Studies of neutron emission during the start-up phase of the Alcator C tokamak

Abstract

Alcator C operations commenced with discharge cleaning and tokamak operation using hydrogen filling gas.Prior to and during these experiments no deuterium gas was allowed into the device. The earliest operation resulted in dosimeter readings of a few Roentgen per shot in the vicinity of the limiter and a localized source of neutron emission of up to 109 neutrons per shot which were subsequently identified as having photonuclear origin. After seven months of operation, conditions were achieved that resulted in substantially less photonuclear activity. Subsequently, deuterium fill gas was allowed into the device and measurement of neutron flux and energy spectra indicated that the majority of neutron emissions in Alcator C high density deuterium discharges were consistent with having thermonuclear origins.

The behaviour of fast ions in tokamak experiments

“…(5) is generally of order m, large shifts in the nominal birth energies are possible, particularly for beam-beam and beamplasma reactions. For thermonuclear reactions, the Doppler shifts and broadenings are more modest; for this case, the energy spread (FWHM) of the fusion reaction products is approximately proportional to f l [38, 391. The energy spectrum of 2.5 MeV neutrons produced in thermonuclear reactions has been measured on many tokamaks [16,. Figure 4 shows a representative thermonuclear spectrum from JET.…”

“…During hydrogen minority heating, the resonance layer was scanned outward into the large ripple region. When the resonance layer was at R,,, N 3.0 m the fusion gamma signal produced by energetic protons was insensitive to the number of field coils, but when it was at R,,, = 3.4 m the signal was ~3 times smaller with 16 A detailed study of stochastic ripple diffusion was performed on TFTR [280-2831. In this experiment, scintillators with pitch angle resolution that are located near the edge of the plasma measured escaping 1 MeV tritons and 3 MeV protons produced in d(d, p)t fusion reactions. Figure 37 shows the signal measured by a movable probe mounted just below the midplane (8 N -20") as a function of plasma current.…”

Neutron emission from KSTAR Ohmically heated plasmas

“…The stainless steel limiters and wall were found to be radioactive due to these reactions. In ALCATOR [12], runaway electrons resulted in activation of the molybdenum limiter. In ATF [13,14], studies of photo-nuclear reactions in a runaway suppression device detected activity from 99 Mo, which has a low photonuclear threshold.…”

“…In ALCATOR [12], runaway electrons resulted in activation of the molybdenum limiter. In ATF [13,14], studies of photo-nuclear reactions in a runaway suppression device detected activity from 99 Mo, which has a low photonuclear threshold. However no activity was found in the stainless steel (SS304) vessel and a maximum runaway energy of ∼ 12 MeV could be deduced from the data [15].…”

The Tokamak as a Neutron Source

“…Photoneutrons from a tungsten limiter have been observed on PLT 18 during initial operation, with typical source strength of 10 10 n/s and peak values greater than 10 12 n/s. In AlcatorC 19 photoneutrons were also measured during the start-up phase, when only hydrogen had been used as fill-gas. Up to 10 9 neutrons per shot were observed originating at the limiter, and when the runaway electron production was controlled during later experiments, the observed photonuclear neutron source strengths were typically reduced to less than 10 6 n/s.…”