R. S. Hornady

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.

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.

The TMX heavy ion beam probe

A heavy ion beam probe has been used to measure the radial space potential distribution in the central cell of TMX. This was the first beam probe system to utilize computer control, CAMAC instrumentation, and fast time response for broadband fluctuation capabilities. The fast time response was obtained using off-line processing of the energy analyzer detector signals and wideband transimpedance amplifiers. The on-axis space potential was found to be 300-400 V, with /spl phi//sub e//T/sub ec//spl sim/8. The radial potential profile is parabolic when gas box fueling is used. The frequency of observed fluctuations was found to agree with the E/spl times/B plasma rotation frequency during the discharge. The measured Tl/sup ++/ secondary ion current level is consistent with calculations, given reasonable assumptions for beam attenuation. >

TMX-U thermal-barrier experiments

Thermal-barrier experiments in the Tandem Mirror Experiment Upgrade (TMX-U) are reported, along with progress made at the Lawrence Livermore National Laboratory in plasma confinement and central-cell heating. Thermal barriers in TMX-U improved axial confinement by two orders of magnitude over a limited range of densities, compared with confinement in single-cell mirrors at the same ion temperature. It is shown that central-cell radial nonambipolar confinement scales as neoclassical theory and can be eliminated by floating the end walls. Radial ambipolar losses can also be measured and reduced. The electron energy balance is improved in tandem mirrors to near classical, resulting in T/sub e/ up to 0.28 keV. Electron cyclotron heating (ECH) efficiencies up to 42%, with low levels of electron microinstability, were achieved when hot electrons in the thermal barrier were heated to average betas as large as 15%. The hot-electron distribution was measured from X-rays and is modeled by a Fokker-Planck code that includes heating from cavity radio-frequency (RF) fields. >

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.

Scaling of plasma potentials in TMX-U with the end walls grounded

The scaling of the axial potential profile measured in the Tandem Mirror Experiment-Upgrade (TMX-U) is reported. These measurements were done with the end walls at ground potential. As predicted theoretically, the results suggest that a deep thermal barrier is achieved when the mirror confined electron fraction in the end cells exceeds 90% of the total density and when there is sufficient pumping of cold, potentially confined ions. In operation with shallow thermal barriers, a potential maximum near the inner turning point of the sloshing ions is frequently observed. The central cell confining potential associated with this local maximum is large enough to have a significant effect on the ion axial confinement time. Measurements of the total ion confining potential were difficult. There is evidence of classical scaling of the parallel particle confinement with the magnitude of the ion confining potential. However, in some cases, long axial confinement times are accompanied by enhanced ambipolar radial losses. These losses are interpreted as occurring in the population of passing ions. Finally, it is observed that the overall machine potential is determined by the plasma densities, the thermal barrier depth and the electron cyclotron heating power, consistent with theoretical predictions based only on collisional and radiofrequency diffusion of the electrons.

Axial potential profile measurements in TMX-U

The diagnostics used to determine the axial potential profiles in the Tandem Mirror Experiment-Upgrade (TMX-U) are described, together with the initial results of these measurements. The application of electron cyclotron resonance heating (ECRH) in the anchors is found to create high central-cell potentials. When the end walls are grounded, deep thermal barriers are created by the application of second harmonic ECRH. Rapid decay of these barriers suggests that they are formed by the presence of low temperature, magnetically confined electrons. Axial confinement of the central-cell ions is sometimes poor, even in the presence of a deep thermal barrier. Frequently, a potential maximum is observed near the inner turning point of the anchor sloshing ions. This potential is sometimes large enough to explain the reduced axial current from the central cell.