During lower hybrid current drive (LHCD), toroidally propagating rf waves are absorbed by the plasma electrons and create a high-energy flat tail in their velocity distribution function; this is interpreted as the plateau created by Landau damping of LH waves at the phase velocity of the wave . In collisions with ions, these energetic electrons produce bremsstrahlung radiation as hard x rays with energy up to approximately 500 keV. A g-ray multichannel collimator was constructed and installed on FTU in 1995 to investigate this range of photon energy. It consists of six NaI(Tl) 3-in.-diam scintillator detectors that record photon spectra along six fixed vertical chords of the poloidal cross section of the plasma, as shown in figure 1 . The detectors are shielded against neutrons by a mixture of lithium carbonate powder and polyethylene. Lead walls stop hard x-ray background radiation. A soft iron and m-metal shield is mounted to avoid magnetic disturbance. The total weight of the shield is about 9 tons. The diagnostic is located under port #3 of the FTU machine. The shielding system is shared by the neutron profile diagnostic. Radiation collimation is obtained by a fixed array of lead conic collimators in the bottom part of the line of sight, and a three-position set of lead conic collimators for each detector in the upper part.
The distance between collimators has been optimized to obtain the best transmission. Radiation collimation provides a spot resolution of about 4 cm2 at the midplane of the poloidal section. Pulse height analysis of the resulting signal is performed by giving the temporal evolution of the energy spectra and counting the photon rate for each of the six channels during the plasma shot. A spectrum is obtained by integrating the signal over 30 ms, with an energy resolution of 1 keV. An energy absolute calibration of the spectrum is needed to obtain quantitative information on the energy content in the fast electron population. Investigation of the anisotropy of the distribution function is crucial for estimating the efficiency of the current drive. An understanding of LH-wave physics is fundamental to the study of LH-wave propagation in plasma. It is expected that by changing the discharge conditions and launched LH spectrum and/or by modulating a fraction of the supplementary LH power, the detection of hard x-ray profile emission will give useful information on the energy deposition profile and plasma transport properties [2, 3]. A comparison of calculated distribution functions with the measurements will give information on the basic physic phenomena responsible for the formation of these energetic electrons .
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