H. Igami

I. Ogawa

K. Tanaka

M. Nishiura

S. Kubo

T. Saito

T. Shimozuma

Y. Tatematsu

Y. Yoshimura

National Institute for Fusion Science (NIFS-Repository)

  Simultaneously oscillated 48- and 57-μm CH3ODlasers pumped by 9R(8) CO2 laser have been developed as an optical source of interferometer/polarimeter for high density plasma such as LHD and ITER. The two color interferometer is capable measuring the vibration compensated electron density profiles1). First of all, only the 57-μm laser has been stabilized by using a frequency stabilized CO2 laser. We reported that the performances are power stability of ± 2.4 %/40 min. and beat frequency stability of 765±25 kHz/40 min. in the previous annual report. In this study, the powers and the beat frequencies of the simultaneously oscillated 48- and 57-μm lasers have been stabilized by controlling the cavity length.  Figure 1 shows the feedback stabilization system of the twin type FIR laser. The frequency of the pump 9R(8) CO2 laser has been stabilized by an external Stark-cell modulation. The performances were the power stability of ± 0.74 %/h and the frequency stability of ± 580 kHz/h. The FIR laser is twin type (A, B) owing to a heterodyne beat modulation for the interferometer. The laser tube is a 3.0-m long, 25-mm bore, Pyrex tube with a water jacket. The cavity mirrors consists of an Au coated input mirror with off-axis hole in order to reduce an back-talk from the FIR laser cavity to the CO2 laser cavity and a Si hybrid output mirror. A change of the cavity length is main cause of the output and frequency instability. For passive output and frequency stabilizations, the distance between the laser mirror mounts is fixed by using two Super Invar rods. For active feedback stabilization, the laser cavity length is controlled by a stepping motor. Either 48- or 57-μm laser is used as the feedback signal by using a polarizer and an electronic filter. The output power is stabilized at the slope of the detuning curves (Fig. 2(a)). Two color beat signals (Fig. 3(a)) detected by a Ge:Ga photoconductor are F-V(frequency and voltage) converted after passing through the electronic filters. The FIR laser cavity is tuned so that the beat signal is locked to a set point value. Figure 2(b) and 3(b) show temporal changes of the output powers and the beat frequencies of both lasers in controlling by the 48-μm laser. The stabilities were ± 2.0 %/30 min. and 1260±26 kHz/30 min. for the 57-μm laser and ± 2.6 %/30 min. and 400±16 kHz/30 min. for the 48-μm laser. These instabilities are caused by the back-talk. When there is the stabilization point on the detuning curves in anti-phase, there is sensitive of the pump CO2 laser instability. We were confirmed that the stabilization of the 57-μm laser was also possible by the control of the 48-μmlaser. The back-talk will be reduced by using an optical isolator in future. 1)Kawahata, K. et al.: Rev. Sci. Instrum. 79 (2008) 10E707 Fig. 1. Stabilization systems of the twin type FIR laser. Fig. 2. (a) Detuning curves and (b) output stabilities of simultaneously oscillated 48- and 57-μm CH3OD lasers. Fig. 3. (a) Beat signals and (b) beat frequency stabilities ofsimultaneously oscillated 48- and 57-μm CH3OD lasers.  In FIR center, Univ. of Fukui, sub-THz frequency-range gyrotrons have been developed, and 400 GHz high-power pulse gyrotrons are now being developed.1-5) A feasibility study of a 400 GHz gyrotron as a collective Thomson scattering (CTS) source had been investigated for LHD high density plasma of 1020 m-3. 4-5) In result, it was found that the CTS in this frequency range is very attractive for LHD plasma. The CTS condition is satisfied for large scattering angle larger than 90 degrees, which provides good spatial resolution. The sub-THz wave does not suffer from refraction due to cutoff in plasma of the order of 1020 m−3. Moreover, it is almost free from cyclotron absorption since its frequency is much higher than harmonics of the cyclotron frequency. Therefore, the background ECE is at a very low level. This year, we have investigated the contribution of fast ions to the CTS spectrum and discuss the feasibility of detecting the fast ions with 400 GHz CTS measurement. Figure 1 shows a calculated CTS spectrum measured at the position we have investigated. Here, the frequency and the incident power of the probe beam are assumed as 400 GHz and 100 kW, respectively. Velocity distributions of electrons, bulk ions and fast ions are assumed as Maxwellians with the temperature of 0.5, 0,5 and 100 keV, respectively.  Plasma density of 1020 cm-3 and 1 % density fraction of the fast ions to the total density are assumed. Figure 1 shows that the intensity of the fast ion term is dominant at the frequencies separated about several GHz from the center frequency (400 GHz). Thus, there is a possibility that information of the fast ions is obtained by the CTS measurement with 400 GHz frequency. The usual CTS condition is  >1, where = 1/kλD, k is wave vector of the fluctuation and λD is the Debye length of plasma. However, the condition  >1 only guarantees that spectrum intensity of the bulk ion term is larger than that of the electron term around the center frequency.  Thus,  >1 is the condition for the bulk ions and does not apply to fast ions. Since the temperature (energy) of the fast ions is much higher than that of bulk ions, the spectrum intensity becomes superior at the frequencies separated from the center frequency. Whether the fast ion term becomes dominant depends on the intensity of non collective scattering spectrum (electron term). It depends not only the electron velocity distribution function but also the factor |1-e/L|, where L is the longitudinal part of the dielectric tensor and e  is the electric susceptibility for electrons. The behavior of this factor is complicated, and it depends on directions of the incident probe beam, the scattering wave and magnetic field at the scattering point. Thus it is necessary to choose them properly in order for CTS spectrum of the fast ion term to be dominant and to give us the information of their velocity  References 1) T. Notake et al., Phys. Rev. Lett. 103, 225002 (2009). 2) T. Saito et al., The Joint 34th International Conference on Infrared and Millimeter and Terahertz Waves, 09030089  (2009).  3) T. Saito et al., The Joint 35th International Conference on Infrared and Millimeter and Terahertz Waves, Th-E1.5 (2010). 4) Y. Tatematsu et al., The Joint 33rd International Conference on Infrared and Millimeter and Terahertz Waves, 1281 (2008).  5) Y. Tatematsu et al., 23rd IAEA Fusion Energy Conference,(http://www-pub.iaea.org/mtcd/meetings/cn180_papers.asp),  FTP/P6-31 (2011). Figure 1: Scattering power spectrum in unit band frequency for Te = Tbulk ion = 0.5 and Tfast ion = 100 keV, ne = 1020 m-3 and nbulk ion/ne =0.01. Dot-dashed, long-dashed and solid lines represent power spectrum of bulk ion, electron and fast ion terms, respectively. 10-510-410-310-210-1100101390 395 400 405 410P/f (eV)fs (GHz)totalelectron termbulk ion termfast ion term147§27. Study of Measurement System for 400 GHz Collective Thomson Scattering in LHDTatematsu, Y., Saito, T., Ogawa, I. (FIR, Univ. Fukui),  Kubo, S., Shimozuma, T., Tanaka, K., Nishiura, M., Yoshimura, Y., Igami, H.

Study of Measurement System for 400 GHz Collective Thomson Scattering in LHD

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Study of Measurement System for 400 GHz Collective Thomson Scattering in LHD

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