High Energy Particle Confinement Research Articles

Effects of radial electric field on confinement of high energy particles in advanced fusion mirror reactor

The radial electric field Er in a magnetic confined machine, such as the compact fusion reactor (CFR), the field-reserved configuration (FRC), and the tokamak, plays an essential role in affecting the confinement properties of the high energy particles. The parallel velocities of the high energy particles will be accelerated or decelerated by applying a radial electric field, which could change the loss rate of the high energy particles in the magnetic confined machines. Unlike the fourth-order Runge-Kutta method RK4, the recently-developed Boris method can strictly preserve energy conservation of the high energy particles in the case without radial electric field. The orbit of high energy α particle in compact fusion reactor (CFR) is simulated by solving the equations of motion numerically with the Boris Algorithm. The effect of radial electric field on the orbit of the high energy α particle is investigated and the confinement of plasma in different radial electric fields in the CFR machine is studied in the present paper. By changing the strength of the radical electric field and the particles' radical locations in the middle plane of the CFR configuration, the confinement property of the high energy α particle is studied. The numerical results indicate that both the positive radial electric field and negative electric field can significantly affect the confinement of the high energy α particle. When the radial electric field is increased to a threshold, the high energy α particle could be confined in the central region of the CFR machine for a long enough time. The threshold of the radial electric field depends on the initial parameters of the confined particle. Systematic investigations of the radical electronic field effect will conduce to greatly improving the performance of the designed CFR machines.

Two-dimensional scanning high-energy particle diagnostic system in Large Helical Device

A high-energy neutral particle measurement is one of the important diagnostics for ion temperature and high-energy particle confinement analysis. The neutral particle analyzer in the large helical device is capable of wide range scanning as a feature. We have obtained various data using the horizontal scan of the analyzer. Recently, in addition to the horizontal scan, a high-speed perpendicular scan became possible which enables acquisition of new information in the poloidal direction. Two stainless blocks are set on the opposite sides of the chain in order to balance the weight (700 kg) of the analyzer and reduce the load for the motor. Therefore a very high scan speed of 1°/s can be obtained. The scanning speed is 1°/s. By adding the vertical scan, the ion temperature profile and the radial variation of the signal loss associated with the resonant loss was obtained in preliminary experimental results.

Long-Pulse Operation and High-Energy Particle Confinement Study in ICRF Heating of LHD

Long-pulse operation and high-energy particle confinement properties were studied using ion cyclotron range of frequency (ICRF) heating for the Large Helical Device. For the minority-ion mode, ions with energies up to 500 keV were observed by concentrating the ICRF heating power near the plasma axis. The confinement of high-energy particles was studied using the power-modulation technique. This confirmed that the confinement of high-energy particles was better with the inward-shifted configuration than with the normal configuration. This behavior was the same for bulk plasma confinement. Long-pulse operation for more than 2 min was achieved during the experimental program in 2002. This was mainly due to better confinement of the helically trapped particles and accumulation of fewer impurities in the region of the plasma core, in conjunction with substantial hardware improvements. Currently, the plasma operation time is limited by an unexpected density rise due to outgassing from the chamber materials. The temperature of the local carbon plates of the divertor exceeded 400°C, and a charge-coupled device camera observed the hot spots. The hot spot pattern was well explained by a calculation of the accelerated-particle orbits, and those accelerated particles came from outside the plasma near the ICRF antenna.

Ion cyclotron range of frequencies heating and high-energy particle production in the Large Helical Device

Significant progress has been made with ion cyclotron range of frequencies (ICRF) heating in the Large Helical Device. This is mainly due to better confinement of the helically trapped particles and less accumulation of impurities in the region of the plasma core. During the past two years, ICRF heating power has been increased from 1.35 to 2.7 MW. Various wave-mode tests were carried out using minority-ion heating, second-harmonic heating, slow-wave heating and high-density fast-wave heating at the fundamental cyclotron frequency. This fundamental heating mode extended the plasma density range of effective ICRF heating to a value of 1×1020 m−3. This use of the heating mode was its first successful application in large fusion devices. Using the minority-ion mode gave the best performance, and the stored energy reached 240 kJ using ICRF alone. This was obtained for the inward-shifted magnetic axis configuration. The improvement associated with the axis-shift was common for both bulk plasma and highly accelerated particles. For the minority-ion mode, high-energy ions up to 500 keV were observed by concentrating the heating power near the plasma axis. The confinement properties of high-energy particles were studied for different magnetic axis configurations, using the power-modulation technique. It confirmed that with the inward-shifted configuration the confinement of high-energy particles was better than with the normal configuration. By increasing the distance of the plasma to the vessel wall to about 2 cm, the impurity influx was sufficiently reduced to allow sustainment of the plasma with ICRF heating alone for more than 2 min.

Finite beta effects on the toroidal field ripple in three-dimensional tokamak equilibria

In order to investigate finite beta effects on the toroidal field ripple in a tokamak, three-dimensional free-boundary magnetohydrodynamics (MHD) equilibria are studied for a tokamak configuration that has an aspect ratio of 3.5 and an almost circular cross-section. Equilibria that are consistent with the non-axisymmetric external magnetic field are obtained by the VMEC code with a prescribed safety factor profile. To study tokamak configurations with a free-boundary constraint, we modify the boundary condition to specify the plasma position and size in the VMEC code. From the equilibria obtained, the change of the toroidal field ripple due to the finite pressure is calculated and the three-dimensional MHD effect on the ripple is estimated. In a finite pressure equilibrium, the toroidal ripple is changed by the equilibrium plasma current. The influence of the purely three-dimensional MHD effect on the toroidal field ripple is not large but the ripple is also changed by the variation of the axisymmetric component in finite pressure equilibria. In a medium-beta equilibrium, high energy particle confinement changes significantly due to the variation of the ripple well depth along the field line, because of the Shafranov shift, the change of the magnetic field line pitch due to the Pfirsch–Schlüter current, and the above mentioned finite pressure effect on the field ripple.

Compatibility between high energy particle confinement and magnetohydrodynamic stability in the inward-shifted plasmas of the Large Helical Device

The experimentally optimized magnetic field configuration of the Large Helical Device [A. Iiyoshi et al., Nucl. Fusion 39, 1245 (1999)], where the magnetic axis is shifted inward by 15 cm from the early theoretical prediction, reveals 50% better global energy confinement than the prediction of the scaling law. This configuration has been investigated further from the viewpoints of high energy particle confinement and magnetohydrodynamic (MHD) stability. The confinement of high energy ions is improved as expected. The minority heating of ion cyclotron range of frequency was successful and the heating efficiency was improved by the inward shift. The confinement of passing particles by neutral beam injection was also improved under low magnetic field strength, and there could be obtained an almost steady high beta discharge up to 3% in volume average. This was a surprising result because the observed pressure gradient exceeded the Mercier unstable limit. The observed MHD activities became as high as beta but they did not grow enough to deteriorate the confinement of high energy ions or the performance of the bulk plasma, which was still 50% better than the scaling. According to these favorable results, better performance would be expected by increasing the heating power because the neoclassical transport can also be improved there.

High-energy neutral particle measurement system in the large helical device

The development of a high-energy neutral particle measurement system for ion temperature measurements and high-energy particle confinement analysis during neutral beam injection and ion cyclotron resonance frequency heating experiments in the large helical device (LHD) is described here. We have been improving the time-of-flight neutral particle analyzer for the LHD which had been developed in ENEA Frascati. The control and data acquisition systems were designed to be suitable for long discharges in the LHD. The horizontal and vertical movable stage is prepared to investigate pitch-angle distribution and loss cone. We have resolved many difficulties at installation, for example, the strong leakage magnetic field from the LHD and the limitations of magnetized materials near the LHD, the quench of the superconducting magnetic field, the narrow viewing port, and the fully remote control system. The preliminary results in plasma experiments are also described here.

Dependence of Velocity Space Loss Region ofl=2 Torsatron onεh(a)/εt(a)

Velocity space loss regions are studied by using constants of motion to calculate particle orbits in l =2 torsatrons. A criterion to confine a particle with v // ≃0 initially is obtained in terms of e t ( a ), e h ( a ) and ρ 0 , where v // is a parallel velocity, e t ( a ) and e h ( a ) are magnitudes of toroidal magnetic ripple and helical magnetic ripple, respectively, and ρ 0 is a normalized radius of initial position. In order to confine all particles in ρ 0 ≤0.3, e h ( a )/ e t ( a )≥3-3.5 is required for a model magnetic field described by both e t ( r ) and e r ( r ); however, e h ( a )/ e t ( a ) can be reduced to about 2 by improving particle confinement with appropriate side-band components corresponding to l =1 and l =3 helical fields having the same helical period number as the l =2 helical field. This result increases flexibility for designing a low aspect ratio helical system with good confinement of high energy particles.