Download Design and development of LANGMUIR PROBE CIRCUIT

DESIGN AND DEVELOPMENT OF LANGMUIR PROBE CIRCUIT FOR
ONLINE MEASURMENT OF PLASMA PARAMETER IN
ECR ION SOURCE.
H.Kewlani#, P.Roychowdhury, L.Mishra, S.H. Gharat, Dr. K.C Mittal,
APPD, BARC, Mumbai.
Abstract
The Low Energy High Power Proton Accelerator
(LEHIPA) is under development at BARC. The high
current Electron Cyclotron Resonance (ECR) ion source
is the first component of LEHIPA. In the ion source the
high density plasma is generated to extract tens of mA of
ion beam. The properties of ion beam depend on the
plasma parameters. An Automated Langmuir Probe (ALP)
circuit has been design and developed to measure the
plasma parameters. The plasma parameters of interest are
the following (i) electron / ion density (ne), (ii) electron
temperature (Te) and (iii) plasma potential (Vp). The ALP
circuit was used to characterize the steady state and pulse
ECR plasma. A voltage sweep pulse of –100 V to + 100 V
amplitude and 250 μs to 100 ms duration is applied to
langmuir probe. The probe I–V characteristics is recorded
on DSO and stored in computer. One can very voltage
sweep amplitude range and pulse duration range as per
requirement.
Introduction
ECR proton ion source of 50 keV, 30 mA has been
developed for the LEHIPA [1]. The ion beam current
of 25 mA has been extracted at 25 keV of beam
energy. The three electrode extraction geometry has been
used for ion extraction [2]. In Fig 1 snapshot of ECR ion
source is shown.
of electric current I to an electrical conductor inserted in a
plasma as a function of its potential V [3][6]. The I-V
characteristic of langmuir probe is shown in Figure 3.
Figure 2: Langmuir probe diagnostics
Langmuir probe I-V characteristics
The typical probe current voltage characteristic
is shown in Figure 3. The shape of such curve can be
explained as follows. For the large negative potentials
(relative to the plasma potential) all the positive ions are
collected by the probe and electrons are repelled. This
current is ion saturation current (IIon). The current IIon >>
Ie in this region. With the increase of bias voltage (VBias)
the absolute value of Ie increases and total current(Ip) to
the probe decreases. At some point current through probe
becomes zero, which means that | IIon | = | Ie |. The
potential at this point is called the floating potential Vf.
After floting point as bias voltage increase the
contribution of the ion current to the total probe current is
negligible and probe collects only electrons. At one point
there is a more or less sharp break of current voltage
characteristic. This inflection point of the curve
corresponds to the plasma potential Vp and all the
electrons are collected. Further increase of the probe
voltage leads to saturation of the electron current.
Automated Langmuir Probe Operation
Figure 1: ECR ION SOURCE
Langmuir Probe
Langmuir probe [5] is a small metallic electrode
(Figure 3), usually a wire, inserted into plasma Fig.2. The
probe is attached to a power supply, capable of biasing it
at various voltages positive and negative relative to the
plasma. The probe method is based on the measurement
Experimental setup:
Experimental setup of ALP is shown in Figure 4.
Langmuir probe is inserted 5 mm inside plasma chamber
surface location; other parameters of experiment like
probe tip length is 1 mm and probe tip diameter is 0.5
mm.
Figure 3: Langmuir probe I-V characteristics and langmuir probe
Circuit description:
ALP setup is shown in figure 4. Voltage sweep of 110 V
/110 ms generated using microcontroller [4] and Kepko
Opam; which is applied to the langmuir probe. Langmuir
probe is inserted inside the plasma chamber as discussed
above. Langmuir probe signal is applied to tungsten tip
and langmuir probe ground is connected to plasma
chamber. Langmuir probe current is detected using
current shunt of 472 Ω, 0.5 Watt. Shunt resister is
connected in signal path to avoid stray capacitance effect
which is present in ground path. To avoid the line
frequency pickups shielding and isolation is provided to
the shunt resister. Applied voltage sweep to langmuir
probe and voltage across shunt resister is observed using
Digital Storage Oscilloscope (DSO). Output waveforms
are shown in figure 5 and 6. Voltage and current data of
langmuir probe are taken from DSO which is in the .CSV
file format.
Figure 4: Langmuir Probe Experimental setup
shorter than pulsed plasma duration). Generated voltage
sweep was applied to Langmuir probe and I-V
characteristic has been observed and recorded using
digital storage oscilloscope which is shown in figure 8.
Plasma parameters were deduced from I-V characteristic.
Plasma parameters for hydrogen plasma were electron
temperature 8-10 eV, electron density 1x1011 - 4x1011 per
cm3 and plasma potential 10-15 V.
Figure 5: Voltage sweep and probe current on DSO.
Result
Plasma parameters were deduced from I-V
characteristic figure 6. Plasma parameters for hydrogen
plasma were electron temperature 8-10 eV, electron
density 1x1011 - 4x1011 per cm3 and plasma potential 1015 V.
Figure 8: ALP with pulsed plasma waveform
Acknowledgment
The author wish to thank Dr. L. M. Gantayat, Director,
BTDG, BARC for his keen interest and support for this
work. .
Figure 6: Langmuir probe I-V characteristic
ALP with pulse ECR Plasma
Figure 7: ALP with pulse ECR Plasma
A setup was made to characterize plasma in
pulsed mode operation which is shown in figure 7.
Trigger pulse generator was used to synchronize ALP
with pulsed plasma. Trigger pulse initiate pulsed
magnetron to generate pulsed microwave of 1 second
total time (T) with Ton of 1 milisecond. Simultaneously
with the rising edge of trigger pulse ALP generate voltage
sweep of -100 V to +50 V with 500µs duration (which is
References
[1] P. Singh, Proceedings of the Indian Particle Accelerator
Conference
[2] P. Roychowdhury and D. P. Chakravarthy, Rev. Sci.
Instrum. 80, 123305 (2009) “High intensity electron
cyclotron resonance proton source for low energy high
intensity proton accelerator”.
[3] F.F. Chen 1965, American press inc, “Plasma diagnostic
techniques”.
[4] A. D. Cheetham ,L. Davidson, J. Jakobsen, T. Lund and J.
P. Rayner,Rev Sci. Instrum. 68 (9), September 1997,
“Stand-alone microprocessor controlled fast sweep
Langmuir probe driver”
[5] “Collected works of Irving Langmuir ” Vol 4 , Phys
Review, New York ,1926.
[6] P. Roychowdhury, H. Kewlani, L. Mishra, D. S. Patil, and
K. C. Mittal; Rev. Sci. Instrum. 84, 073303 (2013);
“Langmuir probe diagnostics of plasma in high current
electron cyclotron resonance proton ion source.”