Suppression of MHD Fluctuations Leading to Improved Confinement in a Gun-Driven Spheromak

Abstract

Magnetic fluctuations have been reduced to approximately 1% during discharges on the Sustained Spheromak Physics Experiment by shaping the spatial distribution of the bias magnetic flux in the device. In the resulting quiescent regime, the safety factor profile is nearly flat in the plasma and the dominant ideal and resistive MHD modes are greatly reduced. During this period, the temperature profile is peaked at the magnetic axis and maps onto magnetic flux contours. Energy confinement time is improved over previous reports in a driven spheromak.

Magnetic reconnection during flux conversion in a driven spheromak

“…Plasmas 12, 092503 ͑2005͒ calculations, the boundary matches that used in SSPX except that the coaxial region ͑"gun"͒ between the cathode and anode/flux conserver was shorter in the second simulation. The bias magnetic field is generated by a set of coaxial coils as in SSPX, resulting in the "modified flux" configuration, 6 and the current pulse from the gun is a close approximation to that used in experiments. The poloidal plane is represented by a 16ϫ32 mesh ͑normalϫtangent to the electrode surfaces͒ of bicubic finite elements, and the toroidal direction is represented by finite Fourier series with 0 ഛ n ഛ 5.…”

“…The buildup of poloidal flux in the Sustained Spheromak Physics Experiment 5,6 ͑SSPX͒ is accompanied by voltage spikes on the gun cathode. Similar voltage spikes ͑or oscillations͒ were seen in the Compact Torus Experiment, CTX, when it was driven by a high impedance power source, 7 but their cause was not understood.…”

Controlled and spontaneous magnetic field generation in a gun-driven spheromak

“…Confinement improvements in SSPX give peak T e Ͼ 200 eV and core electron thermal diffusivities of e ϳ 10 m 2 /s ͑previously T e ϳ 120 eV and e ϳ 50 m 2 / s were reported 14 ͒. There is evidence for island formation, indicative of toroidal surfaces, and most SSPX temperature data are bounded by a ␤ e ϳ 5%.…”

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

“…The aforementioned transition has been identified both experimentally [3] and in numerical simulations [4,5]. The parallel confinement time for a poor confinement regime was evaluated in Refs.…”

“…As a characteristic set of parameters, we take the ones presented in Table 1 based on Refs. [3,10,11]. The following notation is used in TABLE 1 and throughout the paper: R 0 is the distance between the geometrical and magnetic axis, a S is characteristic "radius" of the last closed flux surface, n, T e , and Z eff are the density, electron temperature and the parameter characterizing the presence of higher-Z ions, respectively, B T and B P are the toroidal and poloidal magnetic field strengths near the outer wall.…”