Biswa N. Ganguly

A streamer-like atmospheric pressure plasma jet

The properties of an atmospheric pressure plasma jet (APPJ) are examined in a single-cell dielectric capillary configuration. In contrast to some other flow-driven APPJs, this stable, cold plasma jet is electrically driven, composed of rapidly propagating ionization fronts with speeds of the order of 107cm∕s. Using spatially and temporally resolved optical diagnostics, it is demonstrated that the plasma jet is initiated independent of the dielectric barrier discharge inside the capillary. It is also shown that the properties and dynamics of this APPJ are directly analogous to those of positive corona streamer discharges.

Investigation of discharge mechanisms in helium plasma jet at atmospheric pressure by laser spectroscopic measurements

We have measured spatiotemporal structures of excited species by laser spectroscopic methods in a plasma jet, which was driven by a bipolar impulse voltage pulse train of the order of kilohertz repetition rate applied across a pair of electrodes wrapped around a glass tube with a helium gas flow. We noticed the differences between the positive and the negative phases of the voltage applied to the front-side electrode placed closer to the tube exit while the back-side electrode was grounded. The experimental results showed that the radial distribution of the excited species had a hollow shape at the centre in the positive voltage phase, while it had a more uniform shape in the negative phase. The peak density of the helium metastable atom in the positive phase was almost constant irrespective of the peak applied voltage. However, it increased with the increase in the peak applied voltage in the negative phase. The mechanism causing these differences was argued from the respects of positive and negative corona discharges. We have also investigated the property of the plasma plume under conditions similar to material processing with a conductive substrate placed in front of the plasma jet. In this case, the plasma production by electron impact ionization became dominant near the substrate as was revealed from the spatiotemporal distributions of helium metastable atom and nitrogen ion densities.

Comparison of high-voltage ac and pulsed operation of a surface dielectric barrier discharge

A surface dielectric barrier discharge (DBD) in atmospheric pressure air was excited either by low frequency (0.3–2 kHz) high-voltage ac or by short, high-voltage pulses at repetition rates from 50 to 600 pulses s−1. The short-pulse excited discharge was more diffuse and did not have the pronounced bright multiple cathode spots observed in the ac excited discharge. The discharge voltage, current and average power deposited into the discharge were calculated for both types of excitation. As a measure of plasma-chemical efficiency, the ozone number density was measured by UV absorption as a function of average deposited power. The density of ozone produced by ac excitation did not increase so rapidly as that produced by short-pulse excitation as a function of average power, with a maximum measured density of ~3 × 1015 cm−3 at 25 W. The maximum ozone production achieved by short-pulse excitation was ~8.5 × 1015 cm−3 at 20 W, which was four times greater than that achieved by ac excitation at the same power level.

4H–SiC photoconductive switching devices for use in high-power applications

Silicon carbide is a wide-band-gap semiconductor suitable for high-power high-voltage devices and it has excellent properties for use in photoconductive semiconductor switches (PCSSs). PCSS were fabricated as planar structures on high-resistivity 4H–SiC and tested at dc bias voltages up to 1000 V. The typical maximum photocurrent of the device at 1000 V was about 49.4 A. The average on-state resistance and the ratio of on-state to off-state currents were about 20 Ω and 3×1011, respectively. Photoconductivity pulse widths for all applied voltages were 8–10 ns. These excellent results are due in part to the removal of the surface damage by high-temperature H2 etching and surface preparation. Atomic force microscopy images revealed that very good surface morphology, atomic layer flatness, and large step width were achieved.

Behavior of N2+ Ions in He Microplasma Jet at Atmospheric Pressure Measured by Laser Induced Fluorescence Spectroscopy

The behavior of N2+ ions in a low-frequency driven atmospheric pressure He plasma jet effused into ambient air was analyzed from laser induced fluorescence (LIF) spectroscopy measurements. The gas temperature derived from the rotational distribution was kept near room temperature and the drift velocity of N2+ ions estimated from the line shape was almost zero as compared to the apparent speed of the plasma bunch given by the spatiotemporal intensity profile. This shows that the mechanism of moving plasma bunches can be attributed to the ionization wave propagation similar to the streamer in positive corona discharge.

Spatiotemporally resolved Ar (1s5) metastable measurements in a streamer-like He/Ar atmospheric pressure plasma jet

We have applied diode laser absorption spectroscopy to measure the line-integrated Ar (1s5) metastable column density in a streamer-like atmospheric pressure plasma jet with a 5% Ar to He gas flow driven by a 20?ns rise time, positive unipolar voltage pulse. The stability of this source enabled us to isolate Ar (1s5) production separately from the streamer head, a region of enhanced production near the anode, and renewed production in the residual streamer channel, which continued up to 1??s after the discharge was initiated. Over the first 4?mm from the capillary tip, Ar (1s5) production was dominated by processes occurring behind the streamer head with a combined column density as high as 1012?cm?2. Where the residual streamer channel production was significant, the Ar (1s5) column density remained > 1011?cm?2 up to 10??s after the discharge was initiated. This timescale is significantly longer than has been reported to date of He (23S1) measured in a pure He plasma jet and Ar (1s5) in a pure Ar plasma jet using different configurations. The longer effective lifetime of Ar (1s5) in the He/Ar plasma jet significantly extended the timescale for afterglow plasma chemistry.

Rydberg state Stark spectroscopic measurement of electric‐field profile in a glow discharge

The electric‐field profile in the cathode‐fall region of a low‐pressure helium gas discharge has been measured by high‐level Rydberg state Stark spectroscopy using optogalvanic detection. The Stark manifolds of triplet atomic helium Rydberg states with principal quantum numbers n=15 up to 20 have been used to measure the electric‐field vector using both Δmj=0 and ‖Δmj‖=1 laser polarizations. The absolute accuracy of the electric‐field measurement was found to be 5% or better. The ‖Δmj‖=1 polarization Stark spectra exhibit pseudohydrogenic behavior in Stark splitting, as well as in the manifold intensity distribution. The Stark splitting of states corresponding to 1>4 was almost hydrogenic for all the measured principal quantum‐number states and the intensity distribution became approximately hydrogenic for n≥16 at electric fields ≤700 V/cm.

Hydroxyl (OH) distributions and temperature profiles in a premixed propane flame obtained by laser deflection techniques

OH-concentration distributions and temperature profiles have been measured on a premixed propane–air flame by laser deflection techniques. Photothermal deflection spectroscopy has been utilized for the measurement of the OH radical. Both a low-spatial-resolution (near collinear) and high-spatial-resolution (crossed-beam) scheme were used to profile the premixed flame. An optoacoustic deflection technique was utilized for thermometry. Both average-temperature profiles and probability distribution functions were determined by this technique. A comparison with data obtained by the CARS technique demonstrated that no significant flame perturbation was occurring.

Diode laser absorption measurements of metastable helium in glow discharges

A laser-based technique for accurate measurement of the population density profiles of the helium metastable in a parallel-plate, glow discharge is described. The system utilizes a distributed-Bragg-reflector diode laser operating at 1083 nm to perform absorption measurements on the transitions of helium. The narrow bandwidth (<3 MHz) of the laser permits simultaneous measurement of the line-integrated metastable density and gas temperature. Axial and radial distributions of the triplet line-integrated metastable density were measured in normal and abnormal glow discharges. Current densities ranging from (normal glow) to (abnormal glow) were used at static gas pressures of 2 and 5 Torr. Abel inversion of the data provided a spatial distribution of the metastable density that shows a region that has near one-dimensional character. The location of the peak triplet metastable density in the plasma was displaced from the peak of the negative glow emission towards the anode. Axial profiles of triplet metastable density gave values from to for current densities of 8 to , respectively.

Plasma excitation dependence on voltage slew rates in 10–200 Torr argon–nitrogen gas mixture DBD

An argon–nitrogen gas mixture dielectric barrier discharge (DBD) excited by fast rise time (~20 ns) and slow rise time (~100 ns) 3–5 kV high-voltage pulses was investigated in the pressure range 10–200 Torr. Time resolved emission intensities of Ar (2p1–1s2), Ar (2p9–1s5), N2 (C 3Πu–B 3Πg) and were recorded while the applied voltage was kept constant during measurements. The influence of different applied voltages and nitrogen concentrations on the observed emission spectra was also obtained. The reduced electric field, E/n, was derived from the ratio of the measured emission intensities by comparing excitation rates calculated using the BOLSIG+ code. The corresponding electron excitation energies, , were obtained. For the fast-pulse excited DBD, was in the range 6–8 eV and it decreased by about 0.5 eV for the lower voltage slew rate excitation under the same experimental conditions. The ionization rates were also derived from the corresponding E/n. The pulsed DBD plasma processes with different voltage slew rates were analyzed based on the dependence of the observed emission spectra and on the discharge operating parameters. The discharge voltage, current and power deposition were also estimated from applied voltage and current measurements.