Noise mitigation techniques in thermocouple signals in negative ion sources with RF and HV transients

Abstract

Negative ion-based inductively coupled plasma sources operate in a high RF power and HV environment for plasma production and beam transport. Due to plasma power coupling dynamics, RF power mismatch causes large reflected fields which affect all the diagnostic signals, degrading the signal to noise ratio. In addition to RF disturbances, during the beam extraction and acceleration, the diagnostic signals are also prone to suffer during HV breakdowns due to high dV/dt fast switching of the HV system. The breakdowns cause the generation of HV transients which in turn disturb the entire signal referencing system. Such an operational environment poses challenges for front end electronics design for low voltage signals like the one from thermocouple sensors, which are some of the most important diagnostic elements in such sources. The surface-mounted thermocouples referenced to floating potential pick up noise from HV transients and RF noise. A signal conditioning system is therefore needed to arrest the noise sources and provide clean signals for the acquisition and control system. Such signal conditioning needs specific RF filters and PCB design. Special attention is also required for shielding and grounding to help reduce noise interference. The present work discusses the measurements in light of the improvements made to the front end electronics for the thermocouples used on the ROBIN RF-based negative ion source test bed. The source operates at 1 MHz RF and with HV power supplies rated at 11 kV 35 A for extraction and 35 kV 15 A for acceleration. Mismatch fields of the order of 90 V/m have been observed. The overall signal chain from field to presentation layer shall be presented with the measures undertaken to solve the noise interference.