Radio frequency plasma production on the TOMAS device

The study of discharge initiation by an Ion Cyclotron Range of Frequency (ICRF) antenna is of importance for operational scenarios at low magnetic field in e.g., ITER and Wendelstein 7-X. At low magnetic fields ohmic breakdown is difficult because of relatively short connection lengths and the electron cyclotron (EC) systems cannot assist because of a low-density cut-off. In that case ion cyclotron (IC) start-up, with waves in the Radio Frequency (RF) wavelength range, can be a good alternative solution. To explore in more detail the processes at play during the initial breakdown phase and to test measurement requirements, the diagnostic's capabilities of the TOMAS experimental device are being enlarged to allow for IC breakdown studies. A first set of IC breakdown experiments has been performed and data was taken by a newly installed spectrometer and a horizontal triple Langmuir probe. Limitations were found in pressure, ICRF power, and hydrogen concentration. Further diagnostic's upgrades and extensions of the experimental sessions will allow for a more complete study in support of modelling and experiments on other machines.