Speaker
Description
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 by 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 associated with fast turn off and turn on of HV system. The breakdowns cause generation of HV transients which in turn disturb the entire signal reference system.
Such operational environment poses challenges for front end electronics design for low voltage signals like the one from thermocouple sensors, which are 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 [1,2] is therefore needed to arrest the noise sources and provide clean signals for 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 reducing 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 [3] RF based negative ion source test bed. The source operates at 1 MHz RF and with HV power supplies of rating 11 kV 35 A for extraction and 35 kV 15 A for acceleration for which 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.
References:
[1] Henry Ott, Noise Reduction Techniques, Wiley
[2] L.K. Bansal, et. al.,” Noise Mitigation in Thermocouple Signal Conditioning System for Neutral Beam Calorimeter for NBI SST-1”, IEEE Trans on Plasma Sciences, Volume: 42, Issue: 6, June 2014
[3] K Pandya et. al., “First results from negative ion beam extraction in ROBIN in surface mode”, AIP Conference proceedings 1869 (2017), p. 030009
Summary
The abstract describes the techniques used to reduce the mitigate the effect of RF radiation on thermocouple signals.
