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PLASMA CONFINEMENT in ECH BUMPY TORUS Plasma Particles by the Ambipolar Electric Field

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PLASMA CONFINEMENT in ECH BUMPY TORUS Plasma Particles by the Ambipolar Electric Field plasma ring heated by the electron cyclotron resonance using high power microwaves, and the greatly improved confinement of the PLASMA CONFINEMENT IN ECH BUMPY TORUS plasma particles by the ambipolar electric field. The contents of this review are,. (1) characteristics of the bumpy torus mag- M. FUJIWARA, T. KAMIMURA, H. IKEGAMI, NBT GROUP netic configuration and particle drift orbit, (2) plasma trans- port, (3) hot electron annulus and HHD stability, (4) electron Institute or Plasma Physics, cyclotron heating and ion cyclotron heating, and (5) present Nagoya Univt-sity, status and the future research program of the bumpy torus. Nagoya,Japan 2. NUMERICAL STUDIES ON PARTICLE CONFINEMENT IN BUMPY TORUS Bumpy torus is a magnetic confinement system consisted of ABSTRACT: Theoretical and experimental researches are reviewed toroidally connected mirrors, and is considered to be an S. = 0 on plasma confinement in ECH bumpy torus. Numerical study on stellarator with Et « e^, which has no rotational transform of particle confinement found that the particle confinement by magnetic field lines. Typically the magnetic mirror ratio EM " vacuum magnetic field .is the combination of toroidal and mirror- Bmax/Bo of each mirror section is roughly equal to 2, where Bm like trap depending strongly on spatial position. The ambipolar and Bo are the maximum and minimum magnetic field 1strength iosn10 the potential improves confinement greatly and the critical energy of toroidal axis, and the mechanical aspect ratio ci = Ro/rc > confined particles W is up to et'iefy) (here et and $ are the where Ro and rc are major radius and coil radius. inverse aspect ratio and plasma potential well or hill), while W -v e* in tandem mirror configuration. Some useful suggestions The orbit of a charged particle is closed in the poloidal are given to conventional neoclassical transport theory. Experi- plane by the poloidal drift due to the curvature of magnetic mental results are also surveyed, especially focused on the field lines and grad B. The poloidal precessional velocity scaling laws of the toroidal core plasma and hot electron ring depends strongly on the pitch angle of the particle velocity, parameters. The formation of hot electron rings is possible under and especially for v,,/v = 1 it is one order of magnitude, that is, some special conditions where ECH heating overcome the Coulomb by et times, smaller than that of a trapped particle. The orbit drag cooling by background plasmas. In other words this condition of the particle with v,,/v = I/I'SM does not close within the determines the maximum density of toroidal core plasma. Attain- toroidal vessel because the poloidal drift is cancelled out by able beta value or stored energy of hot electron rings is the positive (at the midplane) and negative curvatura (near the summarized by using experimental data of various machines. Brief mirror throat) of the mirror field lines. report is presented on recent experiments of ICH in NBT device. ICH not only contributes to ion heating, but affects the plasma Computed drift orbits in NBT device are shown in Fig.l for potential which has an important effect on transport. Some trapped particles with v,r/v » 0.3015, escaping particles v,,/v * results are also reported with respect to theoretical analysis of 1/i^H and toroidal passing particles v,,/v • 1. The circle of stability and equilibrium. triangular points shows the shadow of the coil casing, i.e., the mirror throat opening'to the midplane along the field line. From the figure, it is concluded that the mirror-trapped particles are well confined in each mirror sector, however, the particle passing through each mirror is rather poorly confined and the area, of the 1. INTRODUCTION closed orbit is narrow and localized in the area close to the inside wall of the torus. Theoretical and experimental research is reviewed on the plasma confinement by the ECH bumpy torus system which is consid- The spatial dependence of the loss cone in velocity space is ered to be one of the most promising candidates for the fusion shown in Fig.2. The region close to the inside wall of the torus reactor because of the following merits: (1) possible steady is characterized by both mirror and toroidal confinement of state operation, (ii) stable confinement of a high 3 plasma, particles with a narrow loss cone at v,,/v = 1/VEM and the confine- (iii) easy maintenance and good accessibility resulting from the ment of particles in the outside of the torus is similar to that simple structure of the magnetic coil systems. of an ordinary mirror machine which has the loss cone at v»/v < 1/vEjjï except that in case of the bumpy torus configuration, the The bumpy torus is a toroidal magentic trap consisting of a particle in the loss cone v,,/v < I/I'ÊM drift out to the walls set of linked magnetic mirrors. The main characteristic features approximately with the toroidal drift velocity and the loss rate of the ECH bumpy torus are thé HHD stable confinement by the decreases with increasing magnetic field, while in a mirror local magnetic well owing to the high 6, high energy hot electron device particles are rapidly lost in the transit time of L/v, 1S3 which is generally much smaller than the 90" Coulomb scattering plasma, the plasma potential may be 10 kV (* * Te) and it is time. possible to confine most of the fusion plasma particles whose energy W is less than several hundreds of keV. The particles The plasma potential has a strong effect on the confinement with W - 100 keV contribute mainly to the production of fusion characteristics because the E x B drift due to the ambipolar energy. Even alpha particles with the energy as high as 3.5 MeV potential is comparable to the magnetic drift (Er/B = <|>/Ba = could be well confined until they give up most of their energy to T/BRC) for particles with the thermal energy T. The effect of the core plasma. the potential on the particle confinement, that is, on the loss region, is shown in Fig.3 by changing the particle energy and 3. THE SURVEY OF THE EXPERIMENTAL RESULTS pitch angle for both cases of radially inward (N-type) and outward OF NBT-1, EBT-1, and EBT-S (P-type) electric field. The amazing feature is that the loss regions for the particles with 500-1000 eV are considerably The ordinary bumpy torus can confine charged particles as reduced in spite of the 40 V potential well in the N-type, and in is discussed in Section 1, but has been anticipated to be suscep- the P-type with 100 V of peak, potential. Probable reason is that tible to various MHD instabilities. In the past, various methods the critical boundary between confined and unconfined regime is were developed to overcome the difficulties and plasma stabiliza- determined by the condition of balance between toroidal drift and tion has been successfully achieved by the use of internal E x B drift, H/RCB = $/aB. From the result, it is concluded that poloidal rings which can produce rings of null magnetic field most of the plasma particles with the energy W < (R0/a)e<p are surrounding the plasma poloidally. In case of Electron Cyclotron well confined in the whole cross-sectional area of the torus. Heated Bumpy Torus, energetic electrons are produced to form a When compared with the potential plugging of the tandem mirror ring, which has been known as a hot electron ring. The ring is concept, the drastic difference exists in the fact that the observed to have a similar effect to the poloidal ring coil in critical energy of confined particles in tandem mirror is H = et such a way that the plasma is stabilized by the min.B configura- while in bumpy torus it is W = (e*)(Ro/a) [1]. tion [3]. The feature is visualized in Figs.4(a) and (b). Figure 4(a) Bumpy torus plasmas employing electron cyclotron heating shows loss cones expressed in the perpendicular and parallel (ECH) are currently being studied in the ELMO Bumpy Torus (EBT) energy space for various radial positions in the N-type potential, [4] at Oak Ridge National Laboratory and in the Nagoya Bumpy Torus and Fig.4(b) is in the P-type potential. All these calculations (NBT) [5] at the Institute of Plasma Physics, Nagoya University. have been carried for ions. The magnetic drifts tend to be A consistent and expanding data base exists for the two machines cancelled by Er x B drift in the N-type potential, while both — EBT, which has been operating since December 1973, and NBT drifts are added in the P-type. For electrons, the situations in since February 1978. Similar plasma characteristics are observed the N and P types are reversed except the lack of potential rim in both devices, so that a generic machine can be defined. in the reversed P type for electrons. In the numerical studies, the potential height and profile are adopted to best fit the Both NBT and EBT consist of 24 mirror sectors of nearly potential profile observed in EBT (Type N) and NBT (Type P). identical dimensions (see Table I). The vacuum wall in EBT is Assumptions are that the potential is constant along the magnetic aluminum, while in NBT it is gold-plated stainless steel. For field lines. The plasma potential is possibly determined by the all frequencies except the 28-GHz power on EBT, ECH power is fed balance of loss rate between the ions and the electrons, so that to each mirror sector by means of a fundamental waveguide distri- it is the ambipolar potential.
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