Progress with energy confinement time in the CTX spheromak

Abstract

Large improvements in spheromak parameters and new understanding have been obtained from the CTX experiment at Los Alamos [Phys. Rev. Lett. 51, 39 (1983); 61, 2457 (1988)]. In one experiment the global energy confinement time has been increased an order of magnitude over previous experiments to 0.2 msec and the magnetic-energy decay time increased to 2 msec. These results were achieved in a decaying spheromak by reducing the helicity dissipation in the edge. In another smaller spheromak, record electron temperatures (∼400 eV) and record magnetic field strengths (∼30 kG) have been obtained.

Numerical Studies of Magnetohydrodynamic Activity Resulting from Inductive Transients Final Report

“…Like all other electrostatically driven spheromaks [11-13, for example], there are large n=1 magnetic fluctuations (n is the toroidal Fourier index) during formation, due to an MHD kink instability of the pinched column that carries current from one electrode to the other. Fluctuations decrease during the partial decay, as expected from previous results on decaying spheromaks [11,14]. However, applying the sustainment pulse leads to a relatively quiescent state at the same time that the decay rate of the configuration decreases [15].…”

“…In the sustained open-field configurations, electron temperatures resulting from Ohmic heating with classical parallel transport are limited to tens of electron-Volts [20,21]. Thus, the early simulation results offer an explanation for the high temperatures achieved during the decay phase of some experiments [11,14] but appeared to be inconsistent with the SSPX results where temperatures exceeding 100 eV (and greater than 300 eV more recently) are achieved upon application of a second current drive pulse that follows a brief period of decay [15]. Noting that the second current pulse in SSPX does not keep the magnetic energy content fixed, we developed capabilities for more detailed simulations of SSPX [22] including a realistic geometry, temperature-dependent anisotropic thermal conduction, and a current injection waveform that mimics the formation/sustainment pulsing in SSPX.…”

Energy confinement and magnetic field generation in the SSPX spheromak

“…Previous work at the CTX spheromak at Los Alamos [6] resulted in temperatures of >150 eV for driven spheromaks and toroidal currents of >250 kA in discharges up to 2 ms in duration. Decaying spheromaks on CTX heated to 400eV [7]. The Sustained Spheromak Physics Experiment [8,9] (SSPX) at Lawrence Livermore National Laboratory, built upon these achievements and has attained electron temperatures of >500 eV, I tor > 1 MA, and discharges of 9 ms. [10] We report on magnetic field generation [11] and energy confinement in the SSPX spheromak.…”

Quasistatic compression of a compact torus

“…10,17 (2) The CT is stable within the cone and becomes azimuthally symmetric after the gun turns off. (3) When the gun is turned off, the CT magnetic field decays linearly to zero over a relatively long period (60 jus), making the firing time of the accelerator capacitor bank less critical than when the accelerator must be synchronized within a few //s with a fast gun that produces a CT velocity 1 of typically ~2x 10 7 cm/s.…”

“…Forming a CT with the same diameter as that of the gun was tried, in part, due to the recent success on the CTX facility in making smaller CTs this way. 10,16 An earlier configuration of RACE consisted of a 6-m-long, 0.5-(0.2-) m outer (inner) diameter straight coaxial accelerator, expanded from a 0.32-m gun, with a focusing cone at the end. There are several advantages to employing ring formation in compression cones as the first stage of a CT accelerator.…”