A high‐frequency hot electron instability is observed in ELMO Bumpy Torus (EBT) plasmas when the hot electron‐to‐ion density ratio exceeds 0.4. Both the real frequency and the imaginary frequency are larger than the ion cyclotron frequency. The azimuthal mode number (m) is 7, and the instability rotates in the hot electron curvature drift direction. This instability is identified as a curvature‐driven mode. When it is strongly excited, the equilibrium of the hot electron annuli and confined plasmas are destroyed (disruption). Ion heating and neutron bursts are associated with this instability.

Observation of hot electron ring instabilities in ELMO Bumpy Torus

High power ECH (utilizing a 28-GHz gyrotron at power levels up to 200 kW, cw) and fast-wave ICH (in the frequency range 15-30 MHz at power levels up to 20 kW, cw) experiments on EBT-Scale (EBT-S) have yielded Te approximately 1.1 keV, n approximately 1-2*1018 m-3, and tau E approximately 6 ms. The ring and core loss rates scale as predicted by the neoclassical theory, and the plasma parameters have the predicted values for the densities, temperatures, and confinement times reachable in this small plasma radius (10 cm) device at low field (1.0 T). In addition, the following observations were made: (1) the stable operating window is larger at high power levels, (2) a substantial increase in energetic ion tails (Ti>100 eV) and a doubling of the ring electron stored energy (beta) were seen at the highest ICH power levels, and (3) a significant increase in core density and stabilization of hot electron modes was possible with gas puffing.

Confinement heating and stability in the ELMO bumpy torus (EBT)

The theory of nonlinear mode conversion is reviewed and applied to the ELMO Bumpy Torus (EBT) [Proceedings of the 5th International Conference on Plasma Physics and Controlled Nuclear Fusion Research 1974 (IAEA, Vienna, 1975), Vol. 2, p. 141] environment in the electron‐cyclotron, upper‐hybrid, lower‐hybrid, and ion‐cyclotron frequency ranges. A variety of parametric excitations in these frequency ranges is studied. Based on cascade saturation mechanisms, the parametric turbulence levels and corresponding anomalous absorption rates of the excited modes are evaluated. The relevance of nonlinear mode conversion for contemporary and future EBT experiments is discussed.

Nonlinear mode conversion and anomalous absorption processes during radio‐frequency heating of bumpy torus plasmas

New and improved diagnostic techniques and increased theoretical understanding of electron‐cyclotron heating (ECH) have led to a new picture of the electron distribution function in the ELMO Bumpy Torus (EBT). For electron energies ≤2 keV, the distribution function can be approximated as consisting of two components: a cold isotropic bulk component and a warm anisotropic tail. For typical steady‐state EBT‐Scale (EBT‐S) operation (microwave power input of 100 to 150 kW) in the T‐mode (the operating mode of interest for fusion experiments), bulk densities and temperatures of ne =0.5–0.7×1012 cm−3 and Te ≤100 eV are observed, while the warm tail component has Te =200–800 eV, and densities ≤0.4 that of the cold component. The non‐Maxwellian nature of the distribution function is caused by ECH.

The electron distribution function in a bumpy torus

Flute instability of a magnetoplasma is generated and controlled experimentally by a nonuniform weak radial electric field. Six concentric electrodes, biased separately, are used to change the radial electric field. The instability, which is different from the velocity shear flute instability, is observed in a plasma with well‐type potential and hill‐type density profiles.

Control of low-frequency plasma instabilities by a nonuniform radial electric field

Experiments with 200 kW of applied electron cyclotron heating (ECH) power have demonstrated electron temperatures of about 1 keV in the ELMO Bumpy Torus-Scale (EBT-S) device. Electron densities are in the range of (0.5-1.5)*1018 m-3 and increase as the square root of the applied ECH power. A potential well is present, and its depth in V closely follows the electron temperature expressed in electron volts. Only low charge states of impurities are found, and Zeff approximately=1.0. Data from the electrons can be compared with simple scaling laws when scale lengths are held constant. These comparisons indicate that electron densities, temperatures, and confinement times scale according to neoclassical expectations.

Plasma properties in the ELMO bumpy torus

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Plasma properties in the ELMO bumpy torus