Establishment of Configuration with Optimized Current Profile in Pellet Fuelled Discharge on HL-2A

A sawtooth-free period is produced following the pellet injection in the pellet fuelled discharge of qa = 3.4 （where qa is the safety factor at the plasma boundary） in the HL-2A tokamak. Establishment of the current profile such as in the hybrid scenario is studied under the condition of pellet injection in HL-2A. It is shown that a q-profile of weak negative shear is produced immediately after the pellet injection, and it then evolves to a broad flat profile with q0 〉 1 （where qo is the safety factor at the centre）. The measured MHD mode structures evidence consistencies of the evaluated q-profile with locations of the q = 1 surface in the sawtoothing period and of the q = 2 surface in the sawtooth-free period. TRANSP analysis indicates that the energy confinement is enhanced substantially during and after pellet injection, which is resulted from the q-profile optimization and the plasma density peaking.     

Operation of a tokamak reactor in the radiative improved mode

The operation of a nuclear fusion reactor has been simulated within a model based on experimental results obtained at the TEXTOR-94 tokamak and other facilities in which quasistationary regimes were achieved with long confinement times, high densities, and absence of the edge-localized mode. The radiative improved mode of confinement studied in detail at the TEXTOR-94 tokamak is the most interesting such regime. One of the most important problems of modern tokamaks is the problem of a very high thermal load on a divertor (or a limiter). This problem is quite easily solved in the radiative improved mode. Since a significant fraction of the thermal energy is reemitted by an impurity, the thermal loading is significantly reduced. As the energy confinement time τ_E at high densities in the indicated mode is significantly larger than the time predicted by the scaling of ITERH-98P(y, 2), ignition can be achieved in a facility much smaller than the ITER facility at plasma temperatures below 20 keV. The revealed decrease in the degradation of the confinement time τ_E with an increase in the introduced power has been analyzed.     

Analysis of tokamak plasma confinement modes using the fast Fourier transformation

The Fourier analysis is a satisfactory technique for detecting plasma confinement modes in tokamaks. The confinement mode of tokamak plasma was analysed using the fast Fourier transformation (FFT). For this purpose, we used the data of Mirnov coils that is one of the identifying tools in the IR-T1 tokamak, with and without external field (electric biasing), and then compared it with each other. After the Fourier analysis of Mirnov coil data, the diagram of power spectrum density was depicted in different angles of Mirnov coils in the ‘presence of external field’ as well as in the ‘absence of external field’. The power spectrum density (PSD) interprets the manner of power distribution of a signal with frequency. In this article, the number of plasma modes and the safety factor q were obtained by using the mode number of q = m /n (m is the mode number). The maximum MHD activity was obtained in 30–35 kHz frequency, using the density of the energy spectrum. In addition, the number of different modes across 0–35 ms time was compared with each other in the presence and absence of the external field.     

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 D_α 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 tracing code GENRAY are reported.     

Study of density fluctuation during low hybrid current drive on HT-7 tokamak

An experimental study of medium and high-k density fluctuations has been performed during low hybrid current drive (LHCD) in the HT-7 tokamak. Two series of plasma discharges in different plasma current with a limiter configuration were conducted in which the injected LHW power was gradually increased. The fluctuation level of the mode in channel kθ=12 cm−1 and kθ=20 cm−1 is related to the q profile, and the normalized frequency spectrum is found to be independent of the LHW power.     

Inductance of rf-wave-heated plasmas

The inductance of rf-wave-heated plasmas is derived. This inductance represents the inductance of fast electrons located in a plateau during their acceleration due to electric field or deceleration due to collisions and electric field. This inductance has been calculated for small electric fields from the two-dimensional Fokker-Planck equation as the flux crossing the surface of critical energy mv(2)(ph)/2 in the velocity space. The new expression may be important for radio-frequency current drive ramp-up, current drive efficiency, current profile control, and so on in tokamaks. This inductance may be incorporated into transport codes that study plasma heating by rf waves.     

Physics of spherical tokamaks

Spherical tokamaks are a limiting case of conventional tokamaks, combining simple design with attractive physical characteristics. Being of potential importance for the controlled nuclear fusion program in their own right, spherical tokamaks also contribute much to our understanding of the physics of conventional tokamaks. For instance, they are contributing to the modeling of confinement scalings for ITER and to research on general plasma properties such as energy confinement and MHD processes. Results obtained for the first generation of spherical prototype tokamaks are reviewed and the main trends for further research are indicated. Zh. Tekh. Fiz. 69, 50–57 (September 1999)     

托卡马克等离子体电流爬升阶段逃逸电子行为

 分析了电流爬升阶段等离子体密度和电流爬升率对逃逸电子行为的影响,研究了低杂波辅助电流驱动条件下的逃逸电子辐射行为。结果发现：电流爬升阶段等离子体密度的大小严重影响了电流爬升阶段甚至电流平顶阶段逃逸电子的行为,较低的等离子体密度将会导致放电过程中比较强的逃逸电子辐射;低能逃逸电子辐射随着电流爬升率的增大而增强;低杂波辅助电流爬升可以有效地节约装置的伏秒数;降低放电过程中的环电压,可有效抑制逃逸电子的产生。     

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通过中平面活动10探针组、往复快速扫描4探针和低杂波天线口的固定4探针测量了主等离子体边缘的温度、密度、悬浮电位、径向和极向电场、雷诺协强、径向和极向等离子体流速及其径向分布。用偏滤器靶板上的14组嵌入式静电3探针阵列测量了同一环向截面的内外中性化板上的电子温度、密度、悬浮电位及其分布。比较了在孔栏位形和偏滤器位形下边缘等离子体特性的差异,特别是两种位形下边缘温度和密度衰减长度的变化。分析了在多脉冲超声分子束加料和低杂波注入条件下的边界等离子体特性,以及雷诺协强的径向梯度与极向流和径向电场梯度与湍流损失的相互关系。     

Measurements of Flow Velocity in the Plasma Generator PSI‐2

The plasma flow velocity in the Plasma Generator PSI‐2 has been investigated by using of Mach probe. PSI‐2 is a stationary high‐current arc discharge in which the quasi‐neutral plasma expands along the magnetic field lines. The low‐temperature (T_e < 20 eV), medium density (n_e ∼ 10¹⁸— 10¹⁹ m^—3 ) plasma in the discharge is similar to the plasma in the divertor region of tokamaks. From the ratio of ion saturation currents collected from opposite sides of the probe the flow velocities (Mach numbers) in argon and hydrogen discharges are obtained.     

低杂波控制电流分布与改善约束研究

在HT-7超导托卡马克装置上利用低杂波电流驱动有效地控制了等离子体电流分布,并使等离子体约束性能改善。数值模拟与硬X射线测量结果均表明,低杂波的发射功率谱、纵场和等离子体密度对改变等离子体电流分布有明显的影响。在优化低杂波电流驱动实验参数的条件下,等离子体密度、温度分布发生了理想的变化。在电子和离子温度分布上出现了内部输运垒,同时等离子体的能量约束时间和粒子约束时间均有提高。     

Tomographic reconstruction of two-dimensional radiated power distribution during impurity injection in KSTAR plasmas using an infrared imaging video bolometer

Based on the two-dimensional radiation images obtained by an infrared imaging video bolometer (IRVB) with tangential view, the two dimensional radiation profiles of plasmas in KSTAR were reconstructed. The IRVB installed on KSTAR has a tangential view of the plasma, and thus tomographic reconstruction of the raw images of radiation profiles was performed to remove the chord-integration effect by using a tomographic reconstruction code based on the Phillips-Tikhonov algorithm. Phantom reconstruction tests with various synthetic images were carried out to validate the accuracy of the reconstruction results. It is found that hollow radiation phantoms with strong divertor radiation were reconstructed with high accuracy. Furthermore, the effects of the number of channels of the IR camera, and the number of pixels of the plasma and of the IRVB on reconstruction performance are studied with phantom tests. Two-dimensionally reconstructed images of KSTAR plasmas demonstrated that radiation loss at the plasma edge and near the divertor region increased significantly after gaseous impurity injection. The total radiated power was up to 1.2 MW at the disruption, which was 40% of the NBI power. After argon and krypton gas injection, total radiated power was increased by 325 kW and 180 kW, respectively.     

Numerical analysis of high Mach flow and flow reversal in the experimental advanced superconducting tokamak divertor

The B2-Eirene (SOLPS 4.0) code package is used to investigate the plasma parallel flow, i.e., the scrape-off layer (SOL) flow, in the experimental advanced superconducting tokamak (EAST) divertor. Simulation results show that the SOL flow in the divertor region can exhibit complex behaviour, such as a high Mach flow and flow reversal in different plasma regimes. When the divertor plasma is in the detachment state, the high Mach flow with approaching or exceeding sonic speed is observed away from the target plate in our simulation. When the divertor plasma is in the high recycling state, the flow reversal with a small Mach number (|M|< 0.2) is observed near the X-point along the separatrix region. The driving mechanisms for the high Mach flow and the reversed flow are analysed theoretically through momentum and continuity equations, respectively. The profile of the ionization sources is shown to be a possible formation condition causing the complex behaviour of the SOL flow. In addition, the effects of the high Mach flow and the flow reversal on the impurity transport are also discussed in this 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.     

Potential Confinement of a High Density Plasma in the GAMMA 10 Tandem Mirror

Potential confinement was demonstrated experimentally under various electron cyclotron resonance heating (ECRH) scenarios. The particle confinement time and the plasma confining potential increased with ECRH power. The plug potential formation by means of only _ce ECRH was studied by Monte-Carlo simulation. Potential confinement experiments have advanced in higher density region up to 4×10¹² cm^-3. The higher density plasma was obtained by high frequency ICRF heating and recently installed neutral beam injector in the central cell. Studies of scaling relation of the potential confinement with respect to the plasma density have started with the high density plasma.     

Electron structure investigations of homogeneous hydrocarbon films formed in plasma conditions of T-10 tokamak

Electron structure investigations of smooth hydrocarbon thick films with a high atomic D/C ratio, redeposited from deuterium plasma discharge onto the vacuum vessel regions of the T-10 tokamak, are continued. For the first time, the investigations of both film sides and in a valence band region were performed by X-ray photoemission spectroscopy, combined with “conventional” measurements of survey spectra, C1s, O1s core levels, and by using Auger X-ray electron spectroscopy for sp ²/sp ³ ratio determination. The distinctions of electron structure of both sides of flakes which were noticed earlier using another techniques, were found, differing from the structure of thin films deposited in cleaning discharges of a low-temperature plasma. It is shown how these differences are connected with different processes of film deposition in tokamaks.     

HL-2A偏压偏滤器物理设计

根据已有的理论模型,推算出了HL-2A偏压偏滤器的理论电流为1．5kA、偏压电压为200V,并对HL-2A偏压偏滤器做出了初步设计。HL-2A装置采用偏压后,有可能降低L-H模转变阈功率,增加偏滤器等离子体密度、压强和滞留时间,改善约束,改善内外靶板功率沉积的非对称性,提高排灰效率及提高偏滤器性能。     

Early nonlinear regime of MHD internal modes: The resistive case

It is shown that the critical layer analysis, involved in the linear theory of internal modes, can be extended continuously into the early nonlinear regime. For the $m=1$ resistive mode, the dynamical analysis involves two small parameters: the magnetic Reynolds number S and the $m=1$ mode amplitude A, that measures the amount of nonlinearities in the system. The location of the instantaneous critical layer and the dominant dynamical equations inside it are evaluated self-consistently, as A increases and crosses some S-dependent thresholds. A special emphasis is put on the influence of the initial q-profile on the early nonlinear behavior. Predictions are given for a family of q-profiles, including the important low shear case, and shown to be consistent with recent experimental observations.     

东方超环上低杂波驱动等离子体环向旋转实验研究

旋转和旋转剪切能抑制磁流体不稳定性和增强等离子体约束.低杂波电流驱动作为未来聚变堆上可能的旋转驱动手段,探索低杂波在现有托卡马克装置上驱动等离子体旋转的驱动机制,可以为未来的聚变堆上旋转预测提供重要参考.在东方超环托卡马克装置上,早期发现了2.45 GHz的低杂波能有效驱动等离子体旋转的现象,认为是边界旋转的改变导致芯部旋转的同电流方向的增加造成的.更高频率下4.6 GHz低杂波电流驱动可以更有效地驱动同电流方向的等离子体旋转.本论文分析在欧姆背景等离子体下,不同功率的低杂波对等离子体环向旋转的影响,研究安全因子剖面变化对环向旋转的关系,利用功率调制获得了低杂波驱动旋转实验中的环向动量输运系数变化情况,发现环向动量扩散系数(χφ)、环向动量箍缩系数(Vpinch)的数值大小趋势是从芯部向靠外的区域逐渐变大.这与低杂波驱动环向旋转时,环向旋转速度由靠外的区域向芯部传递的特性吻合.     

A semi-empirical scaling law for the β-limit in tokamaks

A limited numerical optimization of the current and pressure profiles in tokamaks of various shapes and aspect ratios has led to a simple scaling law for the maximum volume averaged β which can be reached before the onset of ideal MHD instabilities: β < (2.0?2.5)μ₀I/Ba, where I is the total currrent, B the toroidal field at the centre of the plasma and a the minor plasma radius in the equatorial plane. The limit is set by the appearance of an n = 1 free boundary instability, neglecting the possible stabilizing effect of a shell. The highest β's attained in tokamaks equipped with strong auxiliary heating are shown to be very near the limit given by this scaling law in spite of their very different conditions of operation. Possible implications are discussed.