivities as a function of the electron collisionality, there is a trend that the particle diffusivity at the top of the pedestal increases with the electron collisionality. This trend is in an agreement with the expected dependence on the plasma collisionality for the DRIBMs. These results indicate the potential importance of the DRIBMs in highly collisional plasma discharges.. Summary and Discussion A possible role of the DRIBMs in the plasma edge region of five Alcator C-Mod discharges that represent the plasma density scan is investigated in this paper. Two alternative methods are used to illustrate the role of the DRIBMs. The first method is based on the stability analysis of the Alcator C-Mod equilibria using the DRIBM model[]. The second method is based on the interpretive analysis of anomalous transport in the five Alcator C-Mod discharges using the additive flux minimization technique []. It is demonstrated, using the first method, that generally the DRIBMs become more unstable in the discharges with larger electron collisionality. The other major factors affecting the DRIBM growth rates in these discharges are the density gradients and temperatures. It is shown using the second method that the anomalous electron thermal transport at the pedestal top increases with the electron collisionality. There is a significant scatter in the results which

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AUTHOR SUBMITTED MANUSCRIPT - NF-00.R Page 0 of 0 Anomalous transport in the H-Mode Pedestal of Alcator C-Mod Discharges 0 is likely to be associated with the interpretation of experimental data that have relatively large error bars in the plasma edge region. The scatter might be also related to the fact that the analyzed discharges do not represent a pure plasma collisionality scan and the density and temperature gradients at the pedestal top change from one discharge to the other. The conclusions about the collisionality dependence of the anomalous transport can be confirmed by extending this analysis to larger number of discharges so that the statistics of analysis will improve. This consideration indicates a possible future direction of this research. However, even now one can conclude that the collisionality dependence of the plasma edge anomalous transport is in agreement with the previous experimental results from Alcator C-Mod [, ]. However, it should be noted that these experimental observations pertain primarily to the L-mode discharges. This dependence of anomalous transport on collisionality points to the DRIBMs as one of possible instabilities that drive anomalous transport at the plasma edge in the discharges with high electron collisionality. An additional test regarding the role of these modes in other plasma parameter regimes might be achieved by extending the models for the H-mode pedestal with the linear stability condition of the DRIBMs. It should be noted that the E B flow shear effects are large in the H-mode pedestal region, and these effects can significantly quench the anomalous transport. Although E B flow shear effects are not included in this study, it is expected that the discovered dependencies of the growth rates on electron collisionality will remain valid when flow shear effects are included. The growth rates computed in Sec. are expected to be somewhat lower, but they will still increase with increasing electron collisionality. The largest contribution to the E B flow shear in the pedestal region is typically associated with diamagnetic effects. In a pure collisionality scan, the density and temperature profiles change in a way so that the pressure profile remains fixed. In the scan carried out, the pressure gradient does not change. However, due to larger densities at higher collisionality, the diamagnetic contribution to the radial electric field decreases with increasing collisionality. Consequently, the diamagnetic contribution to the E B flow shear rate also decreases with increasing collisionality. The poloidal and toroidal rotation depend on collisionality in a nontrivial way []. However, the diamagnetic term usually provides a dominant contribution to the E B flow shear rates, and the dependence of the growth rates on the electron collisionality will become more pronounced. The interpretive analysis in Sec. yields the particle diffusivites that take into account the E B flow shear effects. Note that the diffusivities continue to increase with the electron collisionality. The anomalous transport in the plasma edge is also affected by other effects including nonlocal effects and intermittent transport []. These effects significantly complicate the analysis of anomalous transport in the H-mode pedestal and can be subjects of future studies. Acknowledgements This material is based upon work supported by the U.S. Department of Energy, Office of Science, under Award Numbers DE-SC00, DE-FG0-ER, DE-SC00, DE-SC000, and DE-FC0-ER, using Alcator C-Mod, a DOE Office of Science

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