波驱动等离子体电流的特征

本文指出,只当u=ω/(k_xv_e)≤2—2.5时,波驱动的电流以共振电子电流为主,波的耗散机制也主要是共振阻尼效应;而当u≥3时,非共振电子电流一般将超过共振电子电流,碰撞耗散也将大于共振耗散。波驱动电流所需功率较高,比相应欧姆功率大得多。这对反应器的设计带来困难,一般说来反应器中选取u≤2.5的共振电子电流较为合适。波驱动电流时将出现等离子体旋转,除环向旋转外,还将引起较大的角向旋转,后者伴有较大的径向电场。 关键词：     

HL-2Aװ�õ�Լ�����ƺͱ߽羻���빦�ʷ���

在HL-2A装置孔栏位形放电的等离子体实验中,电子回旋辅助加热期间观察到了等离子体约束改善的现象,并对等离子体从低约束模式(L模)向约束改善模式转换时的等离子体线平均电子密度、等离子体储能、分界面内辐射功率、能量约束时间、Hα辐射等进行了研究。同时,分析了电子密度和等离子体辐射功率的空间分布随时间的演化。对改善约束的相关功率(辅助加热、欧姆加热功率和损失功率)进行了分析,并研究了等离子体约束改善转换时的边界净输入功率(阈值)与电子线平均密度和环向磁场的关系。     

HL-2A装置的约束改善和边界净输入功率分析

在HL-2A装置孔栏位形放电的等离子体实验中,电子回旋辅助加热期间观察到了等离子体约束改善的现象,并对等离子体从低约束模式(L模)向约束改善模式转换时的等离子体线平均电子密度、等离子体储能、分界面内辐射功率、能量约束时间、H_α辐射等进行了研究。同时,分析了电子密度和等离子体辐射功率的空间分布随时间的演化。对改善约束的相关功率(辅助加热、欧姆加热功率和损失功率)进行了分析,并研究了等离子体约束改善转换时的边界净输入功率(阈值)与电子线平均密度和环向磁场的关系。     

Investigation of impurity transport using laser blow-off technique in the HL-2A Ohmic and ECRH plasmas

Impurity transports in two neighboring discharges with and without electron cyclotron resonance heating(ECRH)are studied in the HL-2A tokamak by laser blow-off(LBO) technique.The progression of aluminium ions as the trace impurity is monitored by soft x-ray(SXR) and bolometer detector arrays with good temporal and spatial resolutions.Obvious difference in the time trace of the signal between the Ohmic and ECRH L-mode discharges is observed.Based on the numerical simulation with one-dimensional(1D) impurity transport code STRAHL,the radial profiles of impurity diffusion coefficient D and convective velocity V are obtained for each shot.The result shows that the diffusion coefficient D significantly increases throughout the plasma minor radius for the ECRH case with respect to the Ohmic case,and that the convection velocity V changes from negative(inward) for the Ohmic case to partially positive(outward) for the ECRH case.The result on HL-2A confirms the pump out effect of ECRH on impurity profile as reported on various other devices.     

Key to understanding interstitial H2 in Si

A new IR absorption line at 3191.1 cm(-1) has been discovered for the interstitial HD molecule in Si. This new line appears for sample temperatures above approximately 20 K and lies 73.9 cm(-1) below the 3265.0 cm(-1) line previously observed for HD. We attribute the 73.9 cm(-1) energy difference to the rotation of the interstitial HD molecule. The selection rules associated with these two lines are consistent with the puzzling absence of an ortho-para splitting in the IR absorption spectra of H2 and D2 in Si.     

托卡马克等离子体不同运行模式下的电子回旋波电流驱动

在给定等离子体密度分布下，从电子、离子的能量方程出发，根据不同运行模式下等离子体的热传导率不同，分别求出了中心负剪切模式，常规剪切H模式和L模式下的等离子体温度分布，然后通过求解波迹方程与相对论情况下的Fokker-Planck方程，分别计算了这些模式下的电子回旋波电流驱动和波功率沉积.得到在中心负剪切下，驱动电流最大，驱动效率最高，功率沉积和电流分布区间跨度大；在常规剪切H模式下，驱动电流较小，分布区间跨度比较窄，驱动效率相对较低；在常规剪切L模式，驱动电流效率最低，分布区间跨度也非常集中. 关键词： 托卡马克 电子回旋波电流驱动 中心负剪切 常规剪切     

Observation of anomalous momentum transport in tokamak plasmas with no momentum input

Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas through analysis of the time evolution of core impurity toroidal rotation velocity profiles. Following the L-mode to EDA (enhanced D(alpha)) H-mode transition, the ensuing cocurrent toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core. The steady state toroidal rotation velocity profiles are relatively flat and the momentum transport can be simulated with a simple diffusion model. Velocity profiles during edge localized mode free (ELM-free) H-modes are centrally peaked, which suggests the addition of inward momentum convection. In all operating regimes the observed momentum diffusivities are much larger than the neoclassical values.     

Measurement of molybdenum ion density for L-mode and H-mode plasma discharges in the EAST tokamak

We report the measurement of total molybdenum ion density for L-mode and H-mode plasmas on EAST using spectral lines observation and calculation based on an impurity transport code. A flat-filed extreme ultraviolet spectrometer with some spatial resolution is used to obtain the radial profiles of molybdenum spectral line emissions. The absolute calibration for the extreme ultraviolet spectrometer is finished by comparing the calculated bremsstrahlung intensity with the readings of CCD detector. Molybdenum ion transport study is performed using the radial ion density profiles and one-dimensional impurity transport code STRAHL. The total molybdenum density profiles are determined from the transport analysis. The molybdenum density during L-mode and H-mode phases are obtained, which are about 3 and 4 orders of magnitude smaller than the electron density, respectively. An inward pinch is found during the H-mode phase that leads to the peaked profile of molybdenum density.     

Microscopic view of charge injection in an organic semiconductor

We have measured the chemical potential and capacitance in a disordered organic semiconductor by electric force microscopy, following the electric field and interfacial charge density microscopically as the semiconductor undergoes a transition from Ohmic to space-charge limited conduction. Electric field and charge density at the metal-organic interface are inferred from the chemical potential and current. The charge density at this interface increases with electric field much faster than is predicted by the standard diffusion-limited thermionic emission theories.     

AgO_x界面插入层对GZO电极LED器件性能的影响

采用磁控溅射的方法在p-GaN上制备了GZO透明导电薄膜,通过在p-GaN和GZO界面之间插入AgOx薄层来改善LED器件的接触性能。研究结果表明:氮气退火后,采用界面插入层的AgOx/GZO薄膜电阻率为5.8×10-4Ω.cm,在可见光的透过率超过80%。AgOx界面插入层有效地降低了GZO与p-GaN之间的接触势垒,表现出良好的欧姆接触特性,同时使LED器件的光电性能获得了显著的提高。在50 mA的注入电流下,相比于常规的GZO电极LED器件,AgOx/GZO电极LED器件的正向电压由9.68 V降至6.92 V,而发光强度提高了13.5%。     

二维磁性光子晶体及其模场分析

利用频域有限差分方法分析了两种二维磁性光子晶体结构(方形和圆形空气孔结构)的模场分布和有效折射率. 结果表明：在该种结构的材料中，原本简并的基模被分为两个不简并的偏振模——左旋模和右旋模，且两种模式的有效折射率不同； 其次，该种结构材料的法拉第旋转角较连续分布的磁光介质有显著增加. 通过选择适当的空气孔参数，方孔结构材料的法拉第旋转角可增大约一个数量级，而圆孔结构可增加4倍左右. 同时发现，当光通过这类材料后偏振态发生了变化，随着法拉第旋转角的增加出射光的椭圆率也在增加. 最后分析了产生这一现象的原因并提出了解决偏振态变化这一问题的方法，即在材料中心处引入缺陷. 关键词： 磁光效应 二维磁性光子晶体 频域有限差分法     

Magneto-optical evidence for a gapped fermi surface in underdoped YBa2Cu3O6+x

The infrared (900-1100 cm(-1)) Faraday rotation and circular dichroism are measured in the normal state of underdoped High T(c) superconductors and used to study the magnetotransport. YBa2Cu3O6+x thin films are investigated in the temperature range 10-300 K in magnetic fields up to 8 T and as a function of oxygen concentration. A dramatic increase of the Hall frequency is observed for underdoped samples, which is not consistent with the approach to a Mott transition but is consistent with a partial gapping of the Fermi surface as predicted in density wave models.     

Characteristics of electron cyclotron resonance plasma formed by lower hybrid current drive grill antenna

A 3.7 GHz system, which is meant for LHCD experiments on ADITYA tokamak, is used for producing ECR discharge. The ECR discharge is produced by setting the appropriate resonance magnetic field of 0.13 T, with hydrogen at a fill pressure of about 5 × 10⁻⁵ Torr. The RF power, up to 10 kW (of which ∼50% is reflected back), with a typical pulse length of 50 ms, is injected into the vacuum chamber of the ADITYA tokamak by a LHCD grill antenna and is used for plasma formation. The average coupled RF power density (the RF power/a typical volume of the plasma) is estimated to be ∼5 kW/m³. When the ECR appears inside the tokamak chamber for the given pumping frequency (f = 3.7 GHz) a plasma with a density (n _e) ∼ 4 × 10¹⁶ m⁻³ and electron temperature ∼8 eV is produced. The density and temperature during the RF pulse are measured by sets of Langmuir probes, located toroidally, on either side of the antenna. H_α signals are also monitored to detect ionization. An estimate of density and temperature based on simple theoretical calculation agrees well with our experimental measurements. The plasma produced by the above mechanism is further used to characterize the ECR-assisted low voltage Ohmic start-up discharges. During this part of the experiments, Ohmic plasma is formed using capacitor banks. The plasma loop voltage is gradually decreased, till the discharge ceases to form. The same is repeated in the presence of ECR-formed plasma (RF pre-ionization), formed 10 ms prior to the loop voltage. We have observed that (with LHCD-induced) ECR-assisted Ohmic start-up discharges is reliably and repeatedly obtained with reduced loop voltage requirement and breakdown time decreases substantially. The current ramp-up rates also decrease with reduced loop voltage operation. These studies established that ECR plasma formed with LHCD system exhibits similar characteristics as reported earlier by dedicated ECR systems. This experiment also addresses the issue of whether ECR plasma formed with grill antenna exhibits similar behavior as that formed by single waveguide ECR antenna. Our experimental observations suggest that the characteristics of (LHCD system-induced) ECR-assisted Ohmic start-up discharges show similar properties, reported earlier with normal ECR-assisted Ohmic start-up discharges and hence LHCD system may be used as ECR system at reduced toroidal magnetic field for other applications like wall conditioning.      

EAST装置上W波段多道极向相关反射仪的研制

在EAST装置上安装了X模极化W波段多道相关反射仪,用于测量等离子体芯部密度涨落。该诊断利用低损耗(<3dB)多工器将4个不同频率(79.2GHz,85.2GHz,91.8GHz和96GHz)的微波耦合在一起,通过同一个天线发射。反射波由两个极向分离(～5cm)的天线接收,通过下变频技术实现外差测量。通过对两个极向天线接收的信号进行相关分析,获得芯部湍流垂直速度。对2018年低约束模式(L模)放电进行分析发现,在电子回旋共振加热(ECRH)等离子体中,芯部湍流垂直速度在电子逆磁漂移方向。而在注入同向中性束(co-NBI)后,芯部湍流垂直速度变为离子逆磁漂移方向。     

Observation of E×B Flow Velocity Profile Change Using Doppler Reflectometry in HL-2A

A broadband, O-mode sweeping Doppler reflectometry designed for measuring plasma E×B flow velocity profiles is operated in HL-2A. The main feature of the Doppler reflectometry is its capability to be tuned to any selected frequency in total waveband from 26-40 GHz. This property enables us to probe several plasma layers within a short time interval during a discharge, permitting the characterization of the radial distribution of plasma fluctuations. The system allows us to extract important information about the velocity change layer, namely its spatial localization. In purely Ohmic discharge a change of the E×B flow velocity profiles has been observed in the region for 28 〈 r 〈 30cm if only the line average density exceeds 2.2×10^19 m^-3. The density gradient change is measured in the same region, too.     

Reduction of impurity confinement time by combined heating of LHW and ECRH in EAST

The core impurity confinement properties are experimentally investigated in the Experimental Advanced Superconducting Tokamak(EAST) plasma heated by lower hybrid wave(LHW) and electron cyclotron resonance heating(ECRH)(LHW+ECRH).It is shown that the impurity confinement time(τ_(imp)) in the L-mode plasma jointly heated by LHW and ECRH is weakly dependent on electron density but strongly dependent on the heating power,thus it is shorter than that in LHW-only heated L-mode plasma with the similar plasma parameters.The combined heating of LHW and ECRH can reduce the collisionality and indicates a more effective heating method for core τimp reduction and normalized poloidal beta(βP) ~(im)provement.It should be emphasized that in this high β_P operation window the small ELM regime can be accessed,and an L-mode level τ_(imp)(40 ms-80 ms) and high β_N(～1.7) can be obtained simultaneously.It means that this typical small ELMy H-mode regime has an advantage in avoiding the serious tungsten accumulation,and will be competitive in future long-pulse steady-state and high-performance operation with high-Z material plasma-facing components.     

Self-acceleration of a tokamak plasma during ohmic H mode

Core plasma rotation is observed to change from counter direction to co-current direction during the transition from low (L) to high (H) confinement mode, in Alcator C-Mod plasmas that are heated purely Ohmically and, hence, have no momentum input. The changes of the toroidal velocities, deduced independently from impurity Doppler measurements and from magnetic perturbations associated with sawteeth, agree. The magnitude of the change is consistent with the previously documented scaling for rotation in ion cyclotron rf-heated H modes. The rotation in this Ohmic experiment is obviously not an rf effect but demonstrates unequivocally a transport effect accelerating the plasma.     

Experimental characterization of the electron heat transport in low-density ASDEX upgrade plasmas.

The electron heat transport is investigated in ASDEX Upgrade conventional L-mode plasmas with pure electron heating provided by electron-cyclotron heating (ECH) at low density. Under these conditions, steady-state and ECH modulation experiments indicate without ambiguity that electron heat transport exhibits a clear threshold in inverted Delta T(e)/T(e) and also suggest that it has a gyro-Bohm character.     

Initiation of ecton processes by interaction of a plasma with a microprotrusion on a metal surface

Evolution of rapid (～10 ns) Ohmic overheating of a microprotrusion on a surface in contact with a plasma by emission current is studied taking into account the energy carried by plasma ions and electrons, as well as Ohmic heating, emissive source of energy release (Nottingham effect), and heat removal due to heat conduction. Plasma parameters were considered in the range of n = 10¹⁴?10²⁰ cm^?3 and T _e = 0.1 eV?10 keV. The threshold value of energy transferred to the surface from the plasma is found to be 200 MW/cm²; above this value, heating becomes explosive (namely, an increase in the temperature growth rate (δ² T/δt ² > 0) and in passing current (δJ/δt > 0) is observed in the final stage at T ～ 10⁴ K and j ～ 10⁸ A/cm²). In spite of the fact that Ohmic heating does not play any significant role for plasmas with a density lower than 10 18 cm^?3 because the current is limited by the space charge of electrons, rapid overheating of top of microprotrusion is observed much sooner (over a time period of ～1 ns) when the threshold is exceeded. In this case, intense ionization of vapor of the wall material leads to an increase in the plasma density at the surface, and the heating becomes of the Ohmic explosion type. Such conditions for the formation of a micr?xplosion on the surface and of an ecton accompanying it can be created during the interaction of a plasma with the cathode, anode, or an insulated wall and may lead to the formation of cathode and anode spots, as well as unipolar arcs.     

Temperature-dependent transport in suspended graphene

The resistivity of ultraclean suspended graphene is strongly temperature (T) dependent for 50.5 x 10(11) cm(-2), the resistivity increases with increasing T and is linear above 50 K, suggesting carrier scattering from acoustic phonons. At T=240 K the mobility is approximately 120,000 cm2/V s, higher than in any known semiconductor. At the charge neutral point we observe a nonuniversal conductivity that decreases with decreasing T, consistent with a density inhomogeneity <10(8) cm(-2).