Fast Wave Current Drive Research Articles

Development and testing of prototype interdigital TWA for fast wave current drive in SST-1 tokamak

Fast Wave Current Drive (FWCD) has the potential to drive non-inductive current in high beta plasmas. A novel interdigital type Travelling Wave Antenna (TWA) was proposed by the authors to launch fast waves inside SST-1 plasma at an operating frequency of 500 MHz and a parallel refractive index of 4.0. A prototype interdigital TWA has been fabricated and tested for its RF response at low power. The simulated and measured frequency responses with vacuum loading are in excellent agreement and the TWA exhibits band-pass like characteristics with a return loss of > 25 dB and insertion loss of 0.45 dB to 1.0 dB. Measurement studies have also been carried out with saltwater loading for various gaps between antenna mouth and saltwater edge and with different conductivity in water. Simulated and measured results are found to be in good agreement with saltwater loading as well. The n∥ spectrum is also measured and it has been observed that n∥ spectrum peaks at a value of 4.0.

Design and Simulation of an Interdigital Travelling Wave Antenna for Fast Wave Current Drive in SST-1 Tokamak

In reactor-grade plasma, where density and temperature are very high, Fast Wave Current Drive (FWCD) technique can be used to launch fast waves inside the plasma to sustain the current for a longer duration. A Interdigital type travelling wave antenna is proposed alternative structure to the combline antenna to launch these fast waves for SST-1 tokamak at an operating frequency of 500 MHz and parallel refractive index of ~ 4.0. Multiphysics analysis is carried out to evaluate the RF and thermal performance of an antenna structure using COMSOL Multiphysics software. The simulation studies have shown that the thermal performance of the interdigital antenna is better than that of combline antenna for high input power. Simulation results show that the antenna exhibits band-pass characteristics with a return loss of less than 30 dB and insertion loss of ~ 0.23 dB. The simulation studies have been carried out with a vacuum and seawater loading in front of the antenna. Steady-state thermal analysis has shown that an appropriate cooling mechanism should be provided to facilitate carrying heat away from the structure if the antenna is operated with high input power for longer durations.

Computational studies on fast wave current drive in high beta SST-1 and SST-2 plasmas

The non-inductive current drive in low toroidal beta (β) steady-state superconducting tokamak-1 (SST-1) is obtained using slow waves or Lower Hybrid waves (LHW). If β of the plasma is increased either by increasing the plasma parameters (density/temperature) or by decreasing the toroidal magnetic field, the non-inductive current drive may be supported by Fast Waves (FWs), as wave accessibility is limited for slow waves at these high β plasmas. Non-inductive current drive using FWs is explored for these high β plasmas to identify the optimized parameters (operating frequency and parallel refractive index (n||)) of the fast wave. The studies indicate that FWs may be launched with an operating frequency of 0.5 GHz RF source and n|| around 4 in SST-1 plasma. The GENRAY code is employed to carry out detailed studies on Fast Wave Current Drive (FWCD) analysis using ray tracing techniques. Computational studies on FWCD are carried out where central electron density (ne0) and central electron temperature (Te0) of 6×1019m−3 and 3 keV respectively is assumed. The FWCD displays central depositions of RF power and is attributed to fast radial propagation compared to the parallel propagation along the toroidal magnetic field, resulting in ray transits with large number of reflections. The off-axis current drive in SST-1 plasma requires reduced radial penetration of the wave and demands unrealistic plasma parameter in terms of β. However, for SST-2 plasmas, having enhanced plasma parameters (βN∼1.8), it is found that if FWs are launched with n||∼3 and f∼1.5 GHz, the radial penetration of the wave is impeded and single pass absorption in toroidal direction is observed, thus providing conducive condition for off axis current drive. It indicates that FWs would drive plasma current of 11 kA/MW at ρ∼0.7 for ne0∼2.2×1020m−3 and Te0∼7.7 keV values.

ICRF fast wave current drive and mode conversion current drive in EAST tokamak

Fast wave in the ion-cyclotron resonance frequency (ICRF) range is a promising candidate for non-inductive current drive (CD), which is essential for long pulse and high performance operation of tokamaks. A numerical study on the ICRF fast wave current drive (FWCD) and mode-conversion current drive (MCCD) in the Experimental Advanced Superconducting Tokamak (EAST) is carried out by means of the coupled full wave and Ehst-Karney parameterization methods. The results show that FWCD efficiency is notable in two frequency regimes, i.e., f ≥ 85 MHz and f = 50–65 MHz, where ion cyclotron absorption is effectively avoided, and the maximum on-axis driven current per unit power can reach 120 kA/MW. The sensitivity of the CD efficiency to the minority ion concentration is confirmed, owing to fast wave mode conversion, and the peak MCCD efficiency is reached for 22% minority-ion concentration. The effects of the wave-launch position and the toroidal wavenumber on the efficiency of current drive are also investigated.

Modeling of neutral beam injection heating and current drive during the ramp-up phase in KSTAR

For flexible control of the plasma pressure and the current profiles, which are essential for a high performance plasma with long pulse operation, KSTAR is going to implement several heating and current systems, which include Neutral Beam Injection (NBI), Ion Cyclotron Resonant Heting (ICRH)/Fast Wave Current Drive (FWCD), Lower Hybrid Current Drive (LHCD), and Eclectron Cyclotron Heating (ECH)/Electron Cyclotron Current Drive (ECCD). Here, the NBI system is typically used for the central heating and current drive. For the time being, only one NBI device (composed of 3 sources) is available in KSTAR. The first two sources were successfully commissioned in 2010 and 2013. The last source will be installed in 2014. In this work, we present a simulation study of the heating and current drive of the first NBI system (3 sources) during the ramp-up phase. We consider two different NBI configurations (low and high beam energy). The simulation is performed with NUBEAM, a well-recognized Monte-Carlo code. Several different types of KSTAR target equilibria (scan from lower to higher plasma density) are used for the calculation of the current drive, the heating and the different NB losses (shinethrough, charge exchange and bad orbit). The study shows the dependency of those quantities on the plasma density, the position of the NB source and the beam energy. It also shows that because of the shinethrough loss is too high, each NB source cannot be used when the plasma density is under a certain threshold. This study can be used to determine the starting time of the different NB sources during the KSTAR ramp-up phase.

The Full Wave Numerical Solution for Fast Wave Current Drive in Tokamak

This paper uses the full wave method to study the fast wave current drive (FWCD) problem for Ion cyclotron resonance frequency (ICRF). It exists in such plasma bodies as circumferential Tokamak. The research has considered finite Larmor radius effect (FLR) and flattened dispersion. It found the physical model for full wave calculation-full wave method. Through the numerical solution of full wave method, the distribution of electric field strength can be obtained, which is stimulated and produced when fast wave stays in plasma body.

Progress towards high-performance steady-state operation on DIII-D

Advanced Tokamak research in DIII-D seeks to develop a scientific basis for steady-state high-performance tokamak operation. Fully noninductive ( f NI ≈ 100%) in-principle steady-state discharges have been maintained for several confinement times. These plasmas have weak negative central shear with q min ≈ 1.5–2, β N ≈ 3.5, and large, well-aligned bootstrap current. The loop voltage is near zero across the entire profile. The remaining current is provided by neutral beam current drive (NBCD) and electron cyclotron current drive (ECCD). Similar plasmas are stationary with f NI ≈ 90–95% and duration up to 2 s, limited only by hardware. In other experiments, β N ≈ 4 is maintained for 2 s with internal transport barriers, exceeding previously achieved performance under similar conditions. This is allowed by broadened profiles and active magnetohydrodynamic instability control. Modifications now underway on DIII-D are expected to allow extension of these results to higher performance and longer duration. A new pumped divertor will allow density control in high triangularity double-null divertor configurations, facilitating access to similar in-principle steady-state regimes with β N > 4. Additional current drive capabilities, both off-axis ECCD and on-axis fast wave current drive (FWCD), will increase the magnitude, duration, and flexibility of externally driven current.

Radio-Frequency Current Drive in DIII-D

Two methods of radio-frequency (rf) current drive that are well suited to controlling and sustaining the current profile in burning plasma experiments have been studied in the DIII-D tokamak. Fast-wave current drive (FWCD) gave centrally peaked current densities that increased linearly with central electron temperature. While high harmonic absorption of the fast waves on energetic beam ions could reduce the available power for current drive, FWCD figures of merit as high as γFW = 0.5 × 1019 A/m2·W were still achieved. Electron cyclotron current drive (ECCD) was shown to be localized to the region of power deposition, with a current drive efficiency that decreased as the magnetic well depth increased. The detrimental effect of the magnetic well could be mitigated by raising the electron beta. ECCD figures of merit as high as γEC = 0.5 × 1019 A/m2·W were measured for central deposition. The experimental FWCD and ECCD were both extensively tested against theoretical models and were found to be in excellent agreement. Validation of these predictive models of rf current drive aids in scenario development for next-step tokamaks.

On the parasitic absorption in FWCD experiments in JET ITB plasmas

Fast wave current drive (FWCD) experiments have been performed in JET plasmas with electron internal transport barriers produced with LHCD. Because of a large fraction of parasitic absorption, owing to weak single pass damping, the inductive nature of the plasma current and the interplay between the RF-driven current and the bootstrap current only small changes are seen in the central current profiles. The measured difference in the central current density for co- and counter-current drive is larger than the response expected from current diffusion calculations, but smaller than the driven currents, suggesting a faster current diffusion than that given by neo-classical resistivity. A large fraction of the power is absorbed by cyclotron damping on residual 3He ions while a significant fraction appears not to have been deposited in the plasma. The strong degradation of heating and current drive occurs simultaneously with strong increases in the Be II and C IV line intensities in the divertor. The degradation depends on the phasing of the antennas and increases with reduced single pass damping which is consistent with RF-power being lost by dissipation of rectified RF-sheath potentials at the antennas and walls. Asymmetries in direct electron heating, lost power and production of impurities, fast ions and gamma-rays are seen for co- and counter-current drive. These differences are consistent with the differences in the absorption on residual 3He ions owing to the RF-induced pinch. Effective direct electron heating, comparable to the indirect electron heating with H-minority heating, occurs for dipole phasing of the antennas without producing a significant fast ion pressure and with low impurity content in the divertor plasma.

Analysis of combined fast wave current drive and neutral beam injection in the DIII-D tokamak

In recent experiments with combined fast wave current drive (FWCD) and deuterium neutral beam injection on the DIII-D tokamak [Luxon et al., Fusion Technol. 8, 441 (1985)], an enhanced fusion reactivity and fast ion energy content have been observed in the presence of FWCD, with a concomitant low FWCD efficiency [Petty et al., Radio Frequency Power in Plasmas (AIP, New York, 1997), p. 225]. In this paper, we investigate whether high-harmonic ion cyclotron damping could be responsible for the low FWCD efficiency in these experiments, since a number of high-harmonic hydrogen and deuterium cyclotron resonance layers existed in the plasma. The main analysis tool is the ion cyclotron range of frequencies code PION [Eriksson et al., Nucl. Fusion 33, 1037 (1993)], modified to allow multiple frequencies simultaneously as was done in the DIII-D experiments. According to the PION modeling, high harmonic damping of fast wave power can give rise to enhanced fusion reactivity and fast ion energy content, which is consistent with the experimental observations.

Fast wave current drive at high ion cyclotron harmonics on DIII-D

Current driven by the fast Alfvén wave is measured at the fourth and eighth harmonics of the deuterium ion cyclotron frequency in identical plasmas on the DIII-D tokamak. In non-sawtoothingdischarges with neutral beam injection heating, the radial profile ofthe fast wave current drive (FWCD) is determined by the response of theloop voltage profile to co- and counter-antenna phasings. The dimensionless current drive efficiency is a factor of two greater for the eighth harmonic case compared to the fourth harmonic case. Modelling of the fast wave absorption using a ray tracing code shows that the decrease in FWCD efficiency for the latter situation can be explained by high harmonic damping of the fast waves on energetic ions.

Fast wave current drive in low aspect ratio tokamaks

The magnetic field of a low aspect ratio tokamak (LART) deviates rather significantly from the approximately 1/R dependence of conventional tokamaks. The parameterization of current drive efficiency on the basis of conventional large aspect ratio tokamaks becomes inaccurate. The physics of high harmonic ion cyclotron fast wave current drive (FWCD) and low frequency FWCD in LARTs is examined using exact evaluations of the current drive efficiencies from the adjoint technique. The results indicate that the high harmonic ion cyclotron current drive cannot penetrate to the plasma core at the high beta values typical of LART reactors, and hence this scheme may only be applicable for off-axis steady state current drive, while the low frequency current drive penetrates well into the plasma core and is appropriate for on-axis current profile control. The implications for advanced scenarios of LART are discussed

Fast wave and electron cyclotron current drive in the DIII-D tokamak

The non-inductive current drive from directional fast Alfven and electron cyclotron waves was measured in the DIII-D tokamak in order to demonstrate these forms of radiofrequency (RF) current drive and to compare the measured efficiencies with theoretical expectations. The fast wave frequency was 8 times the deuterium cyclotron frequency at the plasma centre, while the electron cyclotron wave was at twice the electron cyclotron frequency. Complete non-inductive current drive was achieved using a combination of fast wave current drive (FWCD) and electron cyclotron current drive (ECCD) in discharges for which the total plasma current was inductively ramped down from 400 to 170 kA. For steady current discharges, an analysis of the loop voltage revealed up to 195 kA of non-inductive current (out of 310 kA) during combined electron cyclotron and fast wave injection, with a maximum of 110 kA of FWCD and 80 kA of ECCD achieved (not simultaneously). The peakedness of the current profile increased with RF current drive, indicating that the driven current was centrally localized. The FWCD efficiency increased linearly with the central electron temperature as expected; however, the FWCD was severely degraded in low current discharges owing to incomplete fast wave absorption. The measured FWCD agreed with the predictions of a ray tracing code only when a parasitic loss of 4% per pass was included in the modelling along with multiple pass absorption. Enhancement of the second harmonic ECCD efficiency by the toroidal electric field was observed experimentally. The measured ECCD was in good agreement with Fokker-Planck code predictions

Theory and experiments on r.f. plasma heating, current drive and profile control in Tore Supra

The combination of r.f. waves in the lower hybrid (LH) and ion cyclotron frequency ranges offers a versatile and efficient way of heating tokamak plasmas while controlling their properties and magnetohydrodynamic stability through the control of the current density profile. Experimental and theoretical studies on the applications of such plasma waves have been carried out on Tore Supra during recent years and are reported here. The LH system coupled up to 6.5 MW during 2 s at 3.7 GHz through two multijunction launchers. In the longest plasma shot, the total injected LH energy reaches a record value of 170 MJ during a 62 s LH pulse, at a power level of 2.8 MW, corresponding to an average power density of 17 MW m −2 . The ion cyclotron resonant frequency (ICRF) system (35–80 MHz) is composed of three resonant double-loop antennae. Up to 4 MW have been coupled with a single antenna, allowing a record power density through the Faraday screen of 16 MW m −2 to be reached. 30 s steady-state r.f. pulses have been obtained with up to 54 MJ delivered to the plasma. One of the major observations has been the transition to the so-called “stationary lower hybrid enhanced performance (LHEP) regime” ( I P = 0.8 MA; n e0 = 2.8 × 10 19 m −3 ; P LH = 3.2 MW) in which the (flat)_central current density ( q 0 ∼2) and (peaked) electron temperature profiles ( T e0 ∼ 6–8 keV) are fully decoupled. This regime exhibits a significant improvement of the global confinement (40%) owing to the increase in l i , i.e. in the magnetic shear in the outer half of the discharge, supplemented by a large reduction in the electron thermal diffusivity in the central zone where the magnetic shear vanishes because of the slight off-axis character of the LH power and current deposition. Transp analyses show that LHEP plasmas provide access to the second ballooning stability regime. At higher current and density ( I p = 1.5 MA; n e0 = 6 × 10 3 m −3 ), ICRH stabilization of sawteeth (4 MW) combined with lower hybrid current drive (LHCD) current profile modifications has allowed to extend the stabilized phase for up to 1 s with 3.4 MW of LH power, the duration of sawtooth-free periods increasing with increasing LH power. The dynamical properties of fast electrons during LHCD have been investigated recently on Tore Supra through power modulation experiments. It is shown that slowing down always predominates and, from the long-time evolution of the hard X-ray emission, the radial diffusion rate of the fast electrons is estimated to be 0.1–0.3 m 2 s −1 . Theoretical developments have focused on the modelling of LHCD and also on fast wave current drive (FWCD) and fast wave heating. The effect of intrinsic stochasticity on the propagation of LH waves is discussed and a fully developed statistical theory of stochastic wave diffusion and multipass absorption, with applications to Tore Supra through a wave diffusion-Fokker-Planck (WDFP) numerical code, is briefly presented. This model provides a simple explanation for the temperature dependence of the LHCD efficiency in small tokamaks. The ion cyclotron resonant heating (ICRH) full-wave code Alcyon has been upgraded to compute the power and current deposition profiles from direct electron absorption of the fast wave (electron Landau damping-transit time magnetic pumping). The code has been used to study FWCD in Tore Supra, the Joint European Torus and the International Thermonuclear Experimental Reactor. Finally a new concept of an efficiently cooled reflector antenna for LHCD applications in a steady-state reactor is briefly described.

Evaluation of Current Drive Requirements and Operating Characteristics of a High Bootstrap Fraction Advanced Tokamak Reactor

The reactor potential of some advanced physics operating modes proposed for the TPX physics program [1] are examined. A moderate aspect ratio (A = 4.5 as in TPX), 2 GW reactor (see Table I for parameters) is analyzed because of its potential for steady-state, non-inductive operation with high bootstrap current fraction. Particle, energy and toroidal current equations are evolved to steady-state conditions using the 1-1/2-D time-dependent WHIST transport code [2]. The solutions are therefore consistent with particle, energy and current sources and assumed transport models. Fast wave current drive (FWCD) provides the axial seed current. The bootstrap current typically provides 80–90% of the current, while feedback on the lower hybrid current drive (LHCD) power maintains the total current. The sensitivity of the plasma power amplification factor, Q ≡ Pfus/Paux, to variations in the plasma properties is examined. The auxiliary current drive power, Paux = PLH + PFW; bootstrap current fraction; current drive efficiency; and other parameters are evaluated. The plasma is thermodynamically stable for the energy confinement model assumed (a multiple of ITER89P). The FWCD and LHCD sources provide attractive control possibilities, not only for the current profile, but also for the total fusion power since the gain on the incremental auxiliary power is typically 10–30 in these calculations when overall Q ≍ 30.

Hamiltonian analysis of fast wave current drive in tokamak plasmas

The Hamiltonian formalism is used to address the problem of the direct resonant interaction between the fast magnetosonic wave and the electrons in a tokamak plasma. The intrinsic stochasticity of the electron trajectories in phase space is first derived. Together with extrinsic decorrelation processes, it assesses the validity of the quasilinear approximation for the kinetic studies of fast wave current drive (FWCD). A full-wave solution of the Maxwell–Vlasov set of equations provides the exact pattern of the wave fields in the tokamak geometry, consistent with a realistic antenna spectrum. The local quasilinear diffusion tensor is then derived from the wave fields and the driven current density profile, the power deposition profile and the current drive efficiency are computed, including possible nonlinear effects in the kinetic equation. Several applications of FWCD on existing and future machines are given, and the combination of FWCD with other noninductive current drive methods is investigated. Finally, an analytical expression for the current drive efficiency is derived in the moderate to high single-pass absorption regime.

Current drive with fast waves, electron cyclotron waves, and neutral injection in the DIII-D tokamak

Current drive experiments have been performed on the DIII-D tokamak using fast waves, electron cyclotron waves, and neutral injection. Fast wave experiments were performed using a 4-strap antenna with 1 MW of power at 60 MHz. These experiments showed effective heating of electrons, with a global heating efficiency equivalent to that of neutral injection even when the single pass damping was calculated to be as small as 5%. The damping was probably due to the effect of multiple passes of the wave through the plasma. Fast wave current drive experiments were performed with a toroidally directional phasing of the antenna straps. Currents driven by fast wave current drive (FWCD) in the direction of the main plasma current of up to 100 kA were found, not including a calculated 40 kA of bootstrap current. Experiments with FWCD in the counter current direction showed little current drive. In both cases, changes in the sawtooth behavior and the internal inductance qualitatively support the measurement of FWCD. Experiments on electron cyclotron current drive have shown that 100 kA of current can be driven by 1 MW of power at 60 GHz. Calculation with a Fokker-Planck code show that electron cyclotron current drive (ECCD) can be well predicted when the effects of electron trapping and of the residual electric field are included.

X-Ray Energy Analysis for Radio Frequency Current Drive Experiments in the DIII-D Tokamak

The X-ray energy spectra were measured during electron cyclotron current drive (ECCD) and fast wave current drive (FWCD) experiments in the DIII-D tokamak. High energy electrons, over 100 keV, were observed during ECCD near the plasma center and enhanced radially by the effect of downshifted frequency. Tail formation with the maximum energy of 50 keV was observed clearly during 2 T, 0.8 MA FWCD experiments. For combined FWCD and ECCD, the bulk heating efficiency was increased over the case with FW only, with ηe∼2.3×1019 m-3 keV/MW, and it was stronger for the counter (CTR) case than the co (Co) case. (Here Co corresponds to the direction of launched wave which will enhance the plasma current. CTR refers to the opposite direction.) However, the coupling of FWs to high energy electrons produced by ECH was stronger for the Co case.

Analysis of the coupling properties of the toroidal antenna array in JFT-2M by a code considering the solid septa

The coupling code based on the variational method has been extended so that it can be applied to a toroidal antenna array used in fast wave current drive (FWCD) experiments. The self-consistent poloidal current profile on the current strap can be obtained with the variational method. The current straps are separated by solid septa in order to suppress the mutual coupling between adjacent antennas. The coupling properties of the loop antenna array for FWCD experiments on JFT-2M are in agreement with calculations. One problem is that the high toroidal refractive index spectrum (Nz) excited by the loop antenna array couples to a low density plasma. However, the steep electron density gradient near the plasma edge improves the coupling of the high Nz spectrum

Conceptual Design of a Fast Wave Current Drive (FWCD) Coupler for the PLT

This paper describes the conceptual design of a Fast Wave Current Drive (FWCD) alumina ceramic loaded coupler for the Princeton Large Torus (PLT). An antenna capable of launching RF at 800 MHz was required. While phased waveguide arrays allow the coupling of correctly phased waves to the plasma, their large size (when used in air or vacuum) would preclude their application for PLT. However, the utilization of a dielectric loaded waveguide results in a reduction in size of waveguide elements by the square root of the dielectric constant1,3. A description of the various approaches considered during the RF design, mechanical design, and component fabrication studies is included.