I. Ogawa

K. Kawahata

M." Sato

R. "Pavlichenko

T. Idehara

"Pavlichenko,  R.

Idehara,  T.

Ogawa,  I.

Kawahata,  K.

Sato,  M."

NIFS-Repository (National Institute for Fusion Science)

§43. Thomson Scatteri ng System for Measurement of Ion Temperature Using a Quasi-Optical Gyrotron as a Radiation Source Pavlichenko, R., Idehara, T. (FIR C. Fukui Univ.), Ogawa, 1. (Facul. Eng. Fukui Univ.), Kawahata, K., Sato, M. Collective Thomson scattering (CTS) has been traditionally used for measuring the electron and ion temperature in laboratory plasmas. Thus CTS play an important role in research into magnetic confinement of thermonuclear plasmas. The detected (scattered) signal is the one that originate due to the scattering of the elec-tromagnetic radiation by the Debye cloud of electrons, which effectively surround each ion. The scattered radi-ation power Ps (w) per frequency interval dw into solid angle interval do' is: where Pi is source power, T5 is classical electron radius, W and Lpl are geometrical factors for the incident / scattered microwave beams and w represents the differential fre-quency w = w S - WI (the difference between scattered and incident ones). Incident radiation "'100m corrugated WG Preamplifier and multi- channel filter bank Fig. 1: Schematic layout of the scattering system The proposed system (Fig. 1) for LHD will utilize the quasi-optical (Q. 0.) gyrotron as a power source. The Q. O. gyrotron designed for operation at the fun-damental frequency in the range of 92 ± 5 GHz. The advantages of Q. O. concept are the frequency tunabil-ity and the geometric separation between the spent elec-tron beam and the microwave output. Above mentioned characteristics offers flexibility for experiments on fusion devices, where it may be advantageous to change the lo-calization of power deposition zone (even for diagnostic usage) without changing the magnetic field and therefore affecting the plasma properties. The expected scattered spectrum from LHD neu-tral heated plasma as a function of a Doppler shifted fre-quency from the gyrotron centered frequency is shown on the Fig. 2. For evaluating of the scattered form factor, so-called Salpeter approximation has been used[l]. 10 -11,..-___ ---.-___ ----, ____ -,-___ --, Plasma parameters: Bo=2.75T, ne=5xl013 cm-3, ni=5xl013 cm-3, Te=3 keV, Ti=3 keV, ENBI=150keV. We assume that electrons and bulk ions velocity distribution functions of the plasma are Maxwellian and that a high energetic ions have an isotropically ltv3 slow-down distribution. 10-17 ~ ___ ~ ___ --' ____ --'--_--'L----' o 5 10 15 20 frequency, [MHz] Fig. 2: Expected scattered power for LHD plasma We have performed initial design of the LHD CTS system based on the Q. O. gyrotron as a radiation source. The main advantage of the using gyrotron as a radia-tion source instead CO2 laser is as follows. In the case of lasers, the spectrum changes very rapidly with an-gle, only small-angle scattering can be. used. This com-plicates obtaining high spatial resolution. In contrast, at gyrotron frequencies, much larger scattering angles are available, the spectra change on slowly with angle, and reasonably large collection solid angles can be used - limited only by coherence requirements for heterodyne detection The choice of the scattered angle will depend primarily on effects of plasma refraction. At the present time the spectrum of the bulk ions fracture can be obtained by means of other diagnostics. That is why the proposed diagnostic is targeting to mea-sure 'fast ion fracture' spectrum mainly. Preliminary estimates show that the scattered spectrum mostly con-sist (starting from the 6 MHz in differential frequency) of 'ion feature' that will exclusively gives the ion temper-ature information. The errors in the spectrum that will come from the electron component will be diminished. The middle-power gyratron with power of up to hundred watts, and pulse duration about 1 - 2 sec is a good contender for collective thomson scattering diag-nostic power source and will provide valuable informa-tion on ion spectrum/temperature. Reference [1] E.E. Salpeter, Phys.Rev., 120, (1975), 1528. 253 

Thomson Scattering System for Measurement of Ion Temperature Using a Quasi-Optical Gyrotron as a Radiation Source

Thomson Scattering System for Measurement of Ion Temperature Using a Quasi-Optical Gyrotron as a Radiation Source

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