Discharge cleaning and wall conditioning in a Novillo Tokamak

Our Novillo Tokamak is a small toroidal device magnetically confined defined by the main design parameters: R_o=0.23 m, a_v=0.08 m, a_p=0.06 m, B_T=0.05-0.47 T, I_p=1-12 kA, n_e=1-2×10¹³ cm^-3, T_e=150 eV, T_i=50 eV. For the initial discharge chamber cleaning we have often used vacuum baking up to 100 °C and then conditioning using Taylor discharge cleaning (TDC) in H₂ and He. In this work we report that vacuum baking is effective for obtaining a final total pressure of the order of 1.6×10^-7 Torr. We have found that a single parameter, the performance parameter (PP), can be used to optimize the TDC method. This parameter represents the quantity of electron and ion energy incident on the chamber wall during the Taylor discharge, it is equal to (I_pτ), where I_p is the peak-to-peak plasma current and τ is the plasma current duration. In graphs of PP versus the gas pressure for different oscillator powers, the maximum value of PP indicates the best cleaning conditions when using TDC. The results of the vacuum chamber wall conditioning using this criterion are reported.