Investigation of current on the conducting target biased with a large negative potential in the non-uniform plasma

It was investigated the current on a biased target in the non-uniform plasma. The argon plasma was generated at the low pressure of 0.5-5 mTorr and the stainless steel target was biased with 0.2-6kV negatively. The target current increases with increasing the target voltage due to the non-uniform plasma distributed near the target, which follows the Child law. The secondary emission current, following the square root of target voltage, is superposed to the ion current. For the higher pressures of larger than 1.5 mTorr, the target current is larger than the expected current of Bohm current plus the secondary emission current. The deviation increases drastically with increasing the operating pressure, which may due to the local ionization near the target. This phenomenon is important to Understand more accurately the high voltage sheath formation in practices.