Graphite-made UHFf resonator of fabry-perot type for investigation of the plasma divertor flow in fusion devices and the small-scale particles of a substance

Abstract

The UHF resonator device has some definite advantage comparatively to an interferometer scheme in measuring parameters of plasma flow1. The resonator allows to measure the plasma density of 2-4 orders in magnitude lower than the critical density for the given frequency, and it can be used for thin plasma layer (when layer thickness - wavelength) as well as for investigation of fluctuations in a plasma flow2. There is a reason to change the metallic mirrors of UHF resonator by ones made of graphite with an aim to lower the degradation of these element and to preclude the ingress of heavy metal impurities into the plasma, when the resonator are subjected to fluxes of plasma, neutron, gammas and heating in fusion devices. The measurement of carbon-graphite materials (CGM) plates reflectivity show that it compose 0.8-0.96 of metallic plate reflectivity on different frequencies (30-105 GHZ). The results of plasma influence on reflectors made of CGM showed small increase of their reflectivity with stronger effect for shorter wavelength. The improvement of reflectivity can be explained by decrease of the surface roughness. The test of CGM element under heating up to lOOOC (20 cycles) did not cause degradation of their surface and their reflectivity R increase up to 700-8OOC due to the graphite electroconductivity increase.To simulate effects of neutron irradiation the CGM reflectors were irradiated in the reactor up to F=2~10~~/cm~. The R values were measured at 1=337 wm and dropped at initial 0.9 to 0.78. Thus the CGM can be using for fabricating the UHF resonator mirrors3. To study the properties of resonators made of CGM the transmitted resonator was fabricated with graphite mirrors, with diameter d=70 mm, curvature radius p=100 mm and connection holes of 1.5 mm in diameter. The distance between mirrors, L, were variable in the range 10-100 mm. The frequency of the unloaded resonator (f-36.47 GHz) was not changed significantly when L was changed. However, with L increasing there were observed some increase of the quality factor, Q, and decrease of the resonance amplitude, A. Important fact is that the measured Q for graphite-made resonator was only 5-10 times lower in comparison to Q of the identical copper resonator, in spite of much higher difference predicted on the base of the electrical conductivity ratio: (o~~/ac)~’~=50. It is evident from this fact the contribution of diffraction losses in the case of a graphite resonator is less than in the case of metallic one, where such losses determine the Q value. For L=60 mm and with Q measured, the minimal density is (n&)min=1010/cm2, and the maximum value of (n&) is determined by the frequency sweeping range. Taking into account the realignment range of a