Ohmic Sustainment of Free Boundary Compact Toroids in MRX
Author: Gerhardt P Stefan
Requested Type: Consider for Invited
Submitted: 2006-12-18 15:32:15
Co-authors: M. Inomoto, E.V Belova, M. Yamada, H. Ji, Y. Ren, S. Dorfman, B. McGeehan
Contact Info:
Princeton Plasma Physics Laboratory
PO Box 451
Princeton, NJ 08543
USA
Abstract Text:
An outstanding issue in oblate Field Reversed Configuration (FRC) research is the sustainment of the configuration against both macroscopic MHD instabilities and resistive decay [1]. Previous research in MRX focused on the role of passive stabilization and plasma shaping in achieving improved global stability in decaying plasmas [2]. The study of sustainment has recently begun, utilizing equilibrium field control, the choice of working gas, and an Ohmic solenoid to achieve long-lived FRCs in a configuration without nearby passive stabilizers. The new Ohmic solenoid system is capable of applying a loop voltage of ~100 V for ~0.3msec, utilizing a pre-existing 0.36mF capacitor bank; compensating coils are utilized to cancel the stray Ohmic field. Through judicious series/parallel connections, the four fast coil sets (the Ohmic solenoids, shaping field coils, and PF & TF winding of the flux cores) can be powered by only three high-voltage capacitor banks, though with some loss of detailed FRC shape control. FRCs in MRX are formed by the merging of spheromaks with anti-parallel toroidal fields (counter-helicity merging); merging spheromaks with parallel toroidal fields produces another spheromak (co-helicity merging).
FRC configurations in Argon, which would otherwise decay in ~50μs, can be sustained in this configuration without destructive instability for the entire 300μs Ohmic pulse; there is no observed physics-based limit on the duration of sustainment in this configuration. Neon FRCs, while also sustainable with Ohmic drive, are susceptible to n=1 tilt and n≥2 co-interchange modes, and Helium FRCs are always terminated early in the discharge by these instabilities. In the more stable sustained FRCs, there is a significant density rise during the Ohmic pulse, with Langmuir probe measurements revealing evolution to a peaked electron pressure profile to support the FRC equilibrium. Doppler spectroscopy measurements indicate that the ion-temperature is sustained in the 15-20 eV range throughout the Ohmic pulse. The resistivity in these Argon FRCs is close to the estimated Spitzer perpendicular resistivity. When Ohmic sustainment is applied to an Argon spheromak formed through co-helicity merging, a transition occurs: the plasma density and temperature increase and peak, the value of the Taylor eigenvalue (parallel current density / magnetic field strength) [3] drops from the spheromak value to zero, and an FRC state is achieved. The electron temperature and density profiles in this final state are quite similar to those achieved when the Ohmic current drive is applied to an FRC, indicating the ability of the Ohmic drive to transform the compact toroid plasma to a final FRC state.
[1] FRC whitepaper, available at www.fusionscience.org/FRC
[2] S.P. Gerhardt, et. al., Phys. Plasmas 13, 112508 (2006)
[3] J.B. Taylor, Rev. Modern Physics 58, 741 (1986)
Characterization: A2,A3
Comments:
