Flute growth rate of plasma jet in mirror machine

Abstract

The evolution of flute instability in a cold, high-density hydrogen plasma jet, injected into a mirror machine, is studied. The experiment was designed to minimize the interaction of the plasma with the walls, thus bringing it close to the ideal magnetic Rayleigh-Taylor instability conditions. The modal growth rate was measured in various settings to demonstrate the effects of the finite Larmor radius, Bohm diffusion, conductive limiter, biased limiter and neutral background gas. In this paper we will demonstrate that lowering the magnetic field increases stability, as does the insertion of a conducting ring. However, if the ring is biased, the stability is reduced due to inhomogeneous coupling between the plasma and the limiter. It was also found that heavy background gas dramatically reduces the flute instability growth rate.

Active feedback stabilization of multimode flute instability in a mirror trap

“…The electrode bias voltage required to induce a pre-determined flute mode in the plasma was measured to range between 150 V for m = 1 at B = 1 kG and 400 V for m = 3 at B = 2 kG. The maximal feedback voltage in the following experiments was in the range of 250-350 V. These potentials seem much larger than one would expect from either the plasma temperature, or from the limiting buoyancy velocity of the plasma in these experiments [16]. However, previous experiments [17] show that the dielectric gradients in the plasma, ε p , reduce the potentials sensed by the plasma by two orders of magnitude, which brings it to the scale of temperature and the buoyancy drift.…”

“…The plasma density at the peak of the pulse is n i = 10 11 -10 12 cm −3 and the temperature is 1-3 eV. The number of unstable flute modes is controllable in the range of m max = 0-4 by varying the magnetic field of the trap, with higher modes stabilized by the finite Larmor radius (FLR) effect [16]. Six collimators at the periphery of the plasma couple visible light to the photodiodes that are used to sense the spatial variations of the plasma (figure 1).…”

“…Six collimators at the periphery of the plasma couple visible light to the photodiodes that are used to sense the spatial variations of the plasma (figure 1). The sensors collect mostly H α radiation from neutral hydrogen, which has been shown to be indicative of the plasma shape and evolution [16,17]. The sensors are fed into a digital processing unit with a response time of 5 µs, and the calculated feedback is fed to amplifiers and then to needle electrodes immersed in the plasma (figure 1).…”

“…The experimenters that dealt with feedback stabilization in mirrors in the last decades used only one sensor or actuator, and therefore were limited to rotating perturbations with a low growth rate [13,14]. However, these feedback methods could not stabilize the fastgrowing, slowly rotating perturbations that severely reduce the confinement time of plasmas in mirrors [15,16]. In this letter, we describe the first demonstration of the stabilization of such deleterious perturbations by a fast, multi-input-multi-output (MIMO) feedback system.…”

Feedback effect on flute dynamics in a mirror machine

“…These unstable modes tend to rise at lower gain if the feedback has a large spatial phase, temporal phase or other non-ideal response. Spatial and temporal feedback phase-lag occurs in our system as a results of the finite response time, the rotation of the flute instability and the final number of feedback electrodes, [3,12]. In long, cylindrical geometry and low plasma β (plasma pressure/magnetic pressure), the angular velocity is [13]:…”