# Plasma and Fusion Research

## Volume 9, 3403146 (2014)

# Regular Articles

_{e}/T

_{i}≫ 1, Collisionless ST Plasmas Sustained by RF Electron Heating

- 1)
- Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.
- 2)
- The University of Tokyo, Kashiwa 277-8561, Japan
- 3)
- Nishi-Ku, Niigata 950-2161, Japan
- 4)
- Kyoto University, Kyoto 606-8501, Japan
- 5)
- Kyushu University, Kasuga 816-8580, Japan

### Abstract

A solution of two-fluid (electron and ion), axisymmetric equilibrium is presented that approximates solenoid-free plasmas sustained only by RF electron heating that are recently studied in TST-2, LATE, QUEST.
These plasmas indicate presence of orbit-confined energetic electrons carrying substantial toroidal current outside the last closed flux surface (LCFS); T_{e}/T_{i} ≫ 1 and low collisionality at modest densities within LCFS; and likely a positive plasma potential relative to the conductive vacuum vessel.
A system of nonlinear second-order partial differential and algebraic equations constraining six functionals of poloidal magnetic flux or canonical angular momentum are solved.
An example plasma measured in TST-2 is used to guide, by trial and error, the selection of these functionals to find appropriate solutions, while assuming peaked plasma profiles and 60% toroidal current within the LCFS.
The numerical equilibrium obtained indicates a substantial ion toroidal flow and electrostatic potential so that the ion ∇p_{i}, centrifugal, and electrostatic forces of nearly equal magnitudes combine to balance the J_{i} × B force, differently from the massless electron fluid that satisfies ∇p_{e} = J_{e} × B.
The calculated properties suggest additional measurements needed to refine the choices of the functional forms and improve the two-fluid equilibrium fit to such plasmas.

### Keywords

solenoid-free spherical tokamak plasma, axisymmetric equilibrium, electron-ion two-fluid plasma with flow, heated electron and colder ion, electrostatic potential, nonlinear second order partial differential equation and algebraic equation, functional of poloidal flux and canonical angular momentum, finite differencing, successive over relaxation

### Full Text

### References

- [1] A. Ishida, L.C. Steinhauer and Y.K.M. Peng, Phys. Plasmas 17, 122507 (2010).
- [2] Y. Takase et al., Nucl. Fusion 53, 063006 (2013).
- [3] T. Watatsuki et al., IEEJ Trans. FM 132, 485 (2012).
- [4] A. Ishida and L.C. Steinhauer, Phys. Plasmas 19, 102512 (2012).
- [5] M. Uchida et al., Phys. Rev. Lett. 104, 065001 (2010).
- [6] K. Hanada et al., Plasma Sci. Technol. 13, 307 (2011).
- [7] V. Shevchenko et al., Nucl. Fusion 50, 022004 (2010).
- [8] S.K. Sharma et al., Fusion Eng. Des. 87, 77 (2012).
- [9] F. Shevchenko, T. Bigelow and J. Caughman, private communications.
- [10] L.L. Lao et al., Nucl. Fusion 25, 1611 (1985).

This paper may be cited as follows:

Yueng-Kay Martin PENG, Akio ISHIDA, Yuichi TAKASE, Akira EJIRI, Naoto TSUJII, Takashi MAEKAWA, Masaki UCHIDA, Hideki ZUSHI, Kazuaki HANADA and Makoto HASEGAWA, Plasma Fusion Res. 9, 3403146 (2014).