The ORNL Multicharged Ion Research Facility (MIRF)
IN-SITU ECR PLASMA POTENTIAL DETERMINATION
This document is for showing the experimental setup of the conference contribution submitted to PAC2005 [1]. The probe circuit, high voltage isolation and data acquisition via a wireless connection were described in detail.
High voltage isolation via a wireless connection
An electrical probe operation in an ECRIS is not technically easy because most ECRIS's are operated on high voltage platform from 5 kV to 30 KV, which is for providing extraction field for ion beam. It is almost impossible to operate the probe electronics (BOP, Oscilloscope or Labjack UE9...) on ground potential, if one can; it is not a good approach. It would be a better approach to operate them on the same potential with the ion source, but in this case a proper isolation has to be given between the measuring devices and a user.
In this experiment wireless Ethernet connections were used for the isolation. Figure 1 shows the schematic diagram of probe operation method by the wireless connection in the CAPRIS ECRIS. As shown the figure 1 both Labjack UE9 and TDS3034B oscilloscope were controlled by the laptop computer through the wireless connection: the Labjack UE9 [2] is for providing the current P/S with analog input signals (DAC1) and the HP function generator for digital signals (FIO0), and the TDS3034B oscilloscope [3] is for actually measuring the probe bias and current signals (V and I). Each device should have Ethernet 10Base-T interface or compatible wireless interface, and be linked to an appropriate Ethernet bridge or wireless LAN card. The filament heating P/S could be controlled by Labjack UE9 by sending its analog input signal (0 to 5V) to the input terminals of the P/S. AD210 isolation amplifier [4] was used to isolate the filament heating P/S from the Labjack UE9 analog signal. The Labjack UE9 also controls the HP function generator by making digital signals to trigger the function generator to generate the sawtooth signal, which is then fed into Kepco BOP 100-1M amplifier for sweeping the probe bias. The digital signal simultaneously goes to the oscilloscope, and triggers it. AD210 isolation amplifier was also used to measure the voltage drop across the measuring resistor in conventional emissive probe circuit. Additionally two switches (S1, S2) are installed to change the probe operation modes: Langmuir probe mode and emissive probe mode.
 Fig. 1 The probe circuits and its operation via a wireless connection in ORNL CAPRIS ECR ion source > |
Data processing and its automatic analysis
As shown figure 2 all control and data acquisition were interfaced with a virtual Labview instrument in a laptop computer. Because all these processes from the control to the analysis could be completed within a second, the data were simultaneously acquired, processed, and analyzed in real time.
The acquired data were first digitally processed through the use of modified Savitzky-Golay smoothing filter [5] in order to obtain a smoothed first derivative of the probe characteristic, where the maximum of the differentiation curve was taken as plasma potential. The maximum point is directly related to the apparatus function of the experiment and to the number of points used for the smoothing [6]. The resolution of the measurement, Rm, could be estimated as the product of the number of smoothing points, n and the voltage range of the sweep, dV, divided by the point resolution of the measurement, N:
Rm=ndV/N.
During the measurement Rm was minimized by adjusting the sweeping voltage range and smoothing points, and then maintained to make the value always less than 2/3Te, it is an accepted standard to consider the electron distribution to be reasonably accurate [7].
 Fig. 2 Automated data analysis by Labview virtual instrument. |
Here are the links for a few real time plasma potential measurements in CAPRIS ECR ion source:
- Introduction to the data acquisition and the pressure dependence of plasma potential,
- Power dependence of plasma potential,
- Axial magnetic field dependence of plasma potential.
There are a few phenomena still not understood, for example, second inflections (knees) over the electron saturation region in I-V curves. Several explanations could be attempted.
Any comment or discussion is welcome (H.J. You: factorial@ihanyang.ac.kr , F. W. Meyer: meyerfw@ornl.gov).
Reference
[1] H. J. You, Y. Liu and F. W. Meyer, Proceedings of the 21st Particle Accelerator Conference, Knoxville, 2005.
[5] F. Fujita and H. Yamazaki, Jap. J. Appl. Phys. 29, 2139 (1990).
[6] J.I.F. Palop, J. Ballesteros, V. Colomer, and M.A. ernandez, Rev. Sci. Insrum. 66, 4625(1995).
[7] V.A. Godyak, R.B. Piejak, and B.M. Alexandrovich, J. Appl. Phys. 73, 3657(1993).
Rev: Feb-2007 design: by H.J. You - updated by E. Galutschek