Combined microwave and laser Rayleigh scattering diagnostics for pin-to-pin nanosecond discharges

Abstract

In this work, the temporal decay of electrons produced by an atmospheric pin-to-pin nanosecond discharge operating in the spark regime was measured via a combination of microwave Rayleigh scattering (MRS) and laser Rayleigh scattering (LRS). Due to the initial energy deposition of the nanosecond pulse, a variation in the local gas density occurs on the timescale of electron decay. Thus, the assumption of a constant collisional frequency is no longer applicable when electron number data are extracted from MRS measurements. To recalibrate MRS measurements throughout the electron decay period, temporally resolved LRS measurements of the local gas density were performed over the event duration. The local gas density was calculated to be 30% of the ambient level during the later stages of electron decay, and it recovers at about 1 ms after discharge. A shock front traveling approximately 500 m/s was additionally observed. Coupled with plasma volume calibration via temporally resolved intensified charge-coupled device imaging, the corrected decay curves of the electron number and electron number density are presented with a measured peak electron number density of 4.5 × 1015 cm−3 and a decay rate of ∼(0.1–0.35) × 107 s−1. A hybrid MRS and LRS diagnostic technique can be applied for a broad spectrum of atmospheric-pressure microplasmas where a variation in the gas number density is expected due to energy deposition in the discharge.

Experimental study of atmospheric pressure single-pulse nanosecond discharge in pin-to-pin configuration

“…Even though the LRS signal overlaps with Thomson and Raman scattering, the contribution of Rayleigh scattering was dominant for the experimental conditions in this work when ne < 10 16 cm -3 as described in Ref. [43]. Scattered laser signals were recorded for a range of times (100 ns to 1 ms) after the discharge.…”

“…More details about the necessity to account for ng variation from the unperturbed atmospheric level due to the ns-discharge and validation of combined MRS-LRS diagnostics can be found elsewhere. 43 A calibration of the MRS system was conducted with a Teflon cylinder (1/8'' diameter, 1 cm length) to determine the coefficient A based on the bottom expression in the Equation (1). The calibration coefficient A was then applied to the ns-discharge plasma using top expression in Equation ( 1).…”

“…Interpretation of the measured scattered signal may be affected by dissociation of oxygen. [53][54][55][56] Considering an extreme case of complete oxygen dissociation, one may expect LRS measurement error of about 15%, 43 and, in this work, we have accounted for the possibility of oxygen dissociation via corresponding error bars. One should expect a substantially lower dissociation degree under the conditions of our experiments given that only 30% dissociation was reported for ns-discharges at significantly larger discharge pulse energy of 20 mJ.…”

“…The results are calculated using Equation ( 1) with the input data of Uout, ν, and V obtained using the procedure described in Ref. [43]. The collisional frequency ν was determined via local gas density ng measured by LRS.…”

Electron momentum-transfer collision frequency measurements in small plasma objects via coherent microwave scattering

“…Advantages of CMS over other diagnostic techniques (microwave and laser interferometry, time-of-flight mass spectrometry, laser Thomson scattering, Langmuir probes, etc [2]) often include the capability of time gating due to continuous scanning, the possibility of singleshot acquisition, non-intrusive probing, low shot noise, good temporal resolution (<1 ns), high sensitivity, and simple absolute calibration of the system using a dielectric scattering sample with known properties [3]. These advantages enable both calibrated and uncalibrated use of the diagnostic in applications ranging from photoionization and electron-loss rate measurements [2][3][4][5][6][7][8][9][10][11][12] to trace-species detection/mixture characterization [13][14][15].…”