Heat Diffusion in a Non-Local Tokomak Stochastic Magnetic Field

Heat transport across a non-local stochastic magnetic field was studied for the first time. Eleven incompact low m perturbed magnetic islands were used in our calculation. Parallel heat diffusion coefficient to the perpendicular coefficient was found still to be a key factor in influencing the effective radial heat conductivity and the results in this paper were compared with earlier studies in a local stochastic magnetic field.     

Modification of exposure conditions by the magnetic field configuration in helicon antenna-excited helium plasma

Modification of exposure conditions downstream in the diffusion chamber has been performed in helicon antenna-excited helium plasma by adjusting the magnetic field(intensity and geometry).In the inductively coupled mode(H mode), a reduction in ion and heat fluxes is found with increasing magnetic field intensity, which is further explained by the more highly magnetized ions off-axis around the last magnetic field lines(LMFL). However, in helicon wave mode(W mode), the increase in magnetic field intensity can dramatically increase the ion and heat fluxes.Moreover, the effect of LMFL geometry on exposure conditions is investigated. In H mode with contracting LMFL, off-axis peaks of both plasma density and electron temperature profiles shift radially inwards, bringing about a beam with better radial uniformity and higher ion and heat fluxes. In W mode, although higher ion and heat fluxes can be achieved with suppressed plasma cross-field diffusion under converging LMFL, the poor radial uniformity and a small beam diameter will limit the size of samples suitable for plasma irradiation experiments.     

Study on Electron Heat Diffusion Across Stochastic Magnetic Field Affected by Magnitude of Perturbed Magnetic Field in Tokomak

Electron heat diffusion across stochastic magnetic fields is studied numerically in order to find out how the magnitude of perturbed magnetic field affect the enhanced heat conductivity and its radial profile in tokomak plasma physics. For these purposes, non-local stochastic magnetic fields are chosen as research objects in our simulation work. From our numerical results, we can find that the effects of the perturbed magnetic field magnitude are dominated parameter on the enhance electron heat transport conductivity wherever the magnetic field is single island or full stochastic field. Also, a theoretical analysis is provided and compared with numerical results.     

Magnetic field topology modeling under resonant magnetic perturbations on EAST

In order to understand the mechanism by which the resonant magnetic perturbation (RMP) mitigates or suppresses the edge-localized mode (ELM), the topological study of the edge magnetic field in ELM mitigation or suppression phase is a critical issue. To model the three-dimensional magnetic field topology superposed RMP on Experimental Advanced Superconducting Tokamak, a numerical model using the field line tracing method for both vacuum and ideal plasma response approximations is proposed. Using the numerical model, the topological change and the penetration depth of the stochastic field lines in the edge magnetic field are studied in an RMP experiment. Comparing profiles of minimum ρ on edge stochastic field lines and the particle flux pattern, the ideal plasma response changes the field line penetration depth while remaining similar profile relative to vacuum approximation. To mitigate and suppress ELM strongly, the deep penetration of RMP fields and topological changes of the edge magnetic field is a key from our modeling.     

Formation of the dendrite structure at ion beam synthesis in the external magnetic field

For the first time the dendrite structure on the surface of single-crystal silicon wafers implanted with Fe⁺ at high fluences in the applied magnetic field was revealed by atomic-force microscopy. The nucleation and growth of dendrites were simulated using the modified model of the diffusion limited aggregation at ion implantation. The magnetic dipole–dipole interaction between diffusing implanted atoms and forming ferromagnetic clusters was also considered. In the frame of this model the form of the dendrite structure is mainly determined by the effective magnetic moment and the diffusion coefficient.     

Symplectic Simulation of Fast Alpha Particle Radial Transport in Tokamaks in the Presence of TF Ripples and a Neoclassical Tearing Mode

A Hamiltonian guiding centre drift orbit code based on a symplectic integration algorithm, which enables the efficient calculation of particle trajectories and diffusion coefficients, is applied to fast alpha particle motion in magnetically perturbed tokamak plasmas. In particular, fast ion drift motion is examined in the presence of a stationary, low mode-number MHD magnetic perturbation in a toroidally rippled tokamak with circular flux surface. The main focus of our study is to investigate the dependence of the radial diffusion coefficient of energetic ions on their energy, on the perturbation strength and the localization of the perturbation. As expected, the resonance between bounce motion and toroidal field ripples plays a significant role in this context. For an ensemble of fast ions uniformly distributed in toroidal angle but with a given poloidal starting position their radial transport coefficient takes on higher values in the neighbourhood of resonance speeds and can exhibit there local minima, i.e. it shows an M-shaped speed dependence around resonances for sufficiently strong ripple perturbations. Expectedly, the addition of a modelled low-mode number neoclassical tearing mode perturbation will modify the pure ripple resonance structure of the radial diffusion coefficient. Depending on the strength and localization of the MHD mode it can cause enhancement or degradation of the radial ripple diffusion coefficient.     

HL-2A芯部等离子体ITG模和TE模增长率研究

本文基于双流体模型,通过理论分析获得了芯部离子温度梯度(ITG)模和俘获电子(TE)模的色散关系。在Weiland模型的基础上,考虑了离子平行方向速度和TE的影响,计算了碰撞对ITG模和TE模增长率的影响。对HL-2A等离子体的计算结果表明,芯部等离子体的ITG模和TE模增长率与扰动波数密切相关,磁剪切对ITG模有明显的抑制作用。     

Neutron Source Based on a Counter-Deuterium Beam Linear IEC

A high-efficiency inertial electrostatic confinement (IEC) neutron generator consists of linear IEC fusion chamber and ionization chambers at both ends, connected by an externally applied magnetic field. A pair of deuterium beams is produced in the ionization chambers that are positively biased so as to accelerate deuterium ions along the magnetic field towards the reaction chamber. For the purpose of achieving high-efficiency, the counter-streaming beam column is focused to form a thin filament. Such a configuration must avoid beam-driven instabilities such as the two-beam instability and Weibel’s instability. The stability analysis is performed showing that these modes are stabilized by an externally applied magnetic field parallel to the ion beams. The required magnetic field is less than several of 0.1 Tesla. Thus highly efficient neutron generator is potentially possible with counter-streaming ion beams focused and stabilized by an external magnetic field. An example giving specifications for such a neutron generator is presented.     

Optimization of the electrostatic and magnetic field configuration in the MITICA accelerator

MITICA (Megavolt ITER Injector Concept Advancement) is a test facility for the development of a full-size heating and current drive neutral beam injectors for the ITER Tokamak reactor. The optimized electrostatic and magnetic configuration has been defined by means of an iterative optimization involving all the physics and the engineering aspects. The acceleration grids have been designed considering optical performances and mechanical constraints related to embedded magnets, to cooling channels, to the grid stiffness and manufacturability. A combination of “local” vertical field and horizontal “long range” field has been found to be the most effective set-up for ion extraction, beam focusing and minimization and equalization of thermo-mechanical loads and minimal number of electrons exiting the accelerator.     

Simulations of NBI fast ion loss in the presence of toroidal field ripple on EAST

NBI fast ion losses in the presence of the toroidal field ripple on EAST have been investigated by using the orbit code GYCAVA and the NBI code TGCO. The ripple effect was included in the upgraded version of the GYCAVA code. It is found that loss regions of NBI fast ions are mainly on the low field side near the edge in the presence of ripple. For co-current NBIs, the synergy effect of ripple and Coulomb collision on fast ion losses is dominant, and fast trapped ions located on the low field side are easily lost. The ripple well loss and the ripple stochastic loss of fast ions have been identified from the heat loads of co-current NBI fast ions. The ripple stochastic loss and the collisioninduced loss are much larger than the ripple well loss. Heat loads of lost fast ions are mainly localized on the right side of the radio frequency wave antennas from the inside view toward the first wall. For counter-current NBIs, the first orbit loss due to the magnetic drift is the dominant loss channel. In addition, fast ion loss fraction with ripple and collision for each NBI linearly increases with the effective charge number, which is related to the pitch angle scattering effect.     

Magnet stage optimization of 5 kW multi-cusped field thruster

The multi-cusped field thruster is a unique electric thruster device, which has many advantages such as long lifetime, large-range thrust throttling ability, high thrust density, and low mass. The thruster employs several alternating polarity permanent magnets to create a periodic magnetic field with several cusps. Previous studies have indicated that the basic ionization and acceleration processes are directly related to the electron motion behavior, which mainly depends on the magnetic field characteristics. The magnet number and magnet stage length are two key magnetic field parameters that have important effects on the thruster performances. In this paper, both the magnet number and magnet stage length parameters are studied for the optimization of a 5 k W multi-cusped field thruster. The results indicate that the three-stage thruster has a better electron confinement than the two-stage thruster. It has lower ion energy loss at the wall, and shows a higher ionization rate. Therefore, the three-stage magnetic field is a superior magnetic field configuration. Besides, the three-stage magnetic field simulation results indicate that an optimal accelerating electric field distribution and ionization region distribution could be obtained when the magnet length ratio is 78:25:20.     

Intermittent increase of electron current on a probe during a discharge in a magnetic field

An account is given of preliminary results in a study of the influence of a magnetic field on electron diffusion in a plasma. An intermittent increase was observed in the ratio of the electron current in the probe to the ion current at some critical value of the magnetic field intensity. According to the preliminary data, the critical magnetic field changes proportionally to the gas pressure. These facts evidently indicate the existence of two qualitatively different mechanisms for the transverse movement of the electrons, one of which is diffusion by collision.In conclusion, I wish to thank P. M. Morozov for his assistance and interest in this work, and also S. Sinotov, for taking part in the measurements.     

The dynamics of an ion acting on two monochromatic obliquely propagating Alfvén waves

The interaction between a magnetized ion and two monochromatic shear Alfvén waves propagating obliquely to an ambient magnetic field is investigated both analytically and numerically. According to the Hamiltonian of this system, the invariant of motion at the lowest order and the half-island widths at the corresponding resonances are obtained analytically using the Lie transformation method. It is shown that these theoretical results agree with the numerical ones from the Poincaré surface of section. The regular motions from the invariant and the transition to stochasticity due to resonance overlapping are demonstrated. Compared to the case of a single wave, there may be a lower stochastic threshold in the multiple-wave problem.     

Magnetic field enhanced radio frequency ion source and its application for Siincorporation diamond-like carbon film preparation

Various ion sources are key components to prepare functional coatings,such as diamond-like carbon(DLC)films.In this article,we present our trying of surface modification on basis of Si-incorporation diamond-like carbon(Si-DLC)produced by a magnetic field enhanced radio frequency ion source,which is established to get high density plasma with the help of magnetic field.Under proper deposition process,a contact angle of 111°hydrophobic surface was achieved without any surface patterning,where nanostructure SiC grains appeared within the amorphous microstructure.The surface property was influenced by ion flow parameters as well as the resultant surface microstructure.The magnetic field enhanced radio frequency ion source developed in this paper was useful for protective film applications.     

Simulation of the Radial Electric Field in the Small Size Divertor Tokamak for Discharge with Neutral Beam Injection (NBI)

The radial electric field in the edge plasma of small size divertor tokamak can be simulated by B2SOLPS0.5.2D fluid transport code. The simulation provides the follow results: (1) Switching on and off the part of the parallel plasma viscosity driven by parallel ion diamagnetic heat flux (Bekheit in J. Fusion Energ 27(4), 338–345, 2008; Schneider et al. in Nucl. Fusion 41:387, 2001) and Counter-NBI plasma heating change profile of radial electric field significantly. (2) Switching on and off the parallel plasma viscosity driven by parallel ion diamagnetic heat flux leads to the radial electric field is toroidal magnetic field dependence (3) For the case of counter-NBI plasma heating, the switching on and off the current driven by part parallel plasma viscosity depends on the ion diamagnetic heat flux leads to the ion poloidal velocity is toroidal magnetic field B_T dependence. (4) The profile of the radial electric field in edge plasma of small size divertor tokamak is consistent with poloidal rotation velocity.     

The influence of charge characteristics of suspension droplets on the ion flow field in different temperatures and humidity

In order to clarify the charging characteristics of suspension droplets in ion flow field under different temperatures and humidity, the effective charging factor used to characterize the charging characteristics of suspension droplets is introduced in this paper, and a calculation method of charging factor is proposed based on the upstream finite element method (FEM). Then, the charging factor under different temperatures and humidity is calculated, and the analytic expression of the charging factor considering the influence of temperature and humidity is obtained by fitting the calculation results. The influence of suspension droplets on the ion flow field is analyzed. The results show that the charging factor is small and increases little with the relative humidity when the relative humidity is less than 60%, and the charging factor is large and increases rapidly with the relative humidity when the relative humidity is more than 60%. At the same relative humidity, the charging factor increases linearly with the temperature. The influence of charged suspension droplets on the ion flow field can be ignored when the relative humidity is less than 60% and must be considered under high temperature and humidity. The calculation method and analytic expression of the charging factor proposed in this paper can be used to model of ion flow field considering the influence of temperature and humidity and provide technical support for the construction of HVDC transmission lines across high temperature and humidity.     

Particle-in-cell simulation of magnetic-assisted electrostatic confinement device

In order to enhance the fusion reaction rate in inertial electrostatic confinement devices, it is necessary to increase the ion density with low cathode current and low background pressure. In order to accomplish the requirement, the authors suggest magnetic-assisted electrostatic confinement (MEC) scheme. The MEC relies on controlling the ion motion by applying an axial magnetic field to a system with cylindrical electrodes. In order to clarify the fundamental performances of the MEC device, particle-in-cell simulation was carried out. By reducing the background pressure, the ion confinement was improved resulting in the increase of the ion density. However, the ion density saturated due to space charge limitation. The estimated fusion reaction rate was about 5 × 10⁶ 1/s/m when the cathode voltage was −100 kV, the magnetic field was 200 mT, and the cathode current was 100 mA/m. The reaction rate, however, is expected to become higher since the ion density limitation is moderated by the electron which is not considered in the present analysis.     

轴向磁场中电弧螺旋不稳定性的简化数值分析

在静电近似下，通过数量级的分析，简化了能量方程。用数值方法研究了外加轴向磁场，电流分布、弧电流等对电弧曙旋不稳定性的影响。数值结果表明，在具有外加磁场情况下，当弧柱内均匀电流分布时，正向磁场能够使短波长的扰动稳定，反向磁场能够使长波长的扰动稳定。当弧柱内电流为抛物线形分布时，外加正向磁场对于短波情况下的稳定性影响很小，但对于长波情况，稳定性增强，外加反向磁场，同样能稳定中等和长波情况下的电弧。     

均匀场能谱分析仪的能量刻度

均匀场能谱分析仪常用来测量离子束的能量。在实际工作中常发现,根据加工后分析仪的场分布测量结果所确定的有效边界与理论设计值不一致,此时需对分析仪的能量刻度作重新校正。文章给出在一级近似下这种校正的计算公式。     

Strong Dependency of Ion Acceleration on Ion Beam Divergency in Magnetized Collisionless Plasma Sheath

The ion acceleration inside the collisionless plasma sheath is investigated at the presence of external magnetic field. By using the fluid model, the number and momentum equations of the ions and the Boltzmann and Poisson equations are solved numerically in the case that the ion beam has a small divergency at the plasma sheath boundary. It is shown that the kinetic energy of the ions has a strong dependency to the magnitude of divergency when the magnetic field has a small component parallel to the sheath boundary.