E × B Plasma Rotation and n = 1 Oscillation Observed in the NSTX-CHI Experiments

Abstract

In the National Spherical Torus Experiment (NSTX), a peak plasma current up to 390 kA has been successfully generated by the Coaxial Helicity Injection (CHI) current drive method. The plasma rotation (∼ 20 km/s) driven in the E × B toroidal direction by CHI has been clearly identified by an ion Doppler spectroscopic measurement. The n = 1 mode has been also observed to rotate in the same direction. This rotating kink behavior observed for the first time in NSTX is consistent with the electron locking model developed in the Helicity Injected Torus-II (HIT-II) experiments to explain the mechanism of CHI current drive. Coaxial helicity injection is one of most attractive candidates to resolve the non-inductive current-drive and plasma start-up issues for spherical torus (ST) [1, 2] and spheromak [3]. CHI has been used to successfully produce non-inductive discharges with up to 390 kA of toroidal current in 0.330 s long pulse in the NSTX device [4] at the Princeton Plasma Physics Laboratory (PPPL). During the steady-state CHI operation, a current transfer mechanism relied on MHD relaxation is required to generate closed flux and its identification has been a subject of ongoing research. An electron locking model [1] was proposed as a possible mechanism for current drive with CHI and its validity was confirmed by accounting for experimental observations in the HIT-II device [1]. The two key elements in the model are 1) n = 1 helical magnetic distortion at the edge coupling the open flux to the closed flux, and 2) the toroidal plasma rotation due to E × B, where B is the poloidal magnetic field at the plasma edge. The rotating helical distortion across the separatrix is predicted to convect some of the electron fluid into the closed flux region, resulting in current drive. This paper shows that the E × B plasma rotation along with the n = 1 distortion is verified by ion Doppler spectroscopic measurements for the long pulse high current discharge demonstrated on NSTX. This result obtained on the NSTX is consistent with observations on the HIT-II.The NSTX device parameters are: major/minor radii of 0.85/0.65 m, elongation ≤ 2.5, plasma volume 12.5 m 3author's e-mail: nagata@eng.u-hyogo.ac.jp [5]. The NSTX vacuum vessel is split in two using toroidal ceramic breaks at the top and bottom to electrically insulate the central column and the inner divertor plates from the outer wall and the outer divertor plates as shown in Fig. 1 Schematic showing application of CHI in NSTX

Magnetohydrodynamic Simulation of Kink Instability and Plasma Flow during Sustainment of a Coaxial Gun Spheromak

“…As a result, a magnetic reconnection event occurs at the X-point near the gun muzzle (E), and the positive toroidal flow (yellow) of v t ≈ 37 km/s (F) is driven in the opposite direction to I t , but in the same direction as the E × B plasma rotation. This result is consistent with the flow observed in the Helicity Injected Torus (HIT-II) [12] and the National Spherical Torus Experiment (NSTX) [13]. At t = 1202 τ A , this magnetic reconnection event generates closed poloidal field lines, which are in a partially relaxed state with a strong poloidal flow in the periphery region.…”

Recent results from the HIT-SI experiment

“…This structure is thought to play a role in the relaxation activity needed for driving a current in the closed-flux regions. In the HIT spherical torus experiments an n = 1 mode appears to be locked to the electron fluid suggesting a rotating magnetic field current drive called electron locking [30,36,[39][40][41][42][43]. If the condition δω ce /ν ie > δ/b is met, then the rotating field is strong enough to lock to the electrons and penetrate the plasma, where δω ce = eδB/m e where δB is the rotating field strength, ν ie is the electron momentum loss rate to the ions, δ is the classical skin depth at the rotation frequency and b is the plasma radius [39,40].…”