Magnetic compression heating experiments at the 1 GW level on field-reversed configuration (FRC) compact toroid plasmas are reported. FRC’s formed in a tapered theta-pinch coil have been translated into a single-turn compression coil, where the external magnetic field is slowly raised up to seven times its initial value. Significant electron and ion heating consistent with the expected B4/5 adiabatic scaling is observed, despite significant particle diffusion, which is enhanced during compression. The n=2 rotational instability is enhanced during compression, but has been controlled to an extent by the application of an external quadrupole field. The particle and flux confinement times, τN and τφ, remain approximately equal and decrease roughly with the square of the plasma radius R during compression, implying a constant nonclassical field-null resistivity. The observed τN and τφ magnitudes and scalings are compared with classical and anomalous transport theories, and existing empirical models. Particle diffusion dominates the energy confinement, accounting for three-fourths of the total losses. Upper bounds on the electron thermal diffusivities are estimated.
Feedback stabilization of the Scyllac 120° toroidal sector is reported. The confinement time was increased by 10-20 us using feedback to a maximum time of 35-45 /us, which is over 10 growth times of the long-wavelength m = I instability. These results were obtained after circuits providing flexible waveforms were used to drive auxiliary equilibrium windings. The resultant improved equilibrium agrees well with recent theory. It was observed that normally stable short-wavelength m = I modes could be driven unstable by feedback. This instability, caused by local feedback control, increases the feedback system energy consumption. An instability involving direct coupling of the feedback 1 = 2 field to the plasma 1 = 1 motion was also observed. The plasma parameters were: temperature, T,. a T, a !©0eV; density, n. a 2 X 10" cm" 1 ; radius, a a 1 cm; and P 2 0.7. Beta decreased significantly in 40 ^s, which can be accounted for by classical resistivity and particle loss from the sector ends.
Measurements are reported on the plasma produced in an 830-kJ theta pinch (scylla IV) with a 3-m compression coil in which the compression field is extended to 120 μsec by means of “crowbar” switches. Double-exposure holographic interferometry, side-on streak photographs, and neutron-emission measurements establish the following comparison with the previous version of the experiment: (1) The plasma confinement time and neutron emission have been extended from 3 to 10 μsec. (2) The scaling of confinement time and plasma temperature from the 1-m device is accounted for by simple end-loss and shock-compression considerations. (3) An m = 1 “wobble”of the plasma column is observed in the present experiment and is interpreted in terms of plasma rotation propagating from the ends at approximately the Alfvén speed.
The m=1 ’’wobble’’ instability of the plasma column in a 5-m linear theta pinch has been studied using an axial array of orthogonally viewing position detectors to resolve the wavelength and frequency of the column motion. The experimental results are compared with recent theoretical predictions that include finite Larmor orbit effects. The frequency and wavelength characteristics at saturation agree with the predicted dispersion relation for a plasma rotating faster than the diamagnetic drift speed. Measurements of the magnetic fields at the ends of the pinch establish the existence of currents flowing in such a way that they short out the radial electric fields in the plasma column. The magnitude of rotation, the observed delay in the onset of m=1 motion, and the magnitude of end-shorting currents can all be understood in terms of the torsional Alfvén waves that communicate to the central plasma column the information that the ends have been shorted. The same waves are responsible for the torque which rotates the plasma and leads to the observed m=1 instability. Observations of the plasma in the presence of solid end plugs indicate a stabilization of high-m number modes and a reduction of the m=1 amplitude.
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