A study of the effect of impurity radiation from the peripheral plasma of a tokamak reactor

Abstract

If the power lost by impurity radiation from the low-temperature boundary region of a confined plasma exceeds the power transported from the central region then temperature equilibrium is impossible and the temperature profile will collapse. This situation is studied on the assumption of coronal equilibrium and the results are used to predict the permissible impurity concentration at the edge of a tokamak reactor. The effect of neoclassical impurity transport is studied analytically and a 1D tokamak diffusion code is used to illustrate the detailed collapse of the temperature profile when the plasma density in a radiation-cooled tokamak is increased. Finally, the results are used to predict the limiting density of a radiation-cooled tokamak; this predicted density limit is compared with the density limits observed in current tokamak experiments.

Multifaceted asymmetric radiation from the edge-like asymmetric radiative collapse of density limited plasmas in the Large Helical Device

“…This is the so-called thermal or radiative instability. [5][6][7] In the discharge presented, the trigger of this thermal instability is marked by the sharp jumps in O V and C III signals at about tϭ0.86 s. The enhanced impurity radiation results in a substantial increase of P rad , as seen in Fig. 1͑c͒.…”

“…3,4 This type of density limit is thought to be related to the onset of an edge thermal instability arising from the increase of impurity radiation with reduced temperature. [5][6][7] In tokamaks, radiative collapse may result in a cooling of the plasma boundary resulting in a contraction of the plasma. Additionally, a poloidally symmetric radiating belt is generally established in such radiative collapse discharges.…”

Experimental results from the TFTR tokamak

“…Near the gasfueled density limit, by carefully programming the current and density waveforms, it is possible to establish stable discharges with a cold radiating mantle surrounding the plasma that isolates the hot core from contact with the limiter for many energy confinement times (Strachan et al 1985). The stability of these discharges and the observation that the density limit is different for gas-fueled and pellet-fueled discharges suggests that the density limit is not merely defined by thermal stability due to the temperature dependence of impurity radiation as analyzed previously by Gibson (1976), Rebut and Greene (1977), Ashby andHughes (1981), ohyabu (1979), Roberts (1983), and Perkins and Hulse (1985), but is also affected by the fueling profile and plasma recycling.…”

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.…”