High-Power Electron Landau-Heating Experiments in the Lower Hybrid Frequency Range in a Tokamak Plasma

Abstract

The effectiveness of plasma heating by electron Landau interaction in the lower hybrid range of frequencies in tokamak plasmas is demonstrated. Upon injection of 850 kW of rf power at a density of n e -1.4x 10 14 cm -3 , an electron temperature increase of 1.0 keV and an ion temperature increase of 0.8 keV was achieved. These results are compared with transport and ray-tracing code predictions.PACS numbers: 52.50. Gj, 52.40.Db In this Letter experimental results are presented which demonstrate for the first time that substantial electron Landau heating in the lower hybrid frequency range 1 can be achieved in magnetically confined high-temperature (T e ~ T { > 1 keV) and high-density (« e > lxlO 14 cm" 3 ) plasmas. In the present experiments ///LH(0)^.2 [where AH(0) is the lower hybrid frequency at the plasma center] so that direct ion Landau heating by the waves can be excluded. 1 Bulk ion heating is achieved by collisional equilibration between electrons and ions. In previous electron Landau-heating experiments only 100-200 kW of rf power was injected, mostly at lower densities, and the associated electron heating was modest. 2 "" 4 The present experiments indicate the potential attractiveness of lower hybrid rf heating of fusion-grade plasmas.The experiments were carried out in the Alcator C tokamak (major radius R = 64 cm, minor radius a = 16.5 cm), at magnetic fields in the range B T ~ 7-11 T. Three types of limiter materials were used: molybdenum, graphite, and silicon-carbidecoated graphite. The lower hybrid waves were launched by two 4x4 waveguide arrays located 180° relative to each other around the torus. 5 Each array was fed by four 250-kW, 4.6-GHz Varian klystrons, and adjacent columns of waveguides were phased 180° relative to each other. The power spectrum of the launched waves extended from N u =ck n / co-±2 to Nn = ±4, and had maxima at N u -±3.1. 6 Figure 1 shows the time evolution of plasma parameters during a typical high-rf-power shot in deuterium plasma with silicon-carbide-coated graphite limiters. The ion temperature measurements were carried out by neutron yield analysis (corrected for impurity influx) and by a massresolving charge-exchange fast-neutral analyzer looking perpendicular to the magnetic field. The fast-neutral energy spectrum was measured every FIG. 1. Time history of a typical rf shot. 5 = 9T,D + ions, and n e \4 is in units of 10 14 cm -3 .

Site-Dependent 18O Substitution in YBa2Cu3O7 Studied by Raman Scattering Measurements

“…Since the brightness of impurity lines increases only by 20+30% at most, eve: with the pessimistic assumption that this increase is caused by an increased impurity influx, particle confinement time is found to improve by a factor 1.5i2. This phenomenon has been observed also in other LH experiments in the same electron heating or current drive regime (Imai and others, 1980; Porkolab and others, 1985). The main conclusions from the study of RF heating can be s m r i z e d as follows:…”

Electron acceleration by wave turbulence in a magnetized plasma

“…The lower-hybrid waves have a high phase velocity along the field lines that resonate with the fast moving electrons as well as a low phase velocity across the field lines that resonate with the slow moving ions, allowing for energy transfer between the two species [13]. This convenient property of lower-hybrid waves, as an efficient channel for the acceleration of electrons above the thermal background, is well known in the magnetically confined fusion community [14] where it has been exploited with considerable efficacy in past experiments [15,16]. Although a different mechanism is favoured in toroidal field configurations, for highly oblique shocks such as might be created in the Solar wind [9], supernova explosions [11], or during the formation of galaxy clusters [17], it is thought that the modified two-stream instability [13] driven by reflected ions from the shock front, excites broad-band lower-hybrid modes.…”

Localized Heating of the Martian Topside Ionosphere Through the Combined Effects of Magnetic Pumping by Large‐Scale Magnetosonic Waves and Pitch Angle Diffusion by Whistler Waves

“…Wave-particle interactions can be important sources of heating for space plasmas. Cyclotron resonance and Landau damping, for example, can efficiently heat plasmas in a variety of environments, including tokamak experiments (Adam, 1987;Porkolab et al, 1984;Seo et al, 2000), the terrestrial ionosphere and magnetosphere (André & Yau, 1997;Agapitov et al, 2015Agapitov et al, , 2016, and other planetary magnetospheres and ionospheres such as at Venus (Taylor et al, 1979) and Mars (Ergun et al, 2006;Lundin et al, 2006). Magnetic pumping can also heat plasmas via adiabatically heated anisotropic electron distributions, which relax to Maxwellians through Coulomb collisions and/or efficient pitch angle diffusion (Borovsky & Hansen, 1990;Laroussi & Roth, 1989;Lichko et al, 2017).…”