Detection of RF Perturbations Using an Ion Beam Diagnostic

Abstract

Description:A Heavy Ion Beam Probe, HIBP, has been installed on a helicon plasma device. The objective was to measure plasma fluctuations at the 13.55MHz RF frequency. This offers a unique challenge for the HIBP, because the transit time of the probing ion is long compared to the fluctuations of interest. For previous HIBPs, the transit time has been short compared to the period of the fluctuations which permits one to assume that the magnetic and electric fields are static. Modeling has shown that the diagnostic will still accurately measure the average potential. The fluctuating potential was to be detected but the absolute magnitude is difficult to determine with signal from a single point. However, modeling indicates multipoint measurements will allow one to resolve the absolute fluctuation magnitude. Work supported by DOE Gant No. DE-FG02-99ER5452985 Results:During the funding of this grant, a helicon plasma discharge device was built and operated. A Heavy Ion Beam Probe primary system was installed and operated. A primary beam detector was installed and primary beam was detected both with and without plasma. Attempts were made to detect secondary ions using the primary beam detector, without success. Given the lack of a detectable signal, the energy analyzer of 2 the HIBP system was never installed. It is available for installation if there is a reason to do so in the future.Analysis of the system indicated that the plasma electron temperature, estimated to be a few eV, was the likely reason for the lack of detectable secondary ions. A change of ion species to either Boron or Magnesium would greatly increase the signal, but neither of these ions have been used in a HIBP system. The ion source used in this system is made by using a charge exchange process to create a zeolite loaded with the desired ion.Attempts were made to use charge exchange to load Magnesium into a zeolite, and were not successful. It is felt that Magnesium and/or Boron zeolite sources could be created, but time and funds were exhausted.Modeling was done to show what could be done if signal had been detected. As originally stated in the proposal, the issue to be addressed is that the ion transit time is long compared to a cycle time of the RF drive signal, or long compared to the time scale of the fluctuations of interest. In other words, the system would be able to make the measurement proposed. There would also be a significant path effect which may be difficult to eliminate. Additional work is required to use simultaneous multipoint measurements to separate the local and path effects.The conclusions of the work are: 1) HIBP measurements on such a low temperature plasma require a change in the probing ion.3 2) Even though the standard electro-static approximation used with HIBP systems isn't valid for high frequency fluctuations, the change in energy of the secondary ions is strongly related to the plasma potential at the ionization location. Background -why is there interest in HIBP measurements of high frequency