Softening of Magnetic Excitations Leading to Pressure-Induced Quantum Phase Transition in Gapped Spin System KCuCl3_3

KCuCl$_3$ is a three dimensionally coupled spin dimer system, which undergoes a pressure-induced quantum phase transition from a gapped ground state to an antiferromagnetic state at a critical pressure of $P_{\rm c} \simeq 8.2$ kbar. Magnetic excitations in KCuCl$_3$ at a hydrostatic pressure of 4.7 kbar have been investigated by conducting neutron inelastic scattering experiments using a newly designed cylindrical high-pressure clamp cell. A well-defined single excitation mode is observed. The softening of the excitation mode due to the applied pressure is clearly observed. From the analysis of the dispersion relations, it is found that an intradimer interaction decreases under hydrostatic pressure, while most interdimer interactions increase.Comment: 4 pages, 5 figures, 1 table, jpsj2.cls, to be published in J. Phys. Soc. Jpn. Vol.76 (2007), the graphic problem of Fig.2 was fixe

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

Homodyne reflectometer for neutral beam injection interlock on large helical device

Neutral beam injection (NBI) into low-density plasmas can cause serious damage to the vacuum vessel wall. It is necessary to stop the NBI when the plasma terminates. This needs a reliable density monitor for NBI interlock. A three-channel homodyne reflectometer, installed on a large helical device was used for an NBI interlock. Microwaves of 28.5, 34.9, and 40.2 GHz were injected with O mode polarization. At present, a simple homodyne detection scheme is used. The reflected signal consists of a dc component due to local and reflected power and an ac component due to fluctuations in the position of the cutoff layer. Since the change in dc signal was very small, the root mean square value of the ac signal was used as the interlock signal. At present the 34.9 GHz O mode channel, whose O mode cutoff density is 1.5×10^19 m^?3, is used for the interlock. The system has been working since the first NBI experiments on LHD in 1999