Page 11 - Global simulation of ion temperature gradient instabilities in a field-reversed configuration

Physics of Plasmas ARTICLE
FIG. 10. Comparison of 1D parallel mode structures on the diagnostic flux surfaces between different parallel domain simulations. L/qi is the field line distance normal- ized by qi at local flux surface. The diagnostic flux surfaces are shown by the dashed lines in Fig. 9.
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structure are shown in Figs. 9 and 10, respectively. The global FRC
simulation shows that the ITG mode grows along the field line direc-
tion with long parallel wavelength mode structure in SOL crossing the
regions of the central chamber and formation sections with the transi-
tion mirror regions included. The maximum amplitude of this mode
is in the formation region with bad curvature. Meanwhile, in central
FRC, the amplitude is much lower as shown in Fig. 9. In the core, ITG
is stable. This is due to the large electron parallel dynamics, large
Larmor radius effect, and large grad-B drift the direction of which is
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VI. CONCLUSIONS
In this paper, we have studied the global dynamics of drift waves in the FRC plasma that consists of various elements such as the closed field FRC central region with its squeezed SOL, strong mirrors, the for- mation sections, another set of strong mirrors, and divertors. It is important that we can now look at the globally developed drift waves that are extended in the axial direction, while localized only in the SOL region. This study was enabled by developing the global GTC particle model: a new global particle-in-cell code GTC-X. Two sets of coordi- nates, field line coordinates and cylindrical coordinates, are used in the simulation, which enable the maximum numerical efficiency for cross- separatrix simulation. A field aligned mesh is applied to suppress the unphysical short wave length noise and dramatically decrease the computational cost. GTC-X is well benchmarked with theory and ANC for ITG instability with adiabatic electrons. Global FRC simula- tions show that the ITG mode is unstable in the SOL distributed along the field line and stable in the core, which is consistent with local simu- lations and experiments. By extending the simulation domain along the SOL field lines, the parity of the ITG mode changes from even to odd, which shows that the parallel domain size is important for deter- mining the most unstable eigenmode of the drift wave. The nonlinear simulation of ITG transport with kinetic electrons in global FRC geometry will be reported in a future paper.
ACKNOWLEDGMENTS
J.B. and Z.L. would like to acknowledge useful discussions with Dr. L. Shi, Dr. W. L. Zhang, Dr. H. S. Xie, and Dr. Y. Xiao. This work was supported by TAE Grant No. TAE-200441, DOE SciDAC ISEP center, and the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No. XDB16010300. GTC-X simulations used resources on the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory (DOE Contract No. DE- AC05-00OR22725) and the National Energy Research Scientific Computing Center (DOE Contract No. DE-AC02-05CH11231).
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We also find that the ITG parity changes from even to odd by increasing the parallel domain length. For the even parity modes, when increasing the domain size, the fre- quency shows a down shift, while the growth rate first increases and then decreases, which is caused by the balance between good curvature and bad curvature. The odd parity mode becomes more unstable than the even parity mode by including more formation region (Z=R0 􏰅 􏰃10 and Z=R0 􏰊 10) into the parallel domain, and the fre- quency and growth rate vary with the domain size. Furthermore, by comparing the mode structures from simulations with different domain sizes, the maximum amplitude location of the modes is in the formation region. From our global gyrokinetic particle simulation, it shows that ITG type drift waves are stable in the core region and
opposite to ion diamagnetic drift.
unstable in the SOL region in FRC.
Phys. Plasmas 26, 042506 (2019); doi: 10.1063/1.5087079 Published under license by AIP Publishing