Observation of MHD Instabilities Driven by Energetic Electrons in the Large Helical Device

Coherent magnetic fluctuations in an acoustic range of frequency have been regularly observed in low-density(n_e0.2×10~(19)m~(-3))plasmas with strong second harmonic electron cyclotron resonance heating(ECRH)on the Large Helical Device.Hard X-ray measurements indicated that energetic electrons are generated in these ECRH discharges.The magnetic fluctuations are suppressed in higher density discharges where energetic electrons are not present.The ECRH power modulation experiment indicated that the observed magnetohydrodynamic(MHD)mode has an acoustic nature rather than an Alfvenic nature.     

Refueling for Steady-State Plasma by Repetitive Pellet Injection in Large Helical Device

A repetitive pellet injector has been developed for investigation of refueling issues towards the steady-state operation in Large Helical Device (LHD). Continuous operation of more than 10000 pellet launching at 10 Hz has been demonstrated. The maximum repeating rate is 11 Hz. No technical constraint for longer operation has been found. The reliability of pellet launch has exceeded 99.9%. The initial application to the NBI-heated plasmas has been successful in the last experimental campaign of LHD. Although the pulse length is limited by the operational constraint of NBI, the plasma with a density of 8×1019 m-3 has been sustained for 2 s by the pellet injection at 10 Hz. A prospect for the future experiment is discussed on the basis of the initial result.     

Improvement of Plasma Performance Using Carbon Pellet Injection in Large Helical Device

A cylindrical carbon pellet with a size of 1.2L?1.2? to 1.8L?1.8?mm and a velocity of 100 to 300 m/s was injected into Large Helical Device (LHD) for an efficient fueling based on its deeper deposition instead of hydrogen gas puffing and ice pellet injection. Electron density increment of ?ne=1014cm-3 is successfully obtained by single carbon pellet injection without plasma collapse. Typical density and temperature of the ablation plasma of the carbon pellet, e.g., 6.5x1016cm-3 and 2.5eV for CII, are examined respectively by spectroscopic method. A confinement improvement up to 50% compared to ISS-95 stellarator scaling is clearly observed in a relatively low-density regime of ne=2 to 4?1013cm-3, and high ion temperature Ti(0) of about 6keV is also observed with an internal transport barrier at ne=1.2?1013cm-3. In particular, the improvement in the ion temperature largely exceeds that observed in hydrogen gas- puffed discharges, which typically ranges below 3 keV.     

Formation of Edge Transport Barriers by L-H Transition and Large Reversed Plasma Current on LHD

On the Large Helical Device （LHD） where nested magnetic surfaces are surrounded by the ergodic field layer, edge transport barrier （ETB） was produced in neutral-beam-injection （NBI） heated plasmas through transition and non-transition processes. The former case is the ETB formation by L-Htransition, where characteristics of L-H transition observed in a tokamak plasma are clearly recognized. The confinement improvement is the modest （- 10%）, compared with the ISS95 international stellarator scaling. The threshold power for the transition is comparable or slightly lower than the ITER scaling law established by tokamaks and compact tori. The ETB is formed inside the ergodic field layer of the vacuum field. The ETB formation destabilizes edge coherent modes such as m/n = 1/1, 2/3 and 1/2, of which rational surfaces are in the magnetic hill. The formed ETB is partially and transiently destroyed by these coherent edge MHD modes and edge localized modes （ELMs） typically observed in Ha signals. The latter ETB is observed in a plasma with large reversed NBI-driven current more than 100 kA at Bt = 1 T. In these plasmas, the edge magnetic shear is enhanced by the current and the rotational transform in the core region is expected to be appreciably reduced. Thus reduced rotational transform in the plasma central region will enhance outward heat and particle fluxes toward ergodic edge layer. The ETB with steep electron temperature gradient up to - 5 keV/m is formed by blocking enhanced outward heat flux.     

Effects of gamma-ray irradiation on electronic and non-electronic equipment of Large Helical Device

In a deuterium operation on the Large Helical Device,the measurement and control equipment placed in the torus hall must survive under an environment of radiation.To study the effects of gamma-ray irradiation on the equipment,an irradiation experiment is performed at the Cobalt-60 irradiation facility of Nagoya University.Transient and permanent effects on a personal computer,media converters,programmable logic controllers,isolation amplifiers,a web camera,optical flow meters,and water sealing gaskets are experimentally surveyed.Transient noise appears on the web camera.Offset of the signal increases with an increase of the integrated dose on the programmable logic controller.The DeviceNet module on the programmable logic controller is broken at the integrated dose of 72 Gy,which is the expected range of the integrated dose of the torus hall.The other equipment can survive under the gamma-ray field in the torus hall.     

等离子体参数对中性束注入下快离子自举电流影响

本文数值研究了等离子体参数对大纵横比托卡马克装置中性束注入时产生的快离子净电流密度分布的影响。研究表明净电流密度的大小随背景等离子体温度的增大而增大,随等离子体密度的增大而减小,等离子体有效电荷对净电流密度的大小影响较小,但随有效电荷的增大净电流密度的峰值向等离子体边缘偏移。     

强流AVF型回旋加速器中的空间电荷效应和束晕

首先,详细推导了AVF型回旋加速器中束团粒子在曲线坐标系中的动力学方程(考虑和不考虑空间电荷相互作用力两种情况)。然后,在假定动力学方程中各参数值的前提条件下,用Lunge-Kutta方法对考虑空间电荷时的动力学方程进行了数值计算。结果表明,束晕的形成和发展同样也是强流回旋加速器中束流损失的一个主要原因。但束晕形成的机制不同于直线加速器的情况,它不是由共振和混沌引起,而是由于粒子的排斥运动和束团内粒子的涡流运动引起的。     

The Influence of Neutral Beam Injection on the Heating and Current Drive with Electron Cyclotron Wave on EAST

Both neutral beam injection(NBI) and electron cyclotron resonance heating(ECRH) have been applied on the Experimental Advanced Superconducting Tokamak(EAST)in the 2015 campaign. In order to achieve more effective heating and current drive, the effects of NBI on the heating and current drive with electron cyclotron wave(ECW) are analyzed utilizing the code TORAY and experimental data in the shot #54411 and #54417. According to the experimental and simulated results, for the heating with ECW, NBI can improve the heating efficiency and move the power deposition place towards the inside of the plasma. On the other hand, for the electron cyclotron current drive(ECCD), NBI can also improve the efficiency of ECCD and move the place of ECCD inward. These results will be valuable for the center heating, the achievement of fully non-inductive current drive operation and the suppression of magnetohydrodynamic(MHD) instabilities with ECW on EAST or ITER with many auxiliary heating methods.     

Effects of Particle Confinement and Recycling on Thermally Stable Regions in D-T Tokamak Plasma

The water accumulation phenomena in the upper plenum of a PWR was investigated by using the data of the Cylindrical Core Test Facility (CCTF) test.

When the liquid inventory in the upper plenum is small, the liquid carryover rate from the upper plenum was governed by the liquid inlet flow rate. After some amount of the water was accumulated in the upper plenum, the liquid carryover rate was governed by the steam up-flow rate and the liquid inventory in the upper plenum. The liquid carryover rate was higher with the higher steam up-flow rate and the larger liquid inventory. This trend on the liquid carryover rate was also observed in small scale model test.

Based on the CCTF data, the empirical correlation on the liquid carryover rate was obtained.