A new structure for RF-compensated Langmuir probes with external filters tunable in the absence of plasma

Abstract

A new Langmuir probe structure using externally placed filters that can be tuned in the absence of plasma is proposed. The probe design and tuning procedure take into account especially the change in the probe's environment when plasma is turned on, thereby ensuring that the filters do not become detuned in the presence of plasma. Measurement of the RF voltage amplitudes in RF plasma using a calibrated capacitive probe gave, respectively, ≈15.7 V, ≈4.1 V, ≈2.1 V and ≈0.5 V at the fundamental frequency (≈13.56 MHz), the second, third and the fourth harmonics; based on these values a three-stage filter was built at the fundamental and the second and third harmonics. A complete analysis of the probe including its stray capacitance, RF equivalent circuit, filter and plasma impedance has been carried out, from which the maximum RF sheath voltage drop could be estimated as ≈0.27 V, ≈0.34 V and ≈1.30 V at the fundamental, the second harmonic and the third harmonic, respectively; the drop for the latter is somewhat large because of unexpectedly high loss in the filter components at the higher frequencies. I -V characteristics presented show that the floating potential of the probe decreases by ≈60 V, as the probe is detuned progressively from its tuned condition; also, the electron temperature increases from ≈1.7 to 3.5 eV with progressive detuning. It is worth noting here that although the method of calibrating the capacitive probe in this work is accurate for moderately high plasma densities (RF skin depth in plasma (= δ s ) small in comparison with the plasma dimension, L) its accuracy for lower densities (δ s ∼ L) is not too certain. Therefore, although the probe itself can be used at low plasma densities, verifying its efficacy for such cases could be difficult.

Evidence of current free double layer in high density helicon discharge

“…In order to be sure of the reliability of the LP data in the present experiments, we use an RF compensated LP 55 where we have presented a new probe structure and its tuning procedure. RF voltage amplitudes measurement in RF plasma using a calibrated capacitive probe 55 shows % 15.7 V, % 4.1 V, % 2.1 V, and % 0.5 V at the fundamental frequency (% 13.56 MHz), the second, third, and the fourth harmonics. Based on these values, we have made a three stage filter at the fundamental, and the second and third harmonics.…”

“…For instance, it allows one to determine warm electron density, n w and warm electron temperature, T w (if present) directly along with the other plasma parameters, permitting one to generate profiles for these quantities, which makes a simultaneous comparison of all variables relatively easy. It may be noted that RF LP measurements were quite valuable in authors' previous work 49,50,55 for measuring the plasma parameters. LP analysis procedure is efficient to determine the bulk and warm electron populations.…”

Experimental investigation of current free double layers in helicon plasmas

“…It is these high-energy electrons, which need to be confined magnetically using the designed mirror field so that their presence can be detected in the Langmuir probe measurements. [30][31][32][33] To produce the axial magnetic field (Fig. 1), a set of seven electromagnet coils (M/s Tondon and Co., New Delhi) of bore diameter % 26 cm position coaxially over the plasma chamber.…”

“…In order to be sure of the reliability of the LP data in the present experiments, we use an RF compensated LP 33 where we have presented a new probe structure and its tuning procedure. RF voltage amplitudes measurement in RF plasma using a calibrated capacitive probe shows, % 15.7 V, % 4.1 V, % 2.1 V, and % 0.5 V at the fundamental frequency (% 13.56 MHz), the second, third, and the fourth harmonics.…”

“…The details of LP data acquisition and robustness and accuracy of the LP data measurement; and LP analysis of the acquired data is presented elsewhere. [30][31][32][33] Fig. 2 is one of the representative LP data taken on axis, showing the various electron populations at axial location z % 6.5 cm in the axial profiles to be discussed below in Fig.…”