T.A. Casper

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.

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.

Experimental beta limit in an average minimum-B tandem mirror

High density (non-thermal-barrier) operation in the Tandem Mirror Experiment Upgrade (TMX-U) is found to be restricted by a stability limit. This limit is observed in the ratio of the neutral beam sustained central cell beta βc to the hot ion beta βih in the minimum-B anchor cells at both ends of the central cell, qualitatively consistent with a flute interchange stability limit. The ratio is βc/βih = 5, over the range of 0.03 ≤ βc ≤ 0.22, with no apparent reduction due to ballooning at high βc. This is a factor of six below the standard magnetohydrodynamic (MHD) m = 1 stability theory prediction of βc/βih = 33 at βc ≤ 0.1, where ballooning corrections to flute modes are small. The discrepancy could be due to approximations in the theory; however, experimental data indicate that the stability limit is due to drift wave turbulence or to large-m MHD flute or ballooning modes. The experimental beta limit is nearly independent of the hot electron beta in the anchor cells, which is compatible with theoretical predictions that the hot electron beta will decouple from MHD activity.

Propagation of waves generated in tandem-mirror end cells

Slow waves generated by ion cyclotron instabilities in tandem-mirror end cells may propagate into the central cell where the waves may degrade ion confinement. Such propagation and degradation occurred in TMX, but not in TMX-U because of the improved neutral-beam injection geometry of the latter device. Fast waves observed in a central cell may arise by mode conversion near the ends of the central cell if the slow-wave amplitude is large there and if the slow and fast waves have comparable perpendicular wavelengths. Comparable wavelengths were present in TMX, but not in TMX-U; the fast-wave amplitudes observed in TMX-U which are much lower than those in TMX are thus consistent with mode-conversion theory.

Energy confinement studies in the tandem mirror experiment (TMX): Power balance

The power balance in the Tandem Mirror Experiment (TMX) is studied for several days of operation. Between them, these days typified the operating range of TMX. Examining the power balance on axis, it is found that 60% to 100% of the power is carried to the end walls by escaping central‐cell ions. Globally, these calculations account for 70% to 100% of the input power on each of the days studied. Based upon the power balance, the energy confinement times of the particle species are calculated. The end‐cell ion energy confinement time is similar to that achieved in the 2XIIB single‐cell magnetic mirror experiment, whereas the electron energy confinement in TMX was 10 to 100 times better. The central‐cell ion energy confinement in the central flux tube was determined by axial particle loss. At the central‐cell plasma‐edge radial particle transport and charge exchange with the fueling gas are important processes.

Low-frequency stability analysis of the Tandem Mirror Experiment-Upgrade (TMX-U)

The paper presents an MHD stability analysis for low frequency modes observed in the Tandem Mirror Experiment-Upgrade (TMX-U). This MHD flute mode analysis emphasizes two key points concerning TMX-U: (1) the central cell × rotation, with appropriate radial boundary conditions, is a significant instability drive for the m = 1 mode, and (2) the anchor hot electrons decouple from the MHD dynamics. It is found that the stability predictions and the oscillation frequencies are consistent with the selected TMX-U mode of operation that has been analysed. The principal stabilizing effect of hot electrons is to introduce a negative charge uncovering and to increase the effective curvature of the anchor region so that the sloshing ions can have a stabilizing influence. Instability can occur when ion charge uncovering caused by the finite Larmor radius cancels the negative effects of the hot electron charge uncovering.

Measurements of the hot‐electron density during thermal‐barrier operation in a tandem mirror experiment

Thermal‐barrier operation of a tandem mirror requires the generation of a dense population of energetic, mirror‐trapped electrons. This has been confirmed by experimental results from the initial thermal‐barrier experiments in the Tandem Mirror Experiment‐Upgrade [Phys. Rev. Lett. 53, 783 (1984)]. For discharges with similar operating conditions, a dramatic enhancement of the axial confinement time was observed only when the mirror‐confined hot‐electron density was a large fraction of the total electron density at the position of the thermal barrier. These results are in excellent agreement with theoretical predictions.

A high-fluence fusion neutron source

Abstract A conceptual design of a D—T fusion facility for the continuous production of 14 MeV neutron wall loading from 5 to 10 MW/m 2 at the plasma surface is presented. In this design, D—T neutrons are produced in a linear, two-component plasma formed by neutral beam irradiation of a fully ionized warm plasma target. The beam energy, which is deposited in the center, is transferred to the warm plasma mainly by electron drag and is conducted along the target plasma column to end regions where it is absorbed in neutral gas at high pressure. The target plasma is operated in a regime where electron thermal conduction along the column is the controlling energy-loss process. The loss rate is minimized by adjusting the diameter and length of the plasma column. A substantial gradient in T e along the column results in recombination of the plasma to gas in the end regions before impact on the end walls. The resultant hot gas is cooled by contact with large-area heat exchangers. In this way, the large steady-state heat load from the injected neutral beams is diffused and removed at tolerable heat flux levels. The reacting plasma is essentially an extrapolation of the 2XIIB high-β plasma to higher magnetic field, ion energy, and density.