D-T radiation effects on TFTR diagnostics (invited)

Abstract

For a 50%-50% deuterium-tritium plasma, the neutron production is 80× higher and the total-energy release is 200× higher than the same plasma composed only of deuterium. With this increase in radiation, diagnostics which see only negligible amounts of noise during DD operation may find themselves overwhelmed during DT. The neutrons are not only more numerous, but have 6× as much energy, which causes the calculated 2.5× increase in the gamma flux per neutron near TFTR. Here the effects of this increased radiation on the TFTR diagnostics set are reported. The most noticeable effects are luminescence and transmission losses in fiber-optic signal cables. Silicon detectors show signs of neutron interactions as well as gamma response, and microchannel electron multipliers show an increased background due to the gamma flux. Bolometers show n and γ heating, and the Thomson scattering intensifier gate spark gap was unreliable until the gas pressure was adjusted. All of these effects were anticipated, and in many cases shielding or compensation techniques were used. Compensation fibers work satisfactorily at these radiation levels, and the rapid falloff of the radiation as one moves away from the machine makes relocation of fibers and other sensitive components very useful. Conventional shielding attempts worked when streaming through signal penetrations was properly dealt with. In the next generation of DT campaigns and new tokamaks, such problems will be more severe. The implications for these machines are discussed.