Description
It is well known that, in tokamak plasmas, electrons with an energy larger than a certain critical value will be continuously accelerated becoming runaway electrons (REs). Although REs are mostly created during disruptions, there is also concern about the initiation phase in future large tokamaks like ITER [1], which in order to succeed will require low enough prefill pressures, giving rise to low densities during the breakdown and burn-through phases, which can lead to RE generation. Therefore, it is important to be able to design the start-up of a tokamak discharge so that no REs can be produced, or at least in a small enough number so that they will not interfere with the build-up of the thermal plasma current or yield damage on the in-vessel components. The Connor critical electric field [2] is often used as a characteristic parameter determining the existence of REs, although it is known that due to energy losses other than collisions, as the electron synchrotron radiation, the threshold electric field for runaway generation can be usually substantially larger. Hence, in this paper, a parameter space analysis, based on the critical electric field due to radiation [3], allowing the identification of the non-runaway/runaway regions and providing information on the RE energy and generation, is used to track the plasma during the start-up phase of FTU discharges, a device in which REs electrons were routinely produced during plasma initiation and well diagnosed for RE detection. Ohmic as well as EC and LH discharges will be considered. In addition, the mechanisms determining the formation of the RE current during start-up in FTU are also discussed.
[1] P.C. de Vries et al., Nucl.Fusion {\bf 63}, 086016 (2023).
[2] J.W. Connor and R.J. Hastie, Nucl.Fusion {\bf 15} 415 (1975).
[3] J.R. Martin-Solis, R. Sanchez, and B. Esposito, Phys.Rev.Lett {\bf 105} 185002 (2010).
The authors wish to thank Prof. A. de Castro for invaluable support, and Dr M. Lehnen for his wise advice on the RE topic along these years.
This work was done under financial support from Ministerio de Ciencia, Innovaci\'on y Universidades (Spain),
Project No.PID2022-137869OB-I00.