Excited by 5 ns, 8 kV voltage pulses, a 260 μm-diameter, 8 mm long helium plasma jet was generated with a single-electrode configuration in ambient air. Application of fast high voltage pulses (≥1012 V s−1) resulted in rapid acceleration of the microplasma plumes; within 5 ns the plume velocity reached 8 × 105 m/s, almost three times higher than that of the plasma jet generated with the pulsed voltage of the same amplitude but with a lower increase rate (1011 V s−1). Importantly, the ultrashort electric pulses were able to efficiently deposit energy in the plasma during the initiation process, which may be responsible for the rapid acceleration of the ionization wavefronts during the streamer onset, as well as efficient production of reactive plasma species including O(5P) and N2+(B2Σu+) via electron-induced processes. Emission spectral comparison between the plasma jets excited with 5 ns voltage pulses and with 140 ns voltage pulses showed enhanced O(5P) and N2+(B2Σu+) emission by the shorter pulses than the longer ones, while the vibrational and rotational temperature for both plasma jets are at 3000 K and 300 K, respectively.

Dosage control of reactive oxygen and nitrogen species (RONS) is critical to low-temperature plasma applications in cancer therapy. Production of RONS by atmospheric pressure, nonequilibrium plasmas in contact with liquid may be modulated via plasma conditions including plasma treatment time and pulse voltage and repetition frequency. In this study, a terephthalic acid-based probe was used to measure hydroxyl radicals [OH(aq)] in water exposed to plasma and to demonstrate that the OH(aq) concentration increases linearly with treatment time. Fluorometric measurements of hydrogen peroxide concentration in plasma-activated water show a linear relationship between the H2O2 production rate and the pulse repetition frequency of the plasma. In vitro plasma treatment of cancer cells shows that pancreatic (Pan02) and breast (4T1-Luc) cancer cells have different sensitivities to plasma exposure. The dependence of Pan02 cell viability on plasma treatment time or pulse voltage is nonlinear. The system described here for generation and delivery of reactive oxygen species from a nanosecond pulsed plasma jet represents a promising alternative approach to cancer therapy.

The discharge characteristics and mechanism of glow discharges in atmospheric pressure helium excited by repetitive voltage pulses with and without dielectric barriers are numerically studied using a one-dimensional self-consistent fluid model. The waveforms of discharge current density show that one discharge event occurs during the voltage pulse with bare electrodes and two distinct discharge events happen at the rising and falling phases of voltage pulse with dielectric barrier electrodes, respectively. The spatial profiles of electron and electric field at the time instant of discharge current peak reveal that the electrons are trapped in the plasma bulk with bare electrodes, while the electrons are accumulated in the region between the sheath and plasma bulk with dielectric barrier electrodes. Furthermore, the spatio-temporal evolution of electron density and mean electron energy clearly demonstrate the dynamics of discharge ignition, especially the temporal evolution of sheath above the instantaneous cathode. V

Applications of atmospheric pressure non-equilibrium plasma jets for a variety of biomedical applications have motivated fundamental studies of the gas-phase plasma in the presence of water. The initiation of guided streamers, excited with 164 ns, 5.2 kV pulses, as well as the dynamics of the streamer were investigated using a micro-hollow needle-to-plate configuration with and without water. Addition of water to the ground plate results in the enhanced ionization at the needle anode, earlier streamer formation, greater total energy deposition, and stronger emissions by excited states including N2, N2+, OH, and O. Importantly, spatiotemporally-resolved emission spectroscopy reveals that excitation transfer via collisions with heavy particles played an important role in the production of OH(A2Σ) in addition to direct electron impact.

Nanosecond pulsed helium plasma jets impinging on water produce hydroxyl radicals both in gas- and liquid-phase. In this study, the effects of pulse width on a repetitively pulsed plasma jet in contact with water are evaluated via OH(A–X) emission and OHaq production in water for various pulse widths ranging from 200 to 5000 ns. The maximal energy efficiency of OH(A–X) emission is obtained for pulse widths of 600–800 ns whereas the maximal efficiency of OHaq production is at 200 ns. Temporally-resolved emission spectroscopy shows that more than 40% of OH(A–X) emission is produced during the first 200 ns of the voltage pulse regardless of the pulse width. An equivalent circuit model of the plasma jet impinging on water is compiled to understand the charge transfer process, which is important for OHaq production via charge exchange reactions.