Mirror Coils Research Articles

Comparison of NbTi and Nb3Sn Quadrupole Magnet Systems for IMP

IMP (Injection into Microwave Plasma) is a fusion research device which will utilize a superconducting mirror-quadrupole magnet system. The NbTi mirror coils have been built and tested to short sample performance at 60 kG with 104 A/cm2 over-all. Designs of the quadrupole system using NbTi at 8500 A/cm2, or alternatively using Nb3Sn at 13 500 A/cm2, have been examined. Anticipated conductor costs favor NbTi, even though the volume of Nb3Sn is significantly less. However, some details of construction suggest that Nb3Sn would result in easier winding and possibly more reliable operation. Details of these comparisons were given, along with discussions of electrical stability and other technical differences in the two approaches.

Scyllacita—Compact High Voltage Theta Pinch Machine

A high voltage theta pinch (Scyllacita) has been developed especially for application to experiments with explosively compressed theta pinches. The system is relatively economical, simple, and compact. Twenty-four 14 μF, 20 kV, low inductance capacitors (60 kJ) are divided into two banks. Each capacitor has a 3 element spark gap (trigatron) mounted directly on it. Twelve low inductance cables from each capacitor spark gap combination are connected to a mix header which puts the two banks of capacitors in series when the gaps fire. One-hundred low inductance cables connect the mix header to a collector plate and thus to the 8 cm diam, 24 cm long load coil. The source inductance (coil terminals shorted) is 17 nH. Both mirror coils and straight bore coils have been used. With the straight bore coil, the full period of oscillation is 11.7 μsec and the efficiency of energy transfer to the coil is 59%. In second half-cycle operation at 100 mTorr filling pressure with the banks charged to 18 kV, over 107 neutrons per discharge are produced in either mirror or straight coil.

Injection and Trapping of Plasma Vortex Structures

Injection of a high density plasma into a magnetic mirror has been accomplished by generating plasma vortex structures under both mirror coils. Observations indicated that the structures move toward the center of the mirror and collide. Trapped magnetic fields in the structures produced a field reversal at the center of the mirror. The collision was followed by an oscillating motion of the ring currents flowing in the highly ionized plasma. Plasma was trapped inside the mirror for times of the order of milliseconds. There was no recycling of the gas in the mirror system. The plasma thermalized and then left through the escape cones and by diffusion to the walls. The ion temperature was of the order of 200 eV. Plasma volume was approximately 1000 cm3. Confining field was 10 kG.

Experimental Studies of the Penetration of a Plasma Stream into a Transverse Magnetic Field

The interaction of a dense (n ≃ 3 × 1013 cm−3), fast (v ≃ 4 × 107 cm/sec) plasma stream with a transverse magnetic field is studied experimentally by injecting a plasma from a coaxial gun perpendicular to the magnetic field of a pair of mirror coils. The experiments show that the stream from such a gun can penetrate magnetic fields up to ∼9 kG. A rapid field-plasma intermixing takes place and the injected stream crosses the magnetic field by electrically polarizing and producing an E × B drift. It was found that the forward motion of the stream can be stopped by draining the polarization charge by current flow along field lines to a shorting conductor located outside a magnetic mirror. Measurements of stream thickness normal to the magnetic field and the effectiveness of the depolarizing conductors have been made as a function of the strength of the transverse field. The exclusion of the plasma from high magnetic fields, and the variation of stream width with magnetic field intensity are due to plasma polarization effects rather than plasma diamagnetism. Observations on high energy electrons > 200 keV which are produced during the experiment are discussed.

Particle Behavior in Static, Axially Symmetric, Magnetic Mirror and Cusp Geometries

A mean containment time τ of many seconds has been observed for a significant fraction of the energetic (≤ 2.2-MeV) positrons emitted from Ne19 isotropically and uniformly in a mirror geometry with mirror ratio R in the range 1.1–3.7. Long-time containment has been observed for particles that escape through a mirror at radial distances for which the particle orbits do not encircle the axis, as well as for those that escape on or near the axis. The dependence of τ on the Z of the scattering gas, the gas pressure, the positron energy, the coil current, and the diameter and separation of the coils has been investigated. Most of the data were obtained with the two mirror coils separated one coil mean diameter. For sufficiently small values of λ (the ratio of the maximum orbit diameter on axis to the coil mean diameter), τ is independent of λ and its dependence upon the above parameters agrees with the predictions of scattering theory. Also, the fraction of particles trapped as a function of the mirror ratio agrees with calculations based on the loss cone concept. These results demonstrate that the particles ``behave adiabatically'' if λ is less than about 1/10. For larger values of λ the observed τ's are shorter. Results obtained with probes are in agreement with the prediction that the intersections of particle drift surfaces with the plane midway between the mirror coils are concentric circles centered on the axis. In the cusped field resulting from opposite currents in two coils separated by one coil mean radius, containment times of several seconds have been observed at radial distances where the field is of sufficient strength that ``adiabatic behavior'' is expected. Near the axis, however, no long-time (> 0.5-sec) containment has been observed.