R. S. Post

Plasma production and heating in a tandem mirror central cell by radio‐frequency waves in the ion cyclotron frequency range

Plasma production and heating in the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research, 1986, Proceedings of the 11th International Conference, Kyoto, Japan (IAEA, Vienna, 1987), Vol. 2, p. 251] have been studied. Using radio‐frequency excitation by a slot antenna in the ion cyclotron frequency range (ICRF), plasmas with a peak β⊥ of 3%, density of 4×1012 cm−3, ion temperature of 800 eV, and electron temperature of 75–100 eV were routinely produced. The plasma radius decreased with increasing ICRF power, causing reduced ICRF coupling and saturation of the plasma beta. About 70% of the applied ICRF power can be accounted for in direct heating of both ions and electrons. Wave field measurements have identified the applied ICRF to be the slow, ion cyclotron wave. In operation without end plugging, the plasma parameters were limited by poor axial confinement and the requirements for maintenance of magnetohydrodynamic stability and micros...

Experimental studies of divertor stabilization in an axisymmetric tandem mirror

A divertor coil set has been installed on the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research 1984 (IAEA, Vienna, 1985), Vol. 2, p. 285] for stabilization of m=1 flutelike modes. The effectiveness of divertor stabilization is discussed in experiments where m=1 modes are driven to instability by plug electron cyclotron heating (ECH) in an ion cyclotron heated (ICH) plasma. The instability onset is characterized by thresholds in ECH power, fueling rate, ICH power, and mapping radius of the divertor null. In general, the stability is enhanced by mapping the null radially inwards into the plasma. The interdependence of these parameters and their effect on equilibrium profiles and stability boundaries are discussed.

Whistler instability in an electron‐cyclotron‐resonance‐heated, mirror‐confined plasma

The warm‐electron‐driven (2 keV) whistler electron microinstability [Phys. Rev. Lett. 59, 1821 (1987)] of the Constance B electron‐cyclotron‐resonance‐heated (ECRH), quadrupole mirror‐confined plasma experiment has been studied. Experiments show (i) that the instability comes in fairly regular bursts on axis and continuously in time off axis due to the minimum‐B geometry, (ii) a frequency spectrum that is insensitive to changes in the plasma parameters, and (iii) instability‐induced power losses which are not greater than 10% of the ECRH power input for the regimes studied. A linear perturbation analysis of the relativistic Vlasov equation together with Maxwell’s equations has been made. Using the ECRH distribution function, a new distribution function well suited for describing ECRH, mirror‐confined plasmas, the analysis shows the instability frequency spectrum to be insensitive to changes in cyclotron frequency, temperature, and density, in agreement with experimental results, and only sensitive to chan...

Plasma potential enhancement by rf heating near the ion‐cyclotron frequency

The observation of enhanced plasma potentials, i.e., potentials greater than the Boltzmann values, in a mirror device is reported. The potential structure is driven by strong radio frequency heating near the ion‐cyclotron resonance and near the local electron bounce frequency. The potentials and their effect on losses from the central cell of a tandem mirror are discussed.

High‐frequency gyrotron scattering diagnostic for instability studies on TARA

A 1‐ to 10‐kW,>30‐ms pulsed, narrow linewidth (<1 MHz), 137‐GHz gyrotron is being fabricated for collective Thomson scattering plasma diagnostics on the TARA tandem mirror experiment. The drift cyclotron loss cone, the axial loss cone, harmonics of these instabilities, and the ion two stream instability in the TARA plugs will be studied with this diagnostic.

Experimental study of nonlinear M = 1 modes in the Tara tandem mirror

The nature of a rigid, flutelike M=1 instability as seen in the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion 1984 (IAEA, Vienna, 1985), Vol. 2, p. 285] is discussed. Radial density and light emission profiles obtained by inverting chord measurements are compared to end loss radial profiles during the evolution of the mode to its nonlinear saturated state. This final state is characterized by a coherent, flutelike motion of the plasma as a whole about the machine axis.

Experimental study of the hot electron plasma equilibrium in a minimum‐B magnetic mirror

The Constance B mirror [in Plasma Physics and Controlled Nuclear Fusion Research 1984 (IAEA, Vienna, 1985), Vol. II, p. 285] is a single cell quadrupole magnetic mirror in which high‐beta (typically 0.3), hot electron plasmas (Te≂400 keV) are created with up to 4 kW of fundamental electron cyclotron resonance heating (ECRH). Details of the plasma equilibrium profile are quantitatively determined by fitting model plasma pressure profiles to the data from four complementary measurements: diamagnetic loops and magnetic probes, x‐ray pinhole cameras, visible light TV cameras, and thermocouple probes. The experimental analysis shows that the equilibrium pressure profile of an ECRH generated plasma in a baseball magnetic mirror is hollow and the plasma is concentrated along a baseball‐seam‐shaped curve. The hollowness of the hot electron density profile is 50%±10%. The baseball‐seam‐shaped equilibrium profile coincides with the drift orbit of deeply trapped electrons in the quadrupole mirror field. Particle dri...

137‐GHz gyrotron diagnostic for instability studies in Tara

A narrow linewidth ( 97% TE11 mode output) have been built for collective Thomson scattering diagnostics. The main goal will be to study instability driven ion density fluctuations in the Tara plug such as the drift cyclotron loss cone (DCLC), the axial loss cone (ALC), harmonics of the DCLC and ALC, and the ion two‐stream instability. The heterodyne receiver and signal optics have been installed on Tara. Background electron cyclotron emission (ECE) at 139±1.5 GHz after electron cyclotron resonance heating (ECRH) in the Tara plug corresponded to equivalent blackbody temperatures of 453 and 70 eV for extraordinary and ordinary emission, respectively. The well‐collimated receiver field of view completely through the Tara plug has allowed for excellent polarization discrimination of the ECE. The high‐power capability of this gyrotron will allow weak fluctuation levels (n/n<10−6) to be detected above thi...

Stability of plasmas sustained by ion cyclotron wave excitation in the central cell of the Tara tandem mirror

The stability of plasmas produced by radio‐frequency heating in the ion cyclotron frequency range (ICRF) has been studied in the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research 1986, Proceedings of the 11th International Conference, Kyoto (IAEA, Vienna, 1987), Vol. II, p. 251]. Ion cyclotron wave excitation by a slot antenna provided stability against macroscopic plasma motions in an axisymmetric configuration. The maintenance of macroscopic stability depended on the ICRF power, gas fueling rate, ion cyclotron resonance location, and ω/ωci at the antenna location. The ICRF ponderomotive force model is consistent with many of the observed stability features and predicts that the E+ component of the ion cyclotron wave was responsible for the stabilization. The Alfven ion cyclotron microinstability was observed when the plasma β⊥ and anisotropy were sufficiently high. Magnetic probe measurements of the unstable mode identified it as a...

Electron velocity-space diffusion in a microunstable electron cyclotron resonance heated mirror plasma

An experimental study of the velocity‐space diffusion of electrons in an electron cyclotron resonance heated (ECRH) mirror plasma, in the presence of microunstable whistler radio frequency (rf) emission, is presented. The dominant loss mechanism for hot electrons, with temperatures Th ∼400 keV, is end loss produced by rf diffusion into the mirror loss cone. In a case with 4.5 kW of applied power, this loss limits the stored energy to 120 J with a hot electron energy confinement time τEh ∼40 msec. The corresponding value associated with collisional scattering is 320 msec. Whistler microinstability rf induces up to 25% of the endloss. The hot electron temperature is limited by rf‐induced end loss of high‐energy electrons, and decreases with increasing rf power in strong diffusion regimes. Measurements of collisional loss agree with standard scattering theory. Weaker diffusion is seen in experiments in which the vacuum chamber walls are lined with microwave absorber than in experiments with reflecting walls....