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For the effect of the collisional dissipation of fast electrons driven by the lower-hybrid waves, a predictive simulation is made for the HT-7 plasma. The simulation results show that the dissipation of fast electrons counteracts the effect of radial diffusion to some extent, thereby making the lower-hybrid driven current profile closer to the power deposition profile. So, in the case of an off-axis lower-hybrid wave power launching, the dissipation is helpful in maintaining a center-hollowed current profile in lower hybrid current drive (LHCD) plasmas, and thus possibly maintains the desired reversed magnetic shear.

A simulation on the propagation of lower hybrid wave in EAST is conducted. Ray tracing, evolution of the parallel refractive index, and power deposition profiles in a D-shaped configuration were calculated. Results show that wave propagation is affected by the plasma current, density, magnetic field, and power spectrum, etc. leading to a different power deposition. This is helpful in understanding the LHCD experiments on EAST.

Calculations of the TE10 tapered waveguide, which will be used in the 4.6 GHz lower hybrid current drive (LHCD) launcher on the EAST, are presented. The calculations are performed in two ways, namely by directly solving the classical coupling wave equations, and by using the simulation code of the high frequency structure simulator (HFSS). According to the calculations, the voltage standing wave ratio (VSWR) of the designed tapered waveguide can be less than 1.01 within a bandwidth of ±100 MHz.

In lower hybrid current drive (LHCD) experiments in HT-7, LH wave coupled to the plasma from 12 klystrons is launched by a phased multi-junction grill. High power microwave reflection or arc, which threatens the klystrons, the wave-guide ceramic windows and the vacuum of the tokamak, occur sometimes during the experiments. For the sake of safety, a high power microwave test system to perform simulations was developed. In the test experiments, the klystron and window can be well protected with an optimized microwave monitor and a PIN switch. The layout of the test system and the simulated experiments for HT-7 are described in this paper.

In order to test the klystrons operated at a frequency of 3.7 GHz in a continuous wave (CW) mode, a type of water load to absorb its power up to 750 kW is presented. The distilled water sealed with an RF ceramic window is used as the absorbent. At a frequency range of 70 MHz, the VSWR (Voltage Standing Wave Ratio) is below 1.2, and the rise in temperature of water is about 30 oC at the highest power level.

A phase-controlled lower hybrid wave (LHW) multi-junction (MJ) coupler (3(rows)×4(columns)×4 (subwaveguides)) has been developed in the HT-7 tokamak. Simulations show that it is more effective for driving plasma current than an ordinary phase-controlled LHW antenna (3(rows)× 12(columns)) (traditional coupler). The plasma–wave coupling experiments show that the reflection coefficient (RC) is below 10%, implying that the MJ grill can launch the wave into the plasma effectively. The effect of power spectrum launched by the MJ coupler on RC indicates that an optimal condition is requisite for a better coupling in the lower hybrid current drive (LHCD) experiments. Studies indicate that the drive efficiency of the MJ antenna is higher than that of the traditional one, which is mainly ascribed to the discrepancy in impurity concentration, plasma temperature, and spectrum directivity. An improved confinement with an electron internal transport barrier is obtained by LHCD. The analysis shows that the modified negative (low) magnetic shear and the change of radial electric field profile due to LHCD are possible factors responsible for the eITB formation.

Preliminary simulation results obtained with the code developed for ion Bernstein wave (IBW) heating in the HT-7 tokamak are presented. Comparison of the simulation of IBW heating and an HT-7 experiment confirms that using IBW of various frequencies can result in local or global plasma heating. The studies suggest that IBW absorption by ions near the ion cyclotron resonant layer and by electrons via electron Landau damping (ELD) around the maximum of n// offers a possible mechanism of plasma heating.

It is well accepted that lower hybrid current drive (LHCD) is the most efficient method for non-inductive current drive in fusion devices and the effect of the current drive is dependent on not only microwave power but also its grill phase shift. This paper presents a new kind of feedback control system for antenna phase difference in LHCD experiments. In this high-speed control system, a lot of new technologies and methods are incorporated. The results of the experiments show a very good agreement with the system design.

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.

Based on the electron's radial force equilibrium, the profiles of radial electric field in OH and LHCD phase are calculated by using a simulation code. The dependences of radial electron field on electron density and its profile and different current ratio, Irf/Ip, are given. The connections between the improvement of plasma confinement and the modified radial electric field by LHCD are discussed by comparing the calculated results with the experimental results.

The radiation constant of the plasma and the temperature distribution at the Lower Hybrid Wave (LHW) antenna on the superconducting tokamak HT-7 are calculated from the engineering point of view and the result provides an important reference to the design of a new antenna.

Effective Lower Hybrid Current Driving (LHCD) and improved confinement experiments in higher plasma parameters (Ip > 200 kA, ne > 2 × 1013 cm-3, Te ≥ 1 keV) have been curried out in optimized LH wave spectrum and plasma parameters in HT-7 superconducting tokamak. The dependence of current driving efficiency on LH power spectrum, plasma density ne and toroidal magnetic field BT has been obtained under optimal conditions. A good CD efficiency was obtained at higher plasma current and higher electron density. The improvement of the energy confinement time is accompanied with the increase in line averaged electron density, and in ion and electron temperatures. The highest current driving efficiency reached ηCD = IpeR/PRF ≈ 1.05 × 1019 Am-2/W. Wave-plasma coupling was sustained in a good state and the reflective coefficient was less than 5%. The experiments have also demonstrated the ability of LH wave in the start-up and ramp-up of the plasma current. The measurement of the temporal distribution of plasma parameter shows that lower hybrid leads to a broader profile in plasma parameter. The LH power deposition profile and the plasma current density profile were modeled with a 2D Fokker-Planck code corresponding to the evolution process of the hard x-ray detector array.

In this paper, the composition of HT-7 LHCD system along with its protection systems is introduced. It is very important for us to run the LHCD system safely and efficiently.

Low voltage start-up was realized in HT-7 discharges under the assistance of lower hybrid (LH) waves. The use of a strong LH wave, 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.

The ICRF (Ion Cyclotron Range Frequency) cleaning technique has been used as a routine wall cleaning method in the HT-7 superconducting tokamak. In a wide range of toroidal field, the removal rate of residual gas by ICRF cleaning was about twenty times higher than that of glow discharge cleaning (GDC). At different gas pressure and RF power levels, the ICRF cleaning is studied carefully. A good impurity cleaning effect and a very high hydrogen removal rate were obtained. The removal rate of hydrogen by 5 kW ICRF cleaning achieved was 1.6 × 10-5 Torr.l/s. And the relationships among pressure P, outgassing rate Q, atomic layers L absorbed on surface and the cleaning mode were discussed briefly.

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 now, 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.

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 charge-exchange 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 eVkW-1 was obtained.

In this paper, the technology of the phase adjustment of a lower hybrid wave on HT-7 superconducting tokamak is described and the calculating method involved in the technology is introduced. The design method of hardware and software is also presented. In addition, a series of experimental results are reported.

This paper analyzes the eddy currents and the electro-magnetic forces on the lower hybrid wave (LHW) launching antenna on the superconducting Tokamak HT-7 by using a finite element circuit 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.

It has been found that lower hybrid waves excite various sawtooth oscillations during the HT-7 tokamak discharges. In this paper we present the results of lower hybrid current drive (LHCD) on sawtooth activity in a series of experiments with 2.45 GHz, 1 MW LH system on HT-7 superconducting tokamak. In these experiments, usually LH power is between 200 similar 400 kW, 2 ≤ N// p ≤ 3 (ΔN// = 0.8), and rf pulse in 500 ms. Sometimes, the sawtooth oscillation is accompanied by a wide range of magnetohydrodynamic activity in the plasma central region, normally with a m = 1/n =1 mode. A LHCD partial suppression of sawtooth, a lengthening sawtooth and a saturation of sawtooth oscillations are also observed during our experiments.

Fundamental experiments on lower hybrid current drive (LHCD) have been undertaken on HT-7 superconducting tokamak. The experiments on LHCD efficiency reveal its dependence 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 of 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.

The behavior of magnetohydrodynamic phenomena associated with disruptions of ohmically heated density limit discharges (qa = 3.9) in the TEXTOR tokamak has been investigated in detail. A minor disruption is demonstrated to be only due to an m = 2 mode instability, possibly due to the magnetic reconnection of an m = 2 island. When the m = 2 mode oscillations cease, the internal m = 1 kink mode instability is drastically enhanced at once, twisting the plasma core considerably. If the twisting is too strong, the twisted plasma core is inevitably displaced to the outside and brings cold plasma to the central region, leading to the energy quench at a major disruption.

High time resolution measurements of the electrostatic fluctuations, radial electric field Er and turbulence-induced electron flux Γe have been performed across the transition of lower hybrid current drive improved confinement with a graphite Langmuir probe array at the last closed flux surface of the HT-7 tokamak. The decrease of Γe is dominated by the suppression of fluctuation levels, which follows the change of Er. A reversal of the poloidal propagation direction of turbulence demonstrates that the poloidal propagation is dominated by Er×B drift. The enhancement of poloidal coherence accompanies the fluctuation suppression, which suggests the subtle variation of turbulence features.

Lower hybrid waves (LHWs) with a selected n|| spectrum have been used to control the energy deposition profiles, and then the wave driven current profiles effectively in tokamak discharges. In our lower hybrid current drive experiment in the HT-7 tokamak, it was found that the set-up of the wave energy deposition profile is a graduation process. In the beginning phase of the wave injection duration, the waves (with different n|| spectra) deposit almost all their energy in the central region of the plasma column, even if their n|| are very different. Up to around one hundred milliseconds, the wave energy deposition profiles can only take their corresponding shapes according to the n|| spectra of LHWs. It also shown that this evolution process is affected obviously by the LHW driven current profile, which has been formed early.

Lower hybrid current drive experiments on the HT-7 device have been carried out by scanning the following parameters: central line averaged electron density (ne = 0.6 - 2.0 × 1019 m-3) and toroidal magnetic field (Bt = 1.62 - 2.0 T). The dependence of current drive efficiency on these parameters has been studied and the experimental curves of current drive efficiency as a function of ne and Bt have also been obtained. From these experimental results, it can be seen that current drive efficiency rises with the increase of toroidal magnetic field. As plasma density increases, the current drive efficiency first increases to a certain value, then gradually decreases, that is, there exists an optimized density regime where a better drive efficiency can be obtained. The analysis shows that the current drive efficiency is mainly affected by wave accessibility and impurity concentration, and the competition of these two factors determines the current drive efficiency.

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.

A compact rectangular TE10–TE30 mode converter is developed for the lower hybrid current drive (LHCD) launcher on the Experimental Advanced Superconducting Tokamak (EAST) at 4.6 GHz. The converter with periodic width perturbation aims to divide the microwave power into three sub-waveguides in the poloidal direction. We present the design and numerical calculation of the mode converter. Calculations are performed on the ripple wall converter by codes based on numerical solving the coupled-mode differential equations and on the simulation of the High Frequency Structure Simulator (HFSS) package. The resulting conversion efficiency from TE10 mode to TE30 mode exceeds 95% within the bandwidth from 4.56 GHz to 4.64 GHz, and the return loss of the oversized transducer can be considerably decreased to 0.068% by means of a capacitive button embedded in the E-plane of the waveguide.

In ion cyclotron resonance heating,a 2-dimension antenna model is used for the c avity loop antenna with comparatively complex bounding conditions,neglecting the influence of loop antenna parallel component for simplicity.The detailed calculation process of a 3-dimension antenna model is given in this paper.This model i s closer to the practical antenna,its calculation results can be used for the re ference of antenna design.

The mechanism of fast wave minority ion heating is discussed in some detail. The ray tracing algorithm is used to simulate fast wave heating for the plasma of two kinds of ions, and the calculation results of different ratios of two ions' density is obtai

The standard normal distribution for particles in tokamak geometry is usually assumed in fast wave heating.In fact,due to the quasi-linear diffusion effect,the parallel and vertical temperatures of the resonant particles are not equal to each other,so,this will bring some error.For this case,the Fokker-Planck equation is introduced in this paper,and iteration algorithm is adopted to solve the problem well.

The properties of the N41-1D oscillations (Mirnov Oscillations) in ohmic TEXTOR tokamak plasmas are systematically described in this paper based on experimental data, including mainly the principal modes of the MHD oscillations and the correlaion among the oscillations, the propagating direction of the MHD oscillations, as well as the correlative properties among the oscillative temperature, density and density fluctuation of the m=2/n=1 mode.