New regime of runaway discharges in tokamaks

Abstract

A new regime of runaway discharges has been found in TCABR (Tokamak Chauffage Alfvén Brésilien). This regime is obtained by initiating the discharge with low filling pressure and, after the initial current rise, maintaining a large filling rate. The line density reaches a maximum value around 2 × 10 19 m −3 , during the current ramp-up phase, and then drops by a factor of around four in the quasi-stationary phase of the discharge, when a new regime is achieved. The most distinctive features of this regime, as compared to 'conventional' runaway discharges reported in the literature, are (i) maintenance of the runaway discharge, with the plasma current almost entirely provided by the runaway beam, in a cold background plasma and with strong neutral gas injection; (ii) enhancement of the relaxation instability with strong spikes in the Hα emission and loop voltage correlated with sawtooth relaxation of the line density; and (iii) plasma detachment from the limiter. A simple phenomenological model, based upon straightforward particle and energy balance calculations, is proposed to explain the experimental observations. According to this model, the plasma is rather cold and the short pulses of gas ionization and the related density spikes are due to sudden plasma heating caused by the relaxation instability. Furthermore, it seems that the runaway generation for the conditions of the experiments can be explained only if the secondary generation process is invoked.

Electrostatic turbulence driven by high magnetohydrodynamic activity in Tokamak Chauffage Alfvén Brésilien

“…The experiments were performed in a hydrogen circular plasma in the Brazilian tokamak TCABR 23,25 ͑major radius R = 61 cm and minor radius a =18 cm͒. The plasma current reaches a maximum value of 100 kA with duration 100 ms.…”

Runaway electron energy measurement using hard x-ray spectroscopy in “Damavand” tokamak

“…11,12 So far in different tokamaks, many experiments have been done to deduce energy of runaway electrons from the energy spectrum of hard x-ray photons. 13,14 Damavand tokamak is a small size machine, which could provide convenient conditions to investigate the mechanisms of production, behavior, prevention, and shutdown of runaway currents in fusion plasmas.…”

Reduction of chaotic particle transport driven by drift waves in sheared flows

“…IV, we use our simple model to interpret recent tokamak experiments performed for studying the transport reduction caused when a biased electrode is inserted into the plasma edge region. Parameters for equilibrium and fluctuating electric field radial profiles are extracted from Tokamak Chauffage Alfvén Brésilien ͑TCABR͒ data, 43 and the model thus obtained is used to calculate transport during standard Ohmic and improved confinement regimes obtained in this tokamak by biasing. A good prediction for the bias voltage necessary for improved confinement is obtained, a prediction consistent with the existence of robust tori.…”