Detection of high k turbulence using two dimensional phase contrast imaging on LHD

High k turbulence, up to 30 cm(-1), can be measured using the two dimensional CO₂ laser phase contrast imaging system on LHD. Recent hardware improvements and experimental results are presented. Precise control over the lens positions in the detection system is necessary because of the short depth of focus for high k modes. Remote controllable motors to move optical elements were installed, which, combined with measurements of the response to ultrasound injection, allowed experimental verification and shot-to-shot adjustment of the object plane. Strong high k signals are observed within the first 100-200 ms after the initial electron cyclotron heating (ECH) breakdown, in agreement with gyrotron scattering. During later times in the discharge, the entire k spectrum shifts to lower values (although the total amplitude does not change significantly), and the weaker high k signals are obscured by leakage of low k components at low frequency, and detector noise, at high frequency

Development of two color laser diagnostics for the ITER poloidal polarimeter

wo color laser diagnostics using terahertz laser sources are under development for a high performance operation of the Large Helical Device and for future fusion devices such as ITER. So far, we have achieved high power laser oscillation lines simultaneously oscillating at 57.2 and 47.7?μm by using a twin optically pumped CH3OD laser, and confirmed the original function, compensation of mechanical vibration, of the two color laser interferometer. In this article, application of the two color laser diagnostics to the ITER poloidal polarimeter and recent hardware developments will be described

Short wavelength far infrared laser polarimeter with silicon photoelastic modulators

A short wavelength far infrared laser whose wavelength lambda is about 50 mu m is preferable for a polarimeter and an interferometer for high density operations in the Large Helical Device (LHD) and on future large fusion devices such as ITER. This is because the beam bending effect (propto lambda^2) in a plasma, which causes fringe jump errors, is small and the Faraday and the Cotton?Mouton effects are moderate.We have developed a polarimeter with highly resistive silicon photoelastic modulators (PEMs) for the CH_3OD laser (lambda =57.2 and 47.7 mu m) . We performed bench tests of the polarimeter with a dual PEM and demonstrated the feasibility for the polarimeter. Good linearity between actual and evaluated polarization angles is achieved with an angular resolution of 0.05° and a temporal resolution of 1 ms. The baseline drift of the polarization angle is about 0.1° for 1000 s

Development of terahertz laser diagnostics for electron density measurements

A two color laser interferometer using terahertz laser sources is under development for high performance operation on the large helical device and for future burning plasma experiments such as ITER. Through investigation of terahertz laser sources, we have achieved high power simultaneous oscillations at 57.2 and 47.6 ?m of a CH3OD laser pumped by a cw 9R(8) CO2 laser line. The laser wavelength around 50 ?m is the optimum value for future fusion devices from the consideration of the beam refraction effect and signal-to-noise ratio for an expected phase shift due to plasma. In this article, recent progress of the terahertz laser diagnostics, especially in mechanical vibration compensation by using a two color laser operation and terahertz laser beam transmission through a dielectric waveguide, will be presented