Reduced electron thermal transport in low collisionality H-mode plasmas in DIII-D and the importance of TEM/ETG-scale turbulence

The first systematic investigation of core electron thermal transport and the role of local ion temperature gradient/trapped electron mode/electron temperature gradient (ITG/TEM/ETG)-scale core turbulence is performed in high temperature, low collisionality H-mode plasmas in the DIII-D tokamak. Wavenumber spectra of L-mode and H-mode density turbulence are measured by Doppler backscattering. H-mode wavenumber spectra are directly contrasted for the first time with nonlinear gyrokinetic simulation results. Core ITG/TEM-scale turbulence is substantially reduced/suppressed by E × B shear promptly after the L–H transition, resulting in reduced electron thermal transport across the entire minor radius. For small kθρs, both experiment and nonlinear gyrokinetic simulations using the GYRO code show density fluctuation levels increasing with kθρs in H-mode (r/a = 0.6), in contrast to ITG/TEM-dominated L-mode plasmas. GYRO simulations also indicate that a significant portion of the remaining H-mode electron heat flux results directly from residual intermediate/short-scale TEM/ETG turbulence. Electron transport at substantially increased electron-to-ion temperature ratio (Te/Ti ⩾ 1, r/a ⩽ 0.35) has been investigated in ECH-assisted, quiescent H-mode plasmas. A synergistic increase in core electron and ion thermal diffusivity (normalized to the gyro-Bohm diffusivity) is found with applied ECH. From linear stability analysis, the TEM mode is expected to become the dominant linear instability with ECH due to increased electron-to-ion temperature ratio and a reduction in the ion temperature gradient. This is consistent with increased electron temperature fluctuations and core electron thermal diffusivity observed experimentally. The reduced ion temperature gradient likely results from a reduction in the ITG critical gradient due to increased Te/Ti and reduced E × B shear. These studies are performed at collisonality ( , r/a ⩽ 0.6) and address transport in electron heat-dominated regimes, thought to be important in ITER due to α-particle heating.

[1]  T. L. Rhodes,et al.  Advances in validating gyrokinetic turbulence models against L- and H-mode plasmas a) , 2011 .

[2]  J. Candy,et al.  Resolving electron scale turbulence in spherical tokamaks with flow shear , 2011 .

[3]  T. L. Rhodes,et al.  Simultaneous measurement of core electron temperature and density fluctuations during electron cyclotron heating on DIII-D , 2010 .

[4]  Scott A Klasky,et al.  Fluctuation characteristics and transport properties of collisionless trapped electron mode turbulence , 2010 .

[5]  J. Deboo.  Probing Plasma Turbulence by Modulating the Electron Temperature Gradient , 2009 .

[6]  Jeff M. Candy,et al.  A unified method for operator evaluation in local Grad–Shafranov plasma equilibria , 2009 .

[7]  T. Rhodes,et al.  A multichannel, frequency-modulated, tunable Doppler backscattering and reflectometry system. , 2009, The Review of scientific instruments.

[8]  F. Jenko,et al.  Multiscale features of density and frequency spectra from nonlinear gyrokinetics , 2008 .

[9]  R. Waltz,et al.  A correlation electron cyclotron emission diagnostic and the importance of multifield fluctuation measurements for testing nonlinear gyrokinetic turbulence simulations. , 2008, The Review of scientific instruments.

[10]  F. Jenko,et al.  Scale separation between electron and ion thermal transport. , 2008, Physical review letters.

[11]  Temperature ratio dependence of ion temperature gradient and trapped electron mode instability thresholds , 2008 .

[12]  R. E. Waltz,et al.  The first transport code simulations using the trapped gyro-Landau-fluid model , 2008 .

[13]  G. Staebler,et al.  Observation of reduced electron-temperature fluctuations in the core of H-mode plasmas. , 2008, Physical review letters.

[14]  Jeff M. Candy,et al.  The effect of ion-scale dynamics on electron-temperature-gradient turbulence , 2007 .

[15]  Jeff M. Candy,et al.  Coupled ion temperature gradient and trapped electron mode to electron temperature gradient mode gyrokinetic simulations , 2007 .

[16]  A. Manini,et al.  Role of Te/Ti and ∇vtor in ion heat transport of ASDEX Upgrade H-mode plasmas , 2006 .

[17]  J. Kinsey,et al.  A theory-based transport model with comprehensive physicsa) , 2006 .

[18]  C. Bourdelle,et al.  Fluctuation spectra and velocity profile from Doppler backscattering on Tore Supra , 2006 .

[19]  R. E. Waltz,et al.  Gyro-Landau fluid equations for trapped and passing particles , 2005 .

[20]  F. Jenko,et al.  Gyrokinetic simulation of collisionless trapped-electron mode turbulence , 2005 .

[21]  E. Asp,et al.  JETTO simulations of Te/Ti effects on plasma confinement , 2005 .

[22]  Frank Jenko,et al.  Linear gyrokinetic stability calculations of electron heat dominated plasmas in ASDEX Upgrade , 2005 .

[23]  Scaling laws of density fluctuations at high-k on Tore Supra , 2004 .

[24]  R. Waltz,et al.  Anomalous transport scaling in the DIII-D tokamak matched by supercomputer simulation. , 2003, Physical review letters.

[25]  F. Jenko,et al.  Prediction of significant tokamak turbulence at electron gyroradius scales. , 2002, Physical review letters.

[26]  L. L. Lao,et al.  Quiescent H-mode plasmas in the DIII-D tokamak , 2002 .

[27]  Jeff M. Candy,et al.  Gyrokinetic turbulence simulation of profile shear stabilization and broken gyroBohm scaling , 2002 .

[28]  Shinichiro Toda,et al.  Theoretical study of the structure of the electric field in helical toroidal plasmas by the use of an anomalous transport model , 2002 .

[29]  J. Kinsey,et al.  The quiescent double barrier regime in the DIII-D tokamak , 2001 .

[30]  Frank Jenko,et al.  Critical gradient formula for toroidal electron temperature gradient modes , 2001 .

[31]  G. Staebler,et al.  Quiescent double barrier regime in the DIII-D tokamak. , 2001, Physical review letters.

[32]  Bill Scott,et al.  Doppler Reflectometry for the Investigation of Propagating Density Perturbations , 2001 .

[33]  M. R. Wade,et al.  Dependence of Heat and Particle Transport on the Ratio of the Ion and Electron Temperatures , 1999 .

[34]  R. L. Miller,et al.  Ion temperature gradient turbulence simulations and plasma flux surface shape , 1999 .

[35]  G. Staebler,et al.  Toroidal Rotation and Core Ion Confinement with RF Heating in DIII-D , 1999 .

[36]  H. Sugama,et al.  Nonlinear electromagnetic gyrokinetic equation for plasmas with large mean flows , 1998 .

[37]  M. Zarnstorff,et al.  Measurements of ion temperature fluctuations in the Tokamak Fusion Test Reactor , 1998 .

[38]  Mike Kotschenreuther,et al.  Comparison of initial value and eigenvalue codes for kinetic toroidal plasma instabilities , 1995 .

[39]  Manickam,et al.  Improved confinement with reversed magnetic shear in TFTR. , 1995, Physical review letters.

[40]  E. Doyle,et al.  LETTER: Edge turbulence reduction at the L-H transition in DIII-D , 1993 .

[41]  Hans Nordman,et al.  Diffusive particle and heat pinch effects in toroidal plasmas , 1989 .

[42]  R. J. Hawryluk,et al.  An Empirical Approach to Tokamak Transport , 1981 .