Effects of low-Z impurities during the start-up phase of a large tokamak

Abstract

The requirements placed on a tokamak Ohmic heating system (i.e. loop voltage) to initiate the plasma become more severe as the size increases because of the current density decrease. During the start-up phase even small concentrations of low-Z impurities can affect the plasma energy balance very substantially and have very important effects on the evolution of the discharge. The start-up phase has been studied using a simple zero-dimensional computer code. Since the dominant energy loss mechanisms during start-up, radiation and ionization, are a volume effect, the zero-dimensional code was adequate to treat this phase. The results of this study which have been applied to TFTR indicate that the plasma evolution is a sensitive function of the applied loop voltage, impurity concentration, initial filling pressure and the manner in which gas is fed into the discharge.

Electron cyclotron/upper hybrid resonant pre-ionization in the ISX-B tokamak

“…One finds 12,13 an optical depth for the extraordinary wave: n_= c2 2 e R cos 2 a (2-a) 3 / 2 (l + a) 2 for e close to 900. For the ordinary wave one finds…”

“…With sufficient power, plasma preheating could cause more rapid 'burn-through" the impurity radiation barrier [2].…”

Influence of plasma surface interactions on tokamak startup

“…14,16,17 Since the impurity concentration is particularly critical in the startup stage, we have taken into account the influx of impurities generated by desorption and sputtering from the wall and limiters by the impact of recycling ions. Thus, the impurity ingress is now dependent on the energy of the impacting ions and cleanliness of the material surface which manifests in our model as surface layer concentration of the adsorbed oxygen species.…”

“…Alternatively, simplified global, or zero-dimensional, models have previously been used to analyse startup in tokamaks. [14][15][16][17][18] This method reduces the overall number of parameters, and calculations become more tractable, particularly when dealing with such timedependent phenomena.…”

Low energy x-ray emission from magnetic fusion plasmas

“…The importance of these effects is indicated by the extensive experimental and theoretical [101][102][103][104][105][106][107][108][109][110][111][112][113][114][115][116][117][118] efforts by many groups and individuals toward understanding impurity transport and impurity effects on the plasma, impurity production mechanisms, and toward developing schemes for reducing impurity influx and removing impurities from the plasma. Recipos for removal of adsorbed oxygen and carbon from the vacuumvessel wall [119] are used for all tokamaks.…”