Page 13 - Drift-wave stability in the field-reversed configuration

092518-13 Onofri et al.
Phys. Plasmas 24, 092518 (2017)
confinement. This in turn decreases the fluctuation level in
the SOL, as observed in Ref. 4, where Ln tends to the critical
gradient length L .44 From Eqs. (22) and (23), we see that crit
the density gradient length Ln ’ Dw. As the parallel confine-
ment improves, both the density scale length Ln and the den-
sity in the SOL increase [Eqs. (22)–(24)]. The increased Ln
in the SOL also affects the density scale length in the core
(see Fig. 16), which leads to the decrease in C in the core, as
C is proportional to L [Eq. (21)]. Furthermore, with a larger n4
Ln, the turbulence in the SOL is reduced and the diffusion coefficient decreases. In the future, this feedback of the par- allel transport on the radial transport needs to be incorpo- rated. One model to do this is suggested in Sec. VI.
The effects of the parallel outflow on the electrostatic potential have been discussed in Sec. V. The parallel outflow is determined by the total pressure gradient, and the electron pressure gives an important contribution to the flow accelera- tion, as we show in Fig. 13. The electron pressure Pe 1⁄4 KBnTe is defined by the density profile, which is determined by the ion dynamics, and the electron temperature, which is determined by the electron heat conduction. The parallel heat conduction in the large mean free path regime is a com- plicated kinetic process. The heat losses are determined by the total electrostatic barrier, which consists of the sheath near the wall and the electrostatic potential profile along the field lines. The MHD model cannot calculate the sheath potential, and it does not give a complete description of the parallel heat flux. However, if the electron parallel thermal conductivity is high enough to produce an electron tempera- ture profile that is approximately uniform on field lines, the potential is given by Eq. (14) and the MHD code can be used to calculate the potential profile before the sheath. Measurements of the potential profile in an expanding mag- netic field were performed in the HELIX experiment,42 where a potential that increases with magnetic expansion was found. Our simulations are in agreement with the results expected from the considerations in Sec. V, showing that the case with cold ions, Ti Te, leads to the formation of a steeper electrostatic potential profile. A similar favorable condition can be obtained when neutral gas is injected near the mirrors. When gas puff is used for fuelling, it is impor- tant to know how it affects the electrostatic confinement and which injection location gives the best effects on the forma- tion of the electrostatic potential. Our simulations show that the electrostatic potential profile can increase with gas puff if the location of the gas injection is chosen correctly. The best location for gas puff is at the mirrors or in the confinement vessel close to the mirrors, while injecting the gas outside the mirrors has a negative effect on the formation of the elec- trostatic potential.
ACKNOWLEDGMENTS
We thank our investors for their support of Tri Alpha Energy, L. Schmitz, S. Gupta, and the TAE team for their contribution to this project.
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