Observation of Lower-Hybrid Current Drive at High Densities in the AlcatorCTokamak

Abstract

A quasi-steady-state lower-hybrid current-drive operation is demonstrated in the Alcator C tokamak at densities up to n e -1 x 10 14 cm" 3 . The current-drive efficiency is measured experimentally over a wide range of densities and magnetic fields. The radial distribution of high-energy x rays indicates that the current-carrying electrons peak near the plasma axis. PACS numbers 52.40.Db, Generation of toroidal currents in tokamaks by injection of traveling lower-hybrid waves is currently of great interest in magnetically confined plasma-fusion research. 1 This interest arises from the possibility of steady-state tokamak operation which would make the tokamak concept a more attractive reactor candidate. 2 To present date, efficient "quasi-steady-state" operation has been demonstrated only at relatively low electron densities, namely at n e < 8x 10 12 cm" 3 in the PLT tokamak. 3 This mode of tokamak operation is achieved by terminating the inductive current sources (Ohmic power) as the rf power is injected. Eventually, a pure rf mode of operation appears feasible. 4The low-density limit in the PLT experiments is believed to be a consequence of the relatively low frequency (/ = 0.8 GHz) used. In other experiments current drive has been demonstrated by injecting rf power during full Ohmic heating (OH) operation. 5 " 8 In these experiments significant dc electric fields often exist during rf injection Avhich may modify the rf current drive figure of merit J/Pj. 1 In particular, the question of the influence of electron tails generated by the OH electric fields which existed before, or during rf injection, arises. Due to the relatively low frequencies (/=^1.3-GHz) even such "OH-assisted" experiments were carried out mostly at low densities (n e <2 xl0 13 cm-3 ).In this Letter we present experimental measurements of the efficiency of lower-hybrid current drive at reactor-relevant densities, namely at 10 13 <. /Hem" 3 ) < 10 14 . The experiments were carried out on the Alcator C tokamak (major radius R =64 cm, minor radius a = 16.5 cm) where the high rf frequency (/ = 4.6 GHz) and high rf powers (P < 1.1 MW) used allowed us to demonstrate rf current-drive operation at high densities. 9 The lower-hybrid waves were launched by two 4x4 wave-guide arrays located 180° relative to each other around the torus. For the current-drive experiments described in this paper the phases of adjacent waveguides in each row were set at 90°. The Brambilla rf power spectrum under such conditions is concentrated between N u = c/cn/co = 1 and 2.5 in the OH electron-drift direction, with approximately 30% of the power being in the opposite (negative) direction. 10 The negative portion of the spectrum is concentrated at relatively high values of N\\ (N n ~~ 6) and is expected to be absorbed by, and heat, the bulk plasma.In the present experiments the primary winding of the Ohmic heating transformer was opencircuited before application of the rf power. After the OH primary is opened the plasma current decays inductively with a typical time scale ...

Current drive at plasma densities required for thermonuclear reactors

“…Th is diagnostic technique has proved to be useful for monitoring wave -plasma edge interactions in LH experiments in tokamaks 8,11,12,18,20 . Figure 4 shows the spectral broadening measurements obtained during two experiments, one in the standard regime ( Fig.…”

“…Using the LH star code, which is described by Cesario et al 11,12 , the PI-induced spectral broadening has been calculated and used to determine the LH deposition in the plasma. Frequencies and growth rates of the ion-sound-quasi-mode-driven LH sideband waves are obtained by solving the parametric dispersion relation 8,19 : where ε is the dielectric function, ω , k indicate, respectively, the complex frequency and the wave vector of the low frequency perturbation, the suffi x i = 0,1,2 refers to the pump, the lower and the upper sidebands, respectively, and μ 1,2 are the coupling coeffi cients referring to the lower and upper sidebands, respectively (see Cesario et al 12 for details). From the measured data in the standard regime, plasma (discharge 32164 in Figure 3 ) high growth rates ( γ / ω 0 ≈ 1 × 10 − 4 ) are obtained for the LH sidebands that are signifi cantly shift ed in frequency with respect to the pump wave ( Δ f ≈ 15 -20 MHz in frequency, and Δ n Խ Խ ≈ 30 in wavenumber).…”

“…Th is implies that, for wave-phase velocity 7 , v Φ Խ Խ ≡ c / n Խ Խ ≈ (2.5 -4) v the , where the suffi x ' Խ Խ ' refers to the direction parallel to the equilibrium magnetic fi eld, n Խ Խ is the wavenumber component, v the and c are the electron thermal velocity and light speed, respectively. Th is mechanism produces the LHCD eff ect 2,4 confi rmed in many experiments 3,8,9 using antennas that launch LH waves with n Խ Խ low enough to satisfy the Landau damping resonance condition. Consequently, the experimental observations of LH power absorption have been interpreted as the consequence of an upshift and broadening of the launched n Խ Խ spectrum produced by wave propagation in the toroidal geometry 10 and by wave plasma interactions occurring at the plasma periphery 11,12 .…”

Energy Confinement of Lower-Hybrid-Current-Driven Tokamak Plasmas

“…On Alcator C, an empirical current drive efficiency, defined as 7 = eIPRo/Prf (10 20 m-2 MA/MW), of 77 =0.12 has been measured for densities up to -ne= 1x10 14 cm-3. 3 Here, ii is the line-averaged electron density, I, is the plasma current, R, is the plasma major radius, and Prf is the net injected rf power. The density scaling of the current drive efficiency is in agreement with the theoretical supposition that the major loss of the superthermal electrons is due to collisions with bulk electrons; i.e.…”

Paraxial WKB Method Applied to the Lower Hybrid Wave Propagation

“…Temperature and density profiles are parabolic following equation (26) T e,edg = 0.5 keV, for all three cases. Plasma parameters mentioned here are based on the Alcator C experiment 41,42 . Regarding the initial conditions of the LH wave beam, a precise matching of the pWKB solution (5) to the electric field generated at the antenna mouth is beyond the scope of this work.…”