Ion kinetic effect on bifurcated relaxation to a field-reversed configuration in TS-4 CT experiment

Abstract

Ion kinetic effect on the bifurcated relaxation of merging spheromaks to a field-reversed configuration (FRC) was studied experimentally using varied € S * which is the ratio of the minor radius to the ion skin depth from 1 to 7. The two merging spheromaks were observed to relax to an FRC or a new spheromak depending on whether the initial poloidal eigen value was smaller or larger than a threshold value. The threshold initial poloidal eigen value for the relaxation to an FRC increased with decreasing € S * value. Decrease in € S * promoted the relaxation to an FRC, annihilating the magnetic helicity, in sharp contrast with the conventional Taylor relaxation. Suppression of the low-n mode by the rotation shear of the toroidal modes is the most probable reason why the low-€ S * condition promotes the relaxation into an FRC. € S * is important factor to measure both ion kinetic effect and two-fluid effect. Ions become unmagnetized and the motion of ions decouples with those of electrons under the small € S * regime, promoting ion kinetic effect and two-fluid effect [6]. Interest has grown in experimentally studying the relaxation to an FRC under varied € S * condition. This non-M HD stability effect closely relates with a high-beta/ high-flow equilibria useful for the future large-scale FRC and the high-beta ST experiments. This paper studies experimentally this bifurcated relaxation of two merging spheromaks in a wide range of € S * numbers. Several ion species were used in the up-scaled TS-4 device in order to vary € S * widely from 1 to 7.

Assessment of Open Magnetic Fusion for Space Propulsion

“…FRC have been formed by counter.helicity merging of two spheromaks (the helicity concept is introduced in the next section). TS-3 has been recently upgradede in the TS-4 facility (Kawamori (2004)) .…”

Formation and sustainment of field reversed configuration (FRC) plasmas by spheromak merging and neutral beam injection

“…Spheromak merging was first studied on the TS-3 device at the University of Tokyo in the late 1980's and then further developed on the MRX device in 1990's [15][16][17][18][19][20][21][22]. These TS-3 and MRX experiments demonstrated that the plasma merging formation scheme could successfully produce both spheromaks and FRCs.…”

“…This configuration utilizes a stabilizing shell and neutral beam injection with a practically broad velocity distribution [10]. Slow formation of an FRC plasma by the merging of spheromak plasmas has been successfully demonstrated in a number of experiments as described above [15][16][17][18][19][20][21]. The experimental observation of the co-interchange instability in MRX [24] yields further motivation for the NBI system included in the SPIRIT concept.…”

A Self-Organized Plasma with Induction, Reconnection, and Injection Techniques: the SPIRIT Concept for Field Reversed Configuration Research

“…Field reversals up to 200 G have been obtained by applying an RMF to pre-ionized plasma in a uniform axial field of 150 G. Although the temperature was relatively low (40 eV), the slow formation of FRC plasmas was clearly demonstrated. An alternative method is based on the merging of counter-helicity spheromaks, a method developed at TS-3/4 [14][15][16][17][18][19][20], as shown in Fig. 4.…”