Khairul Islam

Charge-exchange neutral particle measurements on electron cyclotron resonance heating experiments in hot-ion-mode plasmas of the GAMMA 10 tandem mirror

Charge-exchange (C–X) neutral particle measurements have been carried out in hot-ion-mode plasmas of the GAMMA 10 tandem mirror. In the present experiment, a microwave power of 40 kW in 28 GHz is injected toward a second harmonic ECR layer located in the vicinity of the ICR layer at the central region and the radial profiles of ion temperatures determined from the energy spectrum of the C–X neutrals by using a neutral particle energy analyzer (NPA) are investigated from the viewpoint of ion energy balance. At the onset of the ECRH pulse, a remarkable increase of C–X neutral flux with high energy (few keV to few tens keV) is observed with NPA and the resultant ion temperature on the plasma axis is found to increase from 2.5 to 5.0 keV at the electron line density of 3×1013 cm−2. Based on the measured plasma parameters, radial profiles of ion-energy losses due to classical processes are evaluated and ECRH in the central region is confirmed to reduce the energy loss due to electron drag significantly in the ...

Recent Progress in the GAMMA 10 Tandem Mirror

Following the 2002 Conference on Open Magnetic Systems for Plasma Confinement, a three times progress in the formation of ion-confining potential height is achieved in the hot-ion mode. The advance in the potential formation leads to a finding of remarkable effects of radially produced shear of electric fields dEr/dr on the suppression of not only coherent drift waves but turbulence-like fluctuations for the first time in GAMMA 10. Also, the progress in the potential formation is made in line with the extension of our proposed physics scaling of potential formation covering over representative tandem-mirror operational modes, characterized in terms of (a) a high-potential mode having kV-order plasmaconfining potentials and (b) a hot-ion mode yielding fusion neutrons with 10-20 keV bulk-ion temperatures (Ti).

Investigation of Edge Plasmas in the Anchor Cell Region of GAMMA 10

The first results of Langmuir probe measurements at the outer transition region of the anchor cell of GAMMA10 are given. A probe current asymmetry in vertical direction is found in this region. It is also found that the asymmetry of probe current increases in outward direction and the direction of the asymmetry is independent on movable limiter position. A relation of this plasma asymmetry with the main magnetic field configuration is investigated. Plasma flow through the non-axisymmetric magnetic field configuration region is thought to be the source of plasma asymmetry in this region, i.e., ∇ B and curvature drifts are responsible for the asymmetry. Possibility of cold plasma formation in the anchor cell region is obtained during plug electron cyclotron resonance heating (ECRH) and can be explained with the desorption of particles due to the collision of the drifted out particles with the wall.

Overview of Recent Progress in the GAMMA 10 Tandem Mirror

(1) Four-time progress in ion-confining potentials Φc to 3.0 kV in comparison to Φc attained 1992-2002 is achieved in the hot-ion mode (Ti=several keV). A scaling of Φc, which favorably increases with plug electron-cyclotron heating (ECH) powers (PPECH), is obtained. (2) The advance in Φc leads to a finding of remarkable effects of radially sheared electric fields (dEr/dr) on turbulence suppression and transverse-loss reduction. (3) A weak decrease in Φc with increasing nc to ˜1019 m-3 with the recovery of Φc with increasing PPECH is obtained. (4) The first achievement of active control and formation of an internal transport barrier (ITB) has been carried out with the improvement of transverse energy confinement. Off-axis ECH in an axisymmetric barrier mirror produces a cylindrical layer with energetic electrons, which flow through the central cell and into the end region. The layer, which produces a localized bumped ambipolar potential Φc, generates a strong Er shear and peaked vorticity with the direction reversal of Err × B sheared flow near the Φc peak. Intermittent vortex-like turbulent structures near the layer are suppressed in the central cell. This results in Te and Ti rises surrounded by the layer. The phenomena are analogous to those in tokamaks with ITB. (5) Preliminary central ECH (170 kW, 20 ms) in a standard tandem-mirror operation raises Te0 from 70 to 300 eV together with Ti[perpendicular]0 from 4.5 to 6.1 keV, and Ti//0 from 0.5 to 1.2 keV with τp0=95 ms for Φc (=1.4 kV) trapped ions. The on-axis particle to energy confining ratio of τp0/τE0 is observed to be 1.7 for Φc trapped ions (consistent with Pastukhov’s theory) and 2.4 for central mirror-trapped ions with 240-kW plug ECH and 90-kW ICH (ηICH˜0.3; nlc=4.5×1017 m-2). (6) Recently, a 200 kW central ECH with 430 kW plug ECH produces stable central-cell plasmas (Te=600 eV and Ti=6.6 keV) with azimuthal Er×B sheared flow. However, in the absence of the shear flow, hot plasmas migrate unstably towards vacuum wall with plasma degradation.

The Radial Loss of Ions Trapped in the Thermal Barrier Potential and the Design of Divertor Magnetic Field in GAMMA10

The ion radial loss exists in the presence of a non-axisymmetric electrostatic potential in the end-mirror cells of GAMMA10, which leads to a formation of the thermal barrier potential. The non-axisymmetric electrostatic potential can also exist in the central cell. A design for divertor magnetic field of GAMMA10 is performed, the purpose of which is first to reduce an ion radial transport in the central cell by making electrostatic potential circular and second to assure the macroscopic plasma stability of GAMMA10 without help of non-axisymmetric anchor cells which enhances a neoclassical radial transport.

Optimization of Plasma Production Using Ha Emission Detector and CCD Camera in the GAMMA 10 Tandem Mirror

Abstract Detailed behavior of plasmas has been investigated in the course of an optimization phase of wall conditioning in the central-cell of GAMMA 10 tandem mirror. The results are described based on the visible image measurements using CCD camera together with Hα line-emission measurements in the central-cell. Three limiters are installed in the central-cell and the behavior of the plasmas near the each limiter is precisely observed in response to electron cyclotron heating (ECH) for potential formation. In an early stage of wall conditioning in GAMMA 10, plasma parameters obtained in standard hot ion mode plasmas are compared in cases of sustained plasma in ECH pulses and of collapsed one simultaneously with ECH. Dependence of gas puffing from the mirror throat in the central cell and of the diameter of limiters is also investigated on the plasma behavior in ECH. From the measured results, it is recognized that a significant upward shift of the plasma column and imbalance of the Hα intensity between east and west Iris-limiters cause the plasma collapse in ECH. A systematic analysis of the dependence on plasma durability in the number of wall conditioning shots, the position of the plasma just before the ECH injection and suitable quantity of gas puffing is necessary for stably sustaining the plasma during potential formation by ECH.