Quasistatic compression of a compact torus

Abstract

We have demonstrated the formation of stable, symmetric, compact-torus (CT) plasma rings and the subsequent stable, twofold radial compression in coaxial conical electrodes with the ring accelerator experiment. The CT is compressed by JxB forces from a capacitor bank discharging across the conical electrodes. During compression, the force of the Be acceleration field balances the force of the CT poloidal field against the cones, in good agreement with a 2D MHD code. Power amplification factors of ~ 100 may be possible with an opening switch based on this technique.PACS numbers: 52.55. He, 52.75.Di, 52.75.Kq We report the first quasistatic compression of a compact torus (CT), using the Be toroidal field of a coaxialelectrode, rail-gun-type accelerator to compress the CT through a coaxial cone into a straight coaxial geometry where the CT is accelerated. These experiments were performed in RACE, the ring accelerator experiment. l This is a significant step toward inductive energy storage for acceleration and achieving the higher power densities and magnetic-field values required for many applications of accelerated CTs, 2 including an inertial-confinementfusion driver. Parameters achieved in RACE are already suitable for two applications: (1) The 2-T magnetic fields, 0.16-m diameters, and the 10-100-jUg ring masses are appropriate for fueling and current-drive experiments in tokamaks; 2 " 7 and (2) the large disparity in Bfl-field time scales between the compression phase (10-30 jus) and the acceleration phase (~300 ns) suggests that an "opening switch" 8 or power amplifying device with amplification factors -100 could be possible.Quasistatic compression of a CT, frequently called a spheromak, 9 ' 10 has been demonstrated elsewhere in a different geometry. (We refer to the RACE plasma configuration as a CT or spheromak, even though it has a conductor along the axis, because the essential physics is the same.) Inductively formed CTs in the S-l experiment 9 were compressed by a factor of 1.3-1.6, using a pair of 100-jus-rise-time coils to compress the CT deeper into the cylindrical "vee" formed by a pair of funnelshaped flux conservers. The experiments on RACE achieve a larger compression factor of 2 in the major and minor radii, by forcing the CT through the annulus between two coaxial cones, as shown in Fig. 1. Image currents in the metal walls constrain the CT fields to remain localized radially and axially in the interelectrode gap and help maintain stability during compression. On RACE, the compression ends with the CT entering a straight coaxial acceleration stage where it is free of axial constraints and can be accelerated to serve a variety of purposes.The concept of a CT accelerator, as embodied in RACE, is first to form the CT with a magnetized coaxial plasma gun, and then to accelerate it by the JxB force with separately powered coaxial electrodes.2 The plasma-gun discharge between coaxial electrodes produces a toroidal B e field, coupled with a solenoidal magnetic field that links the electrodes. Unde...

Magnetothermodynamics: measurements of the thermodynamic properties in a relaxed magnetohydrodynamic plasma

“…1988, 1991; Molvik et al. 1991). In these experiments, spheromak-type axisymmetric plasmas were magnetically accelerated to along 6 m long coaxial electrodes and stagnated in a conical taper.…”

“…Over the past decades, numerous experiments (Hammer et al 1988(Hammer et al , 1991Molvik et al 1991;Hurricane et al 2014;Park et al 2014) motivated by fusion applications have been performed to achieve a highly compressed plasma but little has been done to understand the thermodynamics of such systems. Some progress has been made in measuring the EOS of unmagnetized plasma in the context of inertial confinement fusion experiments (Hurricane et al 2014;Park et al 2014).…”

“…Early compression experiments on magnetized plasma were performed at the RACE facility in the 1990s (Hammer et al 1988(Hammer et al , 1991Molvik et al 1991). In these experiments, spheromak-type axisymmetric plasmas were magnetically accelerated to 2500 km s −1 along 6 m long coaxial electrodes and stagnated in a conical taper.…”

Measuring the equations of state in a relaxed magnetohydrodynamic plasma

“…Over the past decades, numerous experiments [5][6][7][8][9] motivated by fusion applications have been performed to achieve a highly compressed plasma but very little has been done to understand the thermodynamics of such systems. Some progress has been made in measuring the EOS of unmagnetized plasmas in the context of inertial confinement fusion experiments [5,6].…”

“…Early compression experiments on magnetized plasma were performed at the RACE facility in the 1990's [7][8][9]. In these experiments, spheromak-type plasmas were accelerated in coaxial electrodes to 2500 km/s and stagnated in a conical taper.…”