K. L. Wong

Harmonic launching of ion Bernstein waves via mode transformation

Ion Bernstein wave excitation and propagation via finite ion-Larmor-radium mode-transformation are investigated theoretically and experimentally. It is shown that in the ion cyclotron range of frequencies omega less than or equal to 4..cap omega../sub i/, with modest ion temperatures (T/sub i/ less than or equal to 10 eV), the finite-Larmor-radius effect removes the wave singularity at lower-hybrid resonance layer, enabling an externally initiated electron plasma wave to transform continuously into an ion Bernstein wave. In an ACT-1 hydrogen plasma (T/sub e/ approx. = 2.5 eV, T/sub i/ less than or equal to 2.0 eV), externally excited ion Bernstein waves have been observed for omega less than or equal to 2..cap omega../sub i/ as well as for omega less than or equal to 3..cap omega../sub i/. The finite ion-Larmor-radius mode transformation process resulting in strong ion Bernstein wave excitation has been experimentally verified. Detailed measurements of the wave dispersion relation and of the wave-packet trajectory show excellent agreement with theory. The dependence of the excited ion Bernstein wave on the antenna phasing, the plasma density, and on the neutral pressure (T/sub i/) is also investigated.

Harmonic generation and parametric decay in the ion-cyclotron frequency range

Harmonic generation and parametric decay are examined in a toroidal ACT‐1 plasma using electrostatic plate antennas. The harmonic generation, which is consistent with sheath rectification, is sufficiently strong that the nonlinearly generated harmonic modes themselves decay parametrically. Resonant and nonresonant parametric decay of the second harmonic are observed and compared with uniform pump theory. Resonant decay of lower‐hybrid waves into lower‐hybrid waves and slow ion‐cyclotron waves is seen for the first time. Surprisingly, the decay processes are nonlinearly saturated, indicating absolute instability.

Parametric excitation of ion Bernstein waves by a fast wave antenna in the ion cyclotron frequency range

Parametric excitation of ion Bernstein waves is observed with an ICRF fast wave induction loop antenna in the ion cyclotron frequency range (..omega../sub o/ approx. 2..cap omega../sub i/ - 4..cap omega../sub i/). Important features of the decay process are investigated and discussed.

Observation of the backward electrostatic ion-cyclotron wave

The backward branch of the electrostatic ion‐cyclotron wave has been observed for the first time. The wave, which was driven by a phased antenna structure inserted in a neon plasma, exists in the parameter ranges 2Ti/mi≪(ω/k∥)2≪2Te/me, nΩi Ωi. Double‐tip probe interferometry data agree with the theoretical dispersion relation. The antenna couples into the wave more readily on the side of the antenna where it has its smallest wavenumber.

Effects of beam plasma instability on current drive via injection of an electron beam into a torus

One‐ and two‐dimensional particle simulations of beam–plasma interaction have been carried out in order to understand current drive experiments that use an electron beam injected into the Advanced Concepts Torus (ACT)‐1 device [Rev. Sci. Instrum. 53, 409 (1982)]. Typically, the initial beam velocity along the magnetic field is V0=109 cm/sec, while the thermal velocity of the background electrons is vt=108 cm/sec. The ratio of the beam density to the background density is about 10% so a strong beam–plasma instability develops, causing rapid diffusion of beam particles. For both one‐ and two‐dimensional simulations, it is found that a significant amount of beam and background electrons are accelerated considerably beyond the initial beam velocity when the beam density is more than a few percent of the background plasma density. In addition, the electron distribution along the magnetic field has a smooth negative slope, f’(v∥) 0 extending to v∥=1.5 V0∼2 V0, which is in sharp contrast to the predict...

Effect of density fluctuations on lower hybrid resonance cone propagation

Experimental measurements show that coherent, azimuthal density fluctuations (1) focus lower hybrid resonance cones azimuthally, and (2) modulate the radial location of the resonance cones. A simple theory based on wave refraction is presented; this theory is in good agreement with the experimental observations. The theory is extended to give a physical description of wave propagation through turbulent, isotropic (perpendicular to B) noise such as has been observed in tokamaks; it is found that the turbulence causes the lower hybrid wave vectors to have an angular spread in the plane perpendicular to B.

Ponderomotive force effects on slow-wave coupling

Localized plasma density depressions are observed to form near a multi‐ring slow‐wave structure when the value of the nonlinearity parameter, s = ω2pe‖Ez‖2/8πω2nκT, is of order unity. Consequent changes in the wave propagation and coupling efficiency are reported. For large enough values of s, the coupling efficiency may be reduced by 50% from the linear value.

Enhancement of drift waves by localized lower hybrid waves

The enhancement of low frequency oscillations by lower hybrid waves at electric fields lower than the thresholds of other parametric decay processes is presented. The frequencies and wavelengths of these oscillations agree with the collisional drift wave dispersion relation. The enhancement is localized deep inside the plasma density gradient. The excited drift waves change the density profile and the lower hybrid wave trajectory.

Lower‐hybrid wave resonance cone detection via CO2‐laser scattering

Lower‐hybrid waves are studied in the Princeton ACT‐I steady‐state toroidal plasma device using a radially scanning CO2 laser scattering system with both amplitude and phase sensitive detection techniques. Clearly defined resonance cones launched from external electrostatic antennas are seen to disappear as the plasma density is raised. Scaling of a lower‐hybrid wave (LHW) laser signal with radio‐frequency (rf) power in the presence of resonance cones shows nonlinearities associated with rf‐induced changes in the effective laser scattering volume. Absolute fluctuation level estimates suggest that this occurs when eΦ/Te≥1. Wave‐front curvature effects can cause a complete loss of resonance cone laser signals, even though probes indicate that cones are still present. Measurements of the wave k⊥ spectrum in the plasma show direct evidence for electron Landau filtering of the original wave k∥ spectrum launched from the antenna at the plasma edge, and strong dependence on antenna phasing. Finally, frequency sh...

Effect of convective loss on the parametric decay of cold electron plasma waves

The resonant decay of cold electron plasma waves was investigated in the Princeton L‐3 device in an argon plasma with n∼1010–1011 cm−3, B∼700 G–2 kG, f0∼50–80 MHz (ω0⩾10 ωlh). The decay waves were identified to be ion‐acoustic waves and cold electron plasma waves. Because of convective losses in the finite extent pump field, the threshold for parametric decay is considerably higher than the collisional threshold for the dominant E0⊥×B coupling. Coupling due to E0∥ can be important in low density, high magnetic field plasmas. At high densities (n≳3×1010 cm−3), E0⊥×B coupling becomes dominant. The E0⊥×B decay convective threshold was measured at various antenna lengths, magnetic field strengths, and plasma densities. The experimental results are in good agreement with theory. Indication of pump depletion and enchanced Landau heating of electrons were observed in the parametric decay process.