ECR heating power modulation as a means to ease the overcoming of the radiation barrier in fusion devices
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ECR Heating Power Modulation as a Means to Ease the Overcoming of the Radiation Barrier in Fusion Devices D. Kh. Morozova, A. A. Pshenova, and A. B. Mineevb a
b
Russian Research Centre Kurchatov Institute, pl. Kurchatova 1, Moscow, 123182 Russia D.V. Efremov Scientific Research Institute of Electrophysical Apparatus, St. Petersburg, 196641 Russia Received February 10, 2009; in final form, May 25, 2009
Abstract—A method is proposed to ease the overcoming of the impurity radiation barrier during current drive in tokamaks, as well as in alternative fusion and plasmochemical systems with ECR plasma heating. The method is based on the fact that the dependence of the ionization rate on the electron temperature is strongly nonlinear and the dependence of the recombination rate on the latter is weaker. The result is that, during tem perature oscillations, the effective temperature for ionization–recombination processes is higher than that in a steady state, so the ionization equilibrium is shifted and strongly emitting ions are stripped more rapidly. Thereby, ECR plasma heating in the initial discharge stage can be made more efficient by modulating the heating power at a low frequency. The evolution of the electron temperature in a homogeneous hydrogen plasma with a carbon impurity and in small ISXscale tokamaks is simulated numerically, as well as the evo lution of the electron and ion temperatures and of the current during discharge startup in the ITER device. Numerical simulations of the effect of modulation of the ECR heating power on the rate of heating of nitro gen, oxygen, and argon plasmas were also carried out. The assumption of coronal equilibrium is not used. It is shown that the lowfrequency modulation of the heating power can substantially ease the overcoming of the radiation barrier. DOI: 10.1134/S1063780X10060012
1. INTRODUCTION For successful ignition of tokamak plasma dis charges, it is necessary to overcome the ionization and radiation barriers [1]. The radiation barrier (socalled “impurity burnthrough”) arises because the intensity of emission from light impurities (such as C and O) has a high maximum in the temperature range below 100 eV. Calculations show that, if the power fed into a tokamak is lower than a certain critical value associ ated with the volume impurity emission, then the vol umeaveraged plasma temperature will not exceed the value corresponding to the maximum and will fail to attain the required level. Hence, it is possible to speak of the existence of a radiation barrier. In small devices, radiation barriers are usually over come by increasing the loop voltage. For large, ITER scale devices, however, this way is inefficient because of both technical restrictions (a breakdown in the insulation of the central solenoid) and the undesirable possibility that the plasma electrons may become run away electrons. As a result, in ITER, the loop voltage is limited to a value of 14 V (in which case the strength of the vortex electric field is approximately Е ≈ 0.3 V/m). The loo
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