Transient confinement of a high-density plasma in a multiple-mirror magnetic-field configuration

Abstract

Multiple-mirror magnetic confinement is investigated experimentally with a pulsed hydrogen plasma produced by a theta-pinch source. The gun output at the end of the guide field between the theta-pinch coil and the multiple-mirror region was a sharp plasma pulse of strength ∼1014 cm−3 with a ratio of drift to thermal velocity close to unity (∼ 10 eV). The seven multiple-mirror cells were stabilized with a linked quadrupole field. Transient experiments were performed with hydrogen in the favourable multiple-mirror regime (n ∼ 1013 cm−3, T ∼ 10 eV) where the ion mean free path is comparable to a cell length. The quadrupole field achieved partial plasma stabilization and the plasma lifetime inside the device was increased from 10–30 μs in the unstabilized simple mirror geometry to 50–80 μs at high values of the stabilizing current. The plasma pulse was successfully injected into the multiple-mirror region but was mainly trapped in the first half of the device, because of a non-uniform introduction region of two cells and because of the low value of the pulse drift velocity. The main features of the plasma build-up and decay were studied analytically and numerically with a simple simulation of the experiment. The numerical results were in qualitative agreement with the density profiles observed experimentally inside the device. The strong density gradients, obtained at the large values of the quadrupole current for which the flux surfaces are strongly elliptical, greatly enhanced the classical radial losses, particularly from ion-ion collisions. When included in the numerical model, the theoretical radial loss rates indicated a significant (up to 30%) decrease of the plasma lifetime inside the multiple mirror for the large values of the quadrupole current necessary for stability, in reasonable agreement with the experimental results.

Rate equations model for multiple magnetic mirrors in various thermodynamic scenarios

“…Therefore the most important parameter is the system collisionality, l/λ , where l is the mirror cell length and λ = v th /ν s is the MFP. [9][10][11][12][13][14][15][16][17][18] Since v th ∝ √ T and ν s ∝ n/T 3/2 (see section II B), the MFP scales as λ ∝ T 2 /n. 30 If the system is isothermal, i.e., the temperature is the same in all cells, the MFP increases as the plasma expands, and the density drops towards the outer MM cells.…”

“…While the flute instability can be controlled with passive, 1,2 RF, 3,4 and active 5,6 methods, the loss cone flux limits the achievable fusion gain to a non-practical level in simple linear machines. In the past, several magnetic configurations have been suggested in order to reduce the axial outward flux including, tandem plugs with thermal barriers, 7,8 multi-mirrors (MM) [9][10][11][12][13][14][15][16][17][18] moving multi-mirrors, 19,20 and Helical mirror with plasma rotation. 21,22 Here, we will focus on the MM configuration.…”

Internal plasma pressure peaking in low-shear open and closed magnetic confinement systems

“…The experimental configuration for such a kind of internal pressure peaking observation can be easily created in various mirror devices, such as tandem mirrors AMBAL-M [5], GAMMA-10 [10] and HANBIT [12], novel axially symmetric mirrors [14,32,33], dipole-based devices Magnetor [11], LDX [13] and RT-1 [15], and field-reversed mirror C-2 [34]. Note that the kinetic approach (2), (3) used is adequate for collisionless hot ECR electrons in dipole-based experiments [11,13,15].…”

Al2O3/InP Structure with Less Oxides of InP Fabricated by Helicon-Wave Exicited O2–Ar Plasma Treatment of Al/InP

“…The relevant problem of MHD-instability, which is absent in the case of a dense plasma confined by walls, is believed to be solved through a complex magnetic system which suppresses the instability [17,18]. The pulsed version of the reactor with 0 < 1 is analysed in Ref.…”