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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.”