F.H. Coensgen

Field-reversal experiments in a neutral-beam-injected mirror machine

Data on field-reversal experiments in the neutral-beam-injected 2XIIB mirror machine are reported. The best result is an estimated field-reversal parameter ζ = ΔB/Bvac = 0.9 ± 0.2 with vacuum field strength Bvac = 4.35 kG. Experiments at higher field strength Bvac = 6.7 kG achieved ζ = 0.6 ± 0.1. Ion energy confinement nτEi for the Bvac = 6.7 kG experiment is less than that predicted by classical Spitzer electron drag. Ion-cyclotron oscillations increasing with injected neutral-beam current suggest that ion-cyclotron losses are present and that ΔB/Bvac could be increased by improving stabilization of the ion-cyclotron oscillations.

Multistage Magnetic Compression of Highly Ionized Plasma

The mean energy of the charged particles of a plasma which is confined in a magnetic field may be increased by increasing the magnitude of the magnetic field in a time which is short with respect to the ion relaxation time (magnetic compression). It is shown that the necessary stored energy can be reduced and the efficiency greatly increased if the magnetic compression is performed in several stages, such that the plasma is compressed and transferred to successively smaller volumes. The predicted transfer behavior has been confirmed by studying the plasma movement in a three‐stage system. In the course of the investigation a plasma source has been developed which provides bursts of plasma of 1016 to 1018 ions with an average velocity of the order of 107 cm/sec.

Ambipolar potential formation and axial confinement in TMX

TMX experimental data on ambipolar potential control and on the accompanying electrostatic confinement are reported. In the radial core of the central cell, measurements of electrostatic potentials of 150 V which augment axial ion confinement are in agreement with predictions using the Maxwell-Boltzmann result. Central-cell ion confinement was observed to scale according to electrostatic potential theory up to average enhancement factors of eight times over mirror confinement alone.

Production of large-radius, high-beta, confined mirror plasmas

This paper reports results of experiments in which mirror-confined plasmas with radii as high as 7 ion gyro-radii are produced and maintained by neutral-beam injection. In these plasmas, betas as high as 0.45 were achieved and limited only by the available neutral-beam power. Electron temperature and ion-energy confinement increased with larger plasma size.

Radial transport in the central cell of the tandem mirror experiment

An experimental study of radial transport in the Tandem Mirror Experiment is reported here. Plasma parameters were measured in a series of well‐diagnosed plasma discharges. A negative electric current (80±40 A within a 30‐cm radius) flowed to the end wall, implying an equal radial loss of plasma ions. The axial losses of plasma ions were 100 A from the same volume. The nonambipolar radial ion flux was of the same order as the flux resulting from resonant‐neoclassical and ion‐neutral transport, but the uncertainties are large. The ambipolar radial transport (of both ions and electrons) was investigated by comparing the observed end losses with calculations of the plasma fueling by gas penetration and neutral beams. The ambipolar radial losses are probably smaller than the loses through other processes and may be as small as the classical losses resulting from Coulomb collisions.

Plasma Injection into a Magnetic Field of Cusped Geometry

A plasma stream was directed from a field‐free region along the axis of symmetry into a magnetic field of biconical cusped geometry. No evidence was found to support the hypothesis that a directed plasma stream can not penetrate a magnetic field whose value exceeds Bc2 = 12πρv2, where ρ is the plasma density and v its directed velocity. As the plasma penetrated the magnetic field, the plasma and field were found to intermix. Large quantities of the plasma which entered the containment region through one point cusp were found to leave promptly through the second point cusp and through the line cusp. Bombardment of the vacuum chamber walls in the vicinity of the line cusp generated sufficient secondary ions to mask any small scale plasma trapping. However, there was no evidence of gross trapping of the injected plasma.

The effect of end-cell stability on the confinement of the central-cell plasma in TMX

In the Tandem Mirror Experiment (TMX), the central-cell losses provide the warm unconfined plasma necessary to stabilize the drift-cyclotron loss-cone instability in the end cells. This places a theoretical limit on central-cell confinement, which is expressed as a limit on the end-cell to central-cell density ratio. As this density ratio increases in a TMX experiment, large increases of end-cell ion-cyclotron-frequency plasma fluctuations are observed. These fluctuations cause the central-cell confinement to decrease, in agreement with a theoretical model.

Beam plasma neutron sources based on beam-driven mirror

The design and performance of a relatively low-cost, plasma-based, 14-MeV D-T neutron source for accelerated end-of-life testing of fusion reactor materials are described in this article. An intense flux (up to 5×1018 n/m2·s) of 14-MeV neutrons is produced in a fully-ionized high-density tritium target (ne ≈ 3×1021 m−3) by injecting a current of 150-keV deuterium atoms. The tritium plasma target and the energetic D+ density produced by D0 injection are confined in a column of diameter ⩽ 0.16 m by a linear magnet set, which provides magnetic fields up to 12 T. Energy deposited by transverse injection of neutral beams at the midpoint of the column is conducted along the plasma column to the end regions. Longitudinal plasma pressure in the column is balanced by neutral gas pressure in the end tanks. The target plasma temperature is about 200 eV at the beam-injection position and falls to 5 eV or less in the end region. Ions reach the walls with energies below the sputtering threshold, and the wall temperature is maintained below 740 K by conventional cooling technology.

A neutral-potassium-beam measurement of plasma density

A 6-keV beam of potassium atoms has been used to measure the integrated line density of a deuterium plasma in which the electron temperature was in the range of a few hundred eV. and the mean ion energy was in the range of 5–10 keV. Line densities from 5 × 1011 to 1014 particles per cm2 can be effectively measured. The various contributions to the attenuation of the neutral beam have been examined in sufficient detail to permit an evaluation of this measurement to a wide range of plasma conditions.

Boundary Condition Effects in a Magnetic Mirror Compression Experiment

The transition from unstable operation to quasi‐stable plasma containment in a plasma‐injected magnetic compression experiment has been found to be dependent upon the density and lifetime of plasma external to the confinement chamber. Transverse motion of the confined plasma is limited and the magnetic compression proceeds nearly adiabatically, provided the density of the external plasma is greater than ∼0.1% of the confined plasma. If the electrical connection to external conducting surfaces is lost after the magnetic compression is well under way (later than 50 μsec after beginning of compression for a magnetic field rise time of 120 μsec), the trapped plasma suddenly moves to the wall between the magnetic mirrors giving dramatic evidence of instability. In any case, during the magnetic compression the shifting external plasma‐wall contact results in intermittent particle and/or energy losses from the hot plasma. Later than 100 μsec, the untrapped portion of the injected plasma which has been cooled, neutralized, and contaminated through its interaction with vacuum chamber walls diffuses into the containment chamber. Thus, at all times, there are loss processes in competition with the magnetic compression.