Y. Higashizono

Progress in potential formation and findings in the associated radially sheared electric-field effects on suppressing intermittent turbulent vortex-like fluctuations and reducing transverse losses

Following the 19th IAEA Fusion Energy Conference (Lyon, 2002), (1) three-time progress in the formation of ion-confining potential heights c including a record of 2.1 kV in comparison to those attained 1992–2002 is achieved for tandem-mirror plasmas in the hot-ion mode with ion temperatures of several kiloelectronvolts. (2) The advance in the potential formation gives the bases for finding the remarkable effects of radially produced shear of electric fields Er, or non-uniform sheared plasma rotation on the suppression of intermittent vortex-like turbulent fluctuations. (i) Such a shear effect is visually highlighted by x-ray tomography diagnostics; that is, spatially and temporally intermittent vortex-like fluctuated structures are clearly observed as two-dimensionally reconstructed visual structures for the first time in kiloelectronvolt order ion-cyclotron heated plasmas having a weak shear in GAMMA 10. (ii) However, during the application of plug electron-cyclotron heatings (ECH), the associated potential rise produces a stronger shear (dEr/dr = several 10 kV m−2) resulting in the disappearance of such intermittent turbulent vortices with plasma confinement improvement. X-ray observations also show elongation of a vortex structure from a circular into an ellipsoidal shape, as depicted in H-mode theories, with an outward shift. (3) For the physics interpretations and control of such potential and the associated shear formation, the validity of our proposed theory of the potential formation is extensionally tested under the conditions with auxiliary heating. The data described above fit well to the extended surfaces calculated from our proposed consolidated theory of the strong ECH theory (plateau formation) with Pastukhovs theory on energy confinement.

Surface modification of tungsten mirrors due to low-energy helium plasma irradiation in the compact PWI simulator APSEDAS

Surface modification of a polycrystalline tungsten (W) mirror due to low-energy helium plasma irradiation has been studied in the compact plasma–wall interaction (PWI) simulator APSEDAS. Many fine irregularities of a few μm are generated on the W surface at Ts~1400 K due to He bubble formation. On the other hand, a distinctive change in the surface morphology cannot be seen from the SEM micrograph on a W mirror exposed at Ts<710 K. In both samples, the optical reflectivity with the incident angle θ=5° decreases more at shorter wavelength. From in situ measurement of the optical reflectivity with θ=75°, it is found that the optical reflectivity at λ~555 nm of the low Ts exposure increased by ~6% and decreased to the original level at a fluence of ~3×1024 He+ m−2, and then increased again by ~6% with the fluence up to ~1.6×1026 He+ m−2. Calculations using a model of the multiple reflection in the modification region suggest that the thickness of the modification layer of the low Ts sample exceeds 70 nm.

Edge plasma measurements near the minimum-B anchor cell of GAMMA10 using Langmuir probe and calorimeter arrays

Edge plasma diagnostic system equipped in minimum-B anchor cell of the GAMMA10 tandem mirror is described together with the measured results. Several sets of the diagnostic element consisting of a Langmuir probe and a calorimeter are placed in array on the conducting plates installed in the east anchor cell. This diagnostic system is designed to be capable of precisely investigating the spatial dependence of the edge plasma parameters. In typical ion cyclotron range of frequency-heated plasmas, detailed behavior of the edge plasma in the anchor transition regions has been measured by using this system. From these measurements a significant asymmetry of spatial profile was observed in the floating potentials. Heat flow profiles also showed the similar dependence to the probe signals. In this region there exists a strong gradient of the magnetic field strength and the direction of the observed shift is found to be consistent with that of gradient B drift for ions.

Two Dimensional Density Fluctuation Measurements During the Non-Inductive Current Ramp-up Phase in the Compact Plasma Wall Interaction Experimental Device CPD

Two-dimensional structure of density fluctuations is examined during the current jump phase, indicating a change from the open magnetic fields to the closed ones. During the smooth current ramp-up phase the two-dimensional contour of the LiI intensity shows vertically alignment, consistent with the magnetic surfaces. At the inflection point in Ip ramp-up the LiI intensity contour becomes flat in the observation regime and then suddenly a steep gradient and higher intensity regime are formed in the vertical direction. This higher intensity corresponds to a burst of LiI waveform. According to these changes in the contour, it is found that, within ±1 ms around the burst of LiI, a low frequency coherent wave with a long wavelength rapidly grows. The relations with other signals (magnetic flux and microwave stray power) are discussed with respect to the topological change in the magnetic configuration and mode conversion of the incident electromagnetic waves.

Neutral Transport Analysis of TRIAM-1M Plasma by Using Monte-Carlo Simulation

Neutral transport in the TRIAM-1M tokamak was analyzed using Monte Carlo simulation code (DEGAS). In the cases of low-density (~2×1012 cm-3) and high-density (~2×1013 cm-3) plasma discharges, the Hα intensity is localized around a neutral source, and in the toroidal direction, the 1/e decay lengths of the Hα intensity in two kinds of discharge differ by a factor of 1.5 to 2. By using a cylindrical mesh model for the DEGAS code, the simulated Hα intensities properly explained the experimental results. A detailed analysis of the simulation results revealed that the Hα profile is significantly affected by the geometric structure of the plasma facing component and the parameters of plasma and neutral particles.

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.

Hα measurements and neutral particle transport in Heliotron J

A multichannel system of the Hα-line emission detector is developed in order to investigate the neutral particle transport in the Heliotron J plasma. The application of a multianode photomultiplier tube to the filter measurement system enables us to observe the Hα emission profile with 32 chords. The absolute calibration with a standard lamp is performed and the cross-talk between each of the adjoining channels is evaluated to be less than 6%, while the photomultiplier tube has a typical cross-talk of 3% in nominal value. The temporal and spatial profiles of the Hα emission are measured in detail in the plasma breakdown by electron cyclotron heating (ECH). The Hα emission profile in the ECH plasma is calculated by the Monte Carlo simulation using a three-dimensional mesh model. The fueling rates of the wall recycling and the gas puffing are estimated by means of the simulation and the measurement.

Analysis of Neutral Transport in the GAMMA10 Anchor-Cell Using H{alpha}-Emission Detectors

The neutral transport was studied in the anchor cell. The Hα line intensities were measured by using axially aligned Hα detectors. It was found that the intensity is considerably dependent on ECRH and GP#3,4. A 5ch Hα detector was newly installed in the outer-transition region of the anchor-cell. From the measurement of the spatial distributions, the vertical intensity profile is estimated to be about 2.5 cm on the half width half maximum, while the horizontal distribution shows roughly flat around Z=-670 cm. The above characteristics were discussed with aid of neutral transport simulation using DEGAS Monte-Carlo Code.

Neutral Beam Injection Experiments and Related Behavior of Neutral Particles in the GAMMA 10 Tandem Mirror

Results of neutral beam injection (NBI) experiments in the GAMMA 10 tandem mirror plasmas are presented together with the neutral particle behavior observed in the experiments. A hydrogen neural beam was injected into the hot-ion-mode plasmas by using the injector installed in the central-cell for the plasma heating and fueling. High-energy ions produced by NBI were observed and its energy distribution was measured for the first time with a neutral particle analyzer installed in the central-cell. The temporal and spatial behavior of hydrogen was observed with axially aligned Hα detectors installed from the central midplane to anchor-cell. Enhancement of hydrogen recycling due to the beam injection and the cause of the observed decrease in plasma diamagnetism are discussed. The Monte-Carlo code DEGAS for neutral transport simulation was applied to the GAMMA 10 central-cell and a 3-dimensional simulation was performed in the NBI experiment. Localization of neutral particle during the beam injection is investigated based on the simulation and it was found that the increased recycling due to the beam injection was dominant near the injection port.

Diagnostic system of Hα emission on neutral beam injection experiments in the GAMMA10 central cell

We newly designed and installed a Hα line-emission detector in order to examine neutral particle transport around the beam line of NBI in the central cell of the GAMMA10 tandem mirror. The Hα detector consists of a Hα interference filter, a lens, an optical fiber, and a photomultiplier. The detector is absolutely calibrated using a standard lamp. The Hα detector is located at the angle of 24° to the beam line, which enabled us to directly view the Hα emission due to neutral beam injection (NBI). In standard hot-ion mode plasmas, we measured the Hα line emission in the case of NBI. The Hα line emission during NBI strongly increased by 550% on the beam line compared with that before NBI. Moreover, it was found that hydrogen recycling played an important role in the neutral transport around the beam line. This diagnostic system is confirmed to be considerably helpful for examining the detailed behavior of the neutral particles related to NBI for the first time.