Spheromak formation and sustainment studies at the sustained spheromak physics experiment using high-speed imaging and magnetic diagnostics

Abstract

A high-speed imaging system with shutter speeds as fast as 2 ns and double frame capability has been used to directly image the formation and evolution of the sustained spheromak physics experiment ͑SSPX͒ ͓E. B. Hooper et al., Nucl. Fusion 39, 863 ͑1999͔͒. Reproducible plasma features have been identified with this diagnostic and divided into three groups, according to the stage in the discharge at which they occur: ͑i͒ breakdown and ejection, ͑ii͒ sustainment, and ͑iii͒ decay. During the first stage, plasma descends into the flux conserver shortly after breakdown and a transient plasma column is formed. The column then rapidly bends and simultaneously becomes too dim to photograph a few microseconds after formation. It is conjectured here that this rapid bending precedes the transfer of toroidal to poloidal flux. During sustainment, a stable plasma column different from the transient one is observed. It has been possible to measure the column diameter and compare it to CORSICA ͓A. Tarditi et al., Contrib. Plasma Phys. 36, 132 ͑1996͔͒, a magnetohydrodynamic equilibrium reconstruction code which showed good agreement with the measurements. Elongation and velocity measurements were made of cathode patterns also seen during this stage, possibly caused by pressure gradients or E ϫ B drifts. The patterns elongate in a toroidal-only direction which depends on the magnetic-field polarity. During the decay stage the column diameter expands as the current ramps down, until it eventually dissolves into filaments. With the use of magnetic probes inserted in the gun region, an X point which moved axially depending on current level and toroidal mode number was observed in all the stages of the SSPX plasma discharge.

Quasi-Static Model of Magnetically Collimated Jets and Radio Lobes. Ii. Jet Structure and Stability

“…In the laboratory, jet propagation experiments have yielded direct information about how jets become collimated. Collimation near r = 0 is evident in Figure 2 showing simulations calibrated to measurements in the SSPX spheromak (Hooper et al 2012), corroborated by photographic studies of the early stages of jet formation in SSPX (Romero-Talams et al 2006). While for accretion disks we found that field lines emerge nearly vertically, the gun used in these experiments has an annular shape, causing some delay before field lines converge and straighten to form the equivalent of the central column in Figure 1.…”

Measurements and phenomenological modeling of magnetic flux buildup in spheromak plasmas

“…A flux rope model initially proposed to explain the amount of helicity needed for the closing of flux surfaces shortly after plasma breakdown in SSPX [5], is extended here to include the stepwise flux buildup seen during runs with multi-pulse gun current waveforms.…”

“…This process had been described previously in Refs. [5,12] from findings using only high-speed images and edge magnetics.…”

A model of plasma rotation in the Livermore spheromak for the regimes of large connection lengths

“…Rotation at the formation phase may also be present (see Refs. [12,13] and references therein), but our model cannot be applied to this case because of a short connection length during the formation phase. We also note that in the NIMROD simulations [9], a slow rotation of the modes was observed when two-fluid effects were included.…”