Design and Simulation for the Pulse High-Voltage DC Power Supply (HVPS) of 1.2 MW/2.45 GHz HT-7U Lower Hybrid Current Drive System

The superconducting tokamak HT-7U [1] has been designed by the Institute of Plasma Physics since 1998 and will be set up before 2003. The 1.2 MW /2.45 GHz HT-7U LHCD (Lower hybrid current drive) system which being the most efficient non-induction device can heat the plasma and drive the plasma current has been efficiently in operation 'owl and a particular design of the 2.8 MW/-35 kV high-voltage DC power supply has been already completed and will apply to the klystron of LHCD on HT-7 and the future HT-7U, and the project of the power supply has been examined and approved professionally by an authorized group of high-level specialist in the institute of Plasma Physics. The detailed design of the power supply and the simulation results are referred in the paper.

Simulation on effect of poloidal power spectrum on lower hybrid wave propagation

The coupling of lower hybrid wave to the plasma is a crucial issue for efficient current drive in tokamaks. This paper establishes a new coupling model which assumes the antenna to be a curved face and the plasma to be a cylinder. Power spectrum considering the coupling between wave-guides in both poloidal and toroidal direction is simply estimated and discussed. The effect of the poloidal wave vector on wave propagation, power deposition and driven current is also investigated with the help of lower hybrid current drive code. Results show that the poloidal wave vector affects the ray tracing, and also has effect on power deposition and driven current. The effect of the poloidal wave vector on power deposition and driven current profile depends on plasma parameters. Preliminary studies suggest that it seems possible to control the current profile by adjusting the poloidal phase difference between the waveguide in poloidal direction.

Optimized calculation of the synergy conditions between electron cyclotron current drive and lower hybrid current drive on EAST

The optimized synergy conditions between electron cyclotron current drive （ECCD） and lower hybrid current drive （LHCD） with normal parameters of the EAST tokamak are studied by using the C3PO/LUKE code based on the under- standing of the synergy mechanisms so as to obtain a higher synergistic current and provide theoretical reference for the synergistic effect in the EAST experiment. The dependences of the synergistic effect on the parameters of two waves （lower hybrid wave （LHW） and electron cyclotron wave （ECW））, including the radial position of the power deposition, the power value of the LH and EC waves, and the parallel refractive indices of the LHW （Nr） are oresented and discussed.

Experimental Research on Lower Hybrid Current Drive in Superconducting Tokamak HT-7

?Fundamental experiments on lower hybrid current drive (LHCD) have been undertaken on HT-7 superconducting tokamak. The experiments on LHCD efficiency reveal its depen- deuce on plasma density and the toroidal magnetic field. Furthermore, the experiments on HT-7 successfully demonstrate the ability for LHCD to sustain long pulse tokamak discharges, such as discharges with full non-inductive current drive for several seconds. The experimental study to improve plasma confinements by LHCD suggests that the improvement should be due to the change o f current profile. It has also been demonstrated by the experiments that the lower hybrid wave may lead to an enhanced ionization of particles in the region where the wave is deposited.

Effect of Wave Accessibility on Lower Hybrid Wave Current Drive in Experimental Advanced Superconductor Tokamak with H-Mode Operation

The effect of the wave accessibility condition on the lower hybrid cm＇rent drive in the experimental advanced superconductor Tokamak （EAST） plasma with H-mode operation is studied. Based on a simplified model, a mode conversion layer of the lower hybrid wave between the fast wave branch and the slow wave branch is proved to exist in the plasma periphery for typical EAST H-mode parameters. Under the framework of the lower hybrid wave simulation code （LSC）, the wave ray trajectory and the associated current drive are calculated numerically. The results show that the wave accessibility condition plays an important role on the lower hybrid current drive in EAST plasma. For wave rays with parallel refractive index n｜｜= 2.1 or n｜｜ = 2.5 launched from the outside midplane, the wave rays may penetrate the core plasma due to the toroida] geometry effect, while numerous reflections of the wave ray trajectories in the plasma periphery occur. However, low current drive efficiency is obtained. Meanwhile, the wave accessibility condition is improved if a higher confined magnetic field is applied. The simulation results show that for plasma parameters under present EAST H-mode operation, a significant lower hybrid wave current drive could be obtained for the wave spectrum with peak value n｜｜ = 2.1 if a toroidal magnetic field BT =2.5 T is applied.

Poloidal rotation induced by injecting lower hybrid waves in tokamak plasma edge

The poloidal rotation of the magnetized edge plasma in tokamak driven by the ponderomotive force which is generated by injecting lower hybrid wave(LHW) electric field has been studied. The LHW is launched from a waveguide in the plasma edge, and by Brambilla's grill theory, analytic expressions for the wave electric field in the slab model of an inhomogeneous cold plasma have been derived. It is shown that a strong wave electric field will be generated in the plasma edge by injecting LH wave of the power in MW magnitude, and this electric field will induce a poloidal rotation with a sheared poloidal velocity.

Study of Bridging of the Spectral Gap in the Lower Hybrid Wave Current Drive in the HT-7 Tokamak

An additional lower hybrid wave （LHW） with a higher refractive index （N//） was investigated in the HT-7 tokamak to bridge the spectral gap. It was found that the spectral gap between the wave and the electrons in the outer region was bridged by the additional wave with a higher N// spectrum. The results showed that the sawteeth oscillation was suppressed by launching the additional wave, and that the power deposition profile was moved outwards and the current profile was broadened due to the application of the additional wave. Our study indicates that the spectral gap may be bridged by an additional wave with a higher N// spectrum in the outer region.

Investigation of lower hybrid current drive during H-mode in EAST tokamak

H-mode discharges with lower hybrid current drive （LHCD） alone are achieved in EAST divertor plasma over a wide parameter range. These H-mode discharges are characterized by a sudden drop in Da emission and a spontaneous rise in main plasma density. Good lower hybrid （LH） coupling during H-mode is obtained by putting the plasma close to the antenna and by injecting D2 gas from a pipe near the grill mouse. The analysis of lower hybrid current drive properties shows that the LH deposition profile shifts off axis during H-mode, and current drive （CD） efficiency decreases due to the increase in density. Modeling results of H-mode discharges with a general ray trueing code GENRAY are reported.

Benchmark Simulations of Gyro-Kinetic Electron and Fully-Kinetic Ion Model for Lower Hybrid Waves in Linear Region

Particle-in-cell （PIC） simulation method has been proved to be a good candidate to study the interactions between plasmas and radio-frequency waves. However, for waves in the lower hybrid range of frequencies, a full PIC simulation is not efficient due to its high computational cost. In this work, a gyro-kinetic electron and fully-kinetic ion （GeFi） particle simulation model is applied to study the propagations and mode conversion processes of lower hybrid waves （LHWs） in plasmas. With this method, the computational efficiency of LHW simulations is greatly increased by using a larger grid size and time step. The simulation results in the linear regime are validated by comparison with the linear theory.

Impact of Toroidal Effect on Lower Hybrid Current Drive in Tokamak

The main topics concerning lower hybrid wave heating （LHH） and lower hybrid current drive （LHCD） in tokamak systems are presented. The inherent properties of tokamak systems give the ‘gap＇ filling on Brambilla＇s spectrum, which are conducive to LHCD, but, on the other hand, induce a consumption of wave energy by the trapped electrons, which reduce the current drive efficiency. The methods for the enhancement of the current drive efficiency may be derived from detailed analyses by drawing upon the ray tracing technology on toroidal geometry and the Fokker-Planck theory on velocity space.

LHCD Properties with a New Lower Hybrid Wave Antenna on HT-7 Tokamak

The structure and the characteristic power spectrum of a new lower hybrid wave antenna on the HT-7 tokamak are briefly described in this paper. The main experimental properties with the new antenna have been demonstrated by showing the dependence of current drive efficiency and particle confinement time on phase difference between adjacent waveguides of the antenna. A few preliminary analyses about the experimental results are also discussed in the paper.

Relativistic effect on synergy of electron cyclotron and lower hybrid waves on EAST

In recent experiments on EAST,the electron temperature at the center can be raised to 9.7 ke V by injecting electron cyclotron(EC)and lower hybrid(LH)waves simultaneously.With such strong core electron heating,the relativistic effect could play an important role in the interactions between the plasma and waves.In order to explore the relativistic effect on synergy between the EC and LH waves on EAST,ray-tracing/Fokker-Planck simulations are conducted to investigate electron heating for a typical discharge with a center electron temperature of 9.7 ke V.It is found that the relativistic effect can cause the EC wave to deposit its power deeper in the plasma core,where the synergy between the EC and LH waves occurs and enhances the absorption of the LH waves.As a result,a high center electron temperature can be achieved.

Parametric decay instabilities of lower hybrid waves on CFETR

The lower hybrid current drive is a potential candidate for sustaining plasma current in tokamak steady-state oper-ations,which could be used in China Fusion Engineering Test Reactor(CFETR)with input power up to a few tens of megawatts.Such high input power could trigger the well-known parametric instabilities(PIs)at the plasma edge affect-ing the propagation and absorption of the lower hybrid pump waves.By analytically solving the nonlinear dispersion relation describing PIs,an explicit expression of the PI growth rate is obtained and analyzed in detail.It is found that pressure is the key parameter determining the PI characteristics.Ion sound quasi-mode is the dominant decay channel in the low-pressure regime,while the ion cyclotron quasi-mode(ICQM),as well as its harmonics,becomes dominant in the intermediate regime.In the high-pressure regime,only one mixed channel is found,which is related to Landau damping by free-streaming ions.Analytical expressions of growth rates of these decay channels are also obtained to show the parameter dependence at different pressure limits.The above analytical results are used to estimate the PIs on a typical profile of CFETR,and verified by corresponding numerical calculations.ICQM is found to be the strongest decay channel with a considerable growth rate for CFETR.

Plasma–wave interaction in helicon plasmas near the lower hybrid frequency

We study the characteristics of plasma–wave interaction in helicon plasmas near the lower hybrid frequency.The(0D)dispersion relation is derived to analyze the properties of the wave propagation and a 1D cylindrical plasma–wave interaction model is established to investigate the power deposition and to implement the parametric analysis.It is concluded that the lower hybrid resonance is the main mechanism of the power deposition in helicon plasmas when the RF frequency is near the lower hybrid frequency and the power deposition mainly concentrates on a very thin layer near the boundary.Therefore,it causes that the plasma resistance has a large local peak near the lower hybrid frequency and the variation of the plasma density and the parallel wavenumber lead to the frequency shifting of the local peaks.It is found that the magnetic field is still proportional to the plasma density for the local maximum plasma resistance and the slope changes due to the transition.

Effects of trapped electrons on off-axis lower hybrid current drive in tokamaks

The effects of trapped electrons on off-axis lower hybrid current drive （LHCD） in tokamaks are studied, A computer code for solving the Fokker-Planck equation in a toroidal geometry is developed and employed. The code is suitable for various auxiliary heating and current drive schemes in tokamak plasmas. The influence of the resonance regime on the current drive efficiency as well as the influence of trapped particle fraction on the current drive efficiency are emphasized. It is shown that, as an electrostatic force, the lower hybrid wave causes some of the trapped electrons to be untrapped and lose their energy, which can cut the LHCD efficiency by about 30%. The ITER scaling law is also used to estimate the trapped electron effects.

A Study of Energy Conversion Efficiency Versus Plasma Density by Lower Hybrid Current Drive in HT-7 Tokamak

Ramp-up experiments by means of lower hybrid wave on HT-7 superconducting tokamak have been performed and analyzed. A ramp-up rate of over 300 kA/s is obtained and a conversion efficiency of over 1.0% has been achieved during the ramp-up phase. The study of the dependence of conversion efficiency on plasma density shows that the conversion efficiency is affected by the driven current, which is mainly dominated by the competition of impurity concentration with wave accessibility condition. In addition, the effect of current profile may play an important role in determining the conversion efficiency.

Analysis of Charge-Exchange Spectrum during Lower Hybrid Current Drive on HT-7 Tokamak

A perpendicular neutral particle analyzer (NPA) is used on HT-7 tokamak experiment to provide the measurements of neutral particle flux and ion temperature. The measured results were compared with calculated data by Monte-carlo method for ohmic heating. In lower hybrid current drive (LHCD) experiments, the ion heating of LHCD was identified. The chargeexchange neutral particle flux enhanced was observed. The energy spectrum clearly showed a high-energy ions tail. The bulk ion temperature increased by 0.4-0.5 keV after the onset of LHCD. Efficient ion heating of 1 eV. kW-1 was obtained.

The Realization of Low Loop Voltage Start-up of HT-7 Tokamak Discharge with the Assistance of Lower Hybrid Wave

Low voltage start-up was realized in HT-7 discharges under the assistance of lower hybrid (LH) waves. The use of a strong LH wavet which can Change its N// spectrum in a large range of several milliseconds, made the loop voltage for start-up reduce from around 20 V to less than 5 V. It means that the electric field for HT-7 start-up decreases from 2.5-3 V/m to 0.6 V/m. Some physical phenomena such as the consumption of magnetic flux in start-up phase and the radiation from the initial plasma were observed in this kind of low voltage start-up discharges.

Eddy Currents and Electro-magnetic Forces on the Lower Hybrid Ware Launching Antenna on the Superconducting Tokamak HT-7

This paper analyzes the eddy currents and the electro-magnetic forces on the lower hybrid wave (LHW) launching antenna on the superconducting Tohamak HT-7 by using a finite element circult method. A new iterative algorithm is developed to analyze the coupled magnetic fields Which are very difficult to be calculated. The method and results obtained are helpful to study the eddy currents and electro-magnetic forces on metal plates which are placed in a rather complicated electro-magnetic environment.

Toroidal rotation induced by 4.6 GHz lower hybrid current drive on EAST tokamak

Using a tangentially viewing x-ray imaging crystal spectrometer, substantial co-current rotation driven by lower hybrid current drive（LHCD） at 4.6 GHz is observed on EAST tokamak. This study presents plasma rotation behaviors with 4.6 GHz LHCD injection. Typically, the 10-20 km/s co-current rotation change and the transport of rotation velocity from edge to core are observed. The relationship between plasma parameters and rotation is also investigated, indicating that rotation decreases with increasing internal inductance（li） and increases with increasing safety factor（q0）. Hysteresis between rotation and Te plasma stored energy is observed, suggesting different response times between the electron heating and rotation acceleration by LHCD. A comparison between the rotations driven by 4.6 G LHCD and 2.45 G LHCD on EAST is also presented, in which higher frequency LHCD could induce more rotation changes.