Runaway-limiter interaction in the FTU tokamak during disruptions

Photoneutron production during disruptions was monitored on the FTU machine by a set of calibrated BF ₃ proportional counters. Both hydrogen and deuterium discharges covering a wide density range were examined. The functional dependence of the photoneutron production on n_e, Z_eff and I_p is in agreement with the Dreicer generation, being the main source of runaway electrons during disruptions in FTU. No detectable photoneutron production was observed at a plasma density higher than 1.4 × 10 ²⁰ m ^-3, corresponding to Z_eff values close to 1. The typical runaway energy was evaluated from the curve of the photoneutron yield per electron vs. energy for disruptions characterized by a hesitation in the plasma current decay. Damage by disruption generated runaways was found to be concentrated around the inboard equatorial plane as suggested by temperature and activation measurements.