Calibration and sensitivity of the infrared imaging video bolometer

The infrared (IR) imaging video bolometer (IRVB) is an imaging bolometer which uses a large (9?cm×9?cm) thin (1 μm) gold foil and an IR camera to provide images of radiation from the plasma. Calibration of the IRVB using a lamp has been performed to compensate for any nonuniformities in the foil’s thickness and its thermal properties due to blackening of the foil with graphite to improve the IR emissivity. This calibration revealed close to expected values for the calibration coefficient proportional to the product of the thermal conductivity and the foil thickness in the central region of the foil, while these values were anomalously high near the foil edge. The calibration coefficient proportional to the thermal diffusivity is a factor of 2 smaller than the expected value at the center and drops further at the edge of the foil. Using a derived expression for the IRVB noise equivalent power, a sensitivity comparison shows the IRVB using current IR technologies to be ? 200 times less sensitive than an equivalent conventional resistive bolometer operating under ideal conditions

Observation of Plasma Flow at the Magnetic Island in the Large Helical Device

Radial profiles of ion temperature and plasma flow are measured at the n/m = 1/1 magnetic island produced by external perturbation coils in the Large Helical Device. The sheared poloidal flows and sheared radial electric field are observed at the boundaries of the magnetic island, because the poloidal flow vanishes inside the static magnetic island. When the width of the magnetic island becomes large, the flow along the magnetic flux surface inside the magnetic island appears around the O point in the direction which reduces the shear of the poloidal flow at the boundary of the magnetic island

Role of Radial Electric Field Shear at the Magnetic Island in the Transport of Plasmas

The structures of radial electric field and transport at the magnetic island are investigated using n/m=1/1 external perturbation coils in the Large Helical Device (LHD). The radial profiles of plasma potential, as well as the electron temperature and density, shows flattening inside the magnetic island and the large shear of the radial electric field is observed at the boundary of the magnetic island. When the current of n/m=1/1 external perturbation coils becomes large enough, the finite radial electric field appears inside the magnetic island. The abrupt appearance of plasma flow inside the magnetic island suggests the non-linearity of the viscous force at the boundary of the magnetic island. The thermal diffusivity perpendicular to the magnetic filed inside the magnetic island is estimated with the cold pulse propagation, which is produced by a tracer-encapsulated solid pellet (TESPEL). The time delay and amplitude of the electron temperature of the cold pulse show much lower thermal diffusivity inside the magnetic island (0.16 m2ls; than that outside the magnetic island (2 m2ls;, which is a clear evidence for the significant reduction of heat transport inside the magnetic island

Formation of electron internal transport barrier and achievement of high ion temperature in Large Helical Device

An internal transport barrier (ITB) was observed in the electron temperature profile in the Large Helical Device [O. Motojima et al., Phys. Plasmas 6, 1843 (1999)] with a centrally focused intense electron cyclotron resonance microwave heating. Inside the ITB the core electron transport was improved, and a high electron temperature, exceeding 10 keV in a low density, was achieved in a collisionless regime. The formation of the electron-ITB is correlated with the neoclassical electron root with a strong radial electric field determined by the neoclassical ambipolar flux. The direction of the tangentially injected beam-driven current has an influence on the electron-ITB formation. For the counter-injected target plasma, a steeper temperature gradient, than that for the co-injected one, was observed. As for the ion temperature, high-power NBI (neutral beam injection) heating of 9 MW has realized a central ion temperature of 5 keV with neon injection. By introducing neon gas, the NBI absorption power was increased in low-density plasmas and the direct ion heating power was much enhanced with a reduced number of ions, compared with hydrogen plasmas

Recent Advance in LHD Experiment

In the first four years of LHD experiment, several encouraging results have emerged, the, most significant of which is that MHD stability and good transport are compatible in the inward shifted axis configuration. The observed energy confinement at this optimal configuration is consistent with ISS95 scaling with an enhancement factor of 1.5. The confinement enhancement over the smaller heliotron devices is attributed to the high edge temperature. We find that plasma with an average beta of 3 % is stable in this configuration even though the theoretical stability conditions of Mercier modes and pressure driven low n modes are violated. In the low density discharges heated by NBI and ECR heatings, ITB(internal transport barrier) and an associated high central temperature (> 10 keV) are seen. The radial electric field measured in these discharges is positive (electron root) and expected to play a key role in the formation of the ITB. The positive electric field is also found to suppress the ion thermal diffusivity as predicted by neoclassical transport theory The width of the externally imposed island (n/m=1/1) is found to decrease when the plasma is collisionless with finite beta and it increases when the plasma is collisional. The ICRF heating in LHD is successful and a high energy tail ( up to 500keV) has been detected for minority ion heating, demonstrating good confinement of the high energy particles. The magnetic field line structure unique to the heliotron edge configuration is confirmed by measuring the plasma density and temperature profiles on the divertor plate. A long pulse (2minute) discharge, with an ICRF power of 0.4 MW has been demonstrated and energy confinement characteristics are almost the same as those in short pulse discharges