25B06
High-Voltage Nanosecond Pulse Discharge in a Low-Pressure PreionizedMediumMikhail Pustylnik1, Lujing Hou1, Alexei Ivlev1, Leonid Vasilyak2, Hubertus Thomas1, Gregor
Morfill1, Vladimir Fortov2
Max-Planck-Institut für Extraterrestrische Physik,
Giessenbachstr. 1, 85741 Garching, Germany
2
Joint Institute for High Temperatures, Russian Academy of Sciences,Izhorskaya 13/19, 125412 Moscow, Russia
1

A disharge, created by superposition of a steady-state RF electric field and single high-voltage pulses, isbeing studied. RF field sustains a capacitively coupled weakly-ionized low-pressure plasma, to which thehigh-voltage pulses are applied. It is demonstrated, that this discharge evolves through two main phases. In
the first phase strong electric field of the pulse sweeps significant part of the plasma electrons out of thedischarge gap and the residual quasi-stationary electric field of the ion cloud gives rise to the rapid increaseof plasma density and intensity of the emission. In the second phase the discharge becomes dark due to thescreening of the RF electric field by the high-density plasm, formed in the first phase. Different physicalmechanisms, underlying the observed phenomena, are discussed

1. Introduction
Interaction of short (tens ns) high-voltage (kVto tens of kV) pulses with the weakly ionized
low-pressure plasma of RF discharge isinvestigated. Such pulses are at present a very
common tool in the low-temperature plasma
physics and are used for e.g. fast ignition of thereactive gas mixtures [1] or in DBD discharges inplasma medicine [2]. They were also used tomanipulate the microparticles, levitating in DC[3] and RF discharges [4].
2. Experimental setup
The experiments are performed in a modified
GEC reference cell (Fig.1). Two disc-shaped
electrodes of 15 cm diameter are separated by a 5cm gap. Top electrode is grounded via a 50 Ωresistor. To the bottom electrode sinusoidal RF
signal (13.56 MHz, 50-100 V peak-to-peak),sustaining a steady-state discharge, is applied. In
addition, to the top electrode single pulses of 20 ns
duration and 1-17 kV amplitude are coupled.
Experiments are performed in argon in the pressure
range of 0.07-3 Pa. The discharge was characterizedboth electrically (by measuring RF voltage andcurrent) and optically (by observing its image with
the ICCD videocamera).
3. Simulations
We performed 1D3V particle-in-cell simulations,based on the code from [5] according to thefollowing procedure: fisrt a steady-state RF
discharge was obtained, then a high-voltage pulse
was applied to the discharge gap and evolution of
the plasma was tracked. RF voltage was kept

pulse generator
vacuumchamber

insulator
electrodes
rf-generator
Fig. 1. Scheme of the experimental setup
constant all through the process.
4. Results
Both experiment and simulation exhibit similar
evolution of the plasma after the pulse. During thepulse majority of the electrons are swept away fromthe discharge gap by the strong electric field of thepulse. After removing the pulse voltage thedischarge is restarted due to the high electric field,
created by the ions, which are practically notaffected by the pulse. Within several hundred ns theexcessive ions leave the discharge gap.

pulse20 ns[ns]

Fig.2. Space-time diagram of an overshoot,following the nanopulse
Quasineutral plasma with the density much higher,
than that of the steady-state discharge, is formed.
This results also in the rapid increase of thebrightness of the discharge. In several µs thedischarge enters a dark phase, where the plasma
density is still high, but the average electron energy
is too small to efficiently produce the excitation ofargon atoms. The dark phase lasts for several
hundred µs and finishes with the relaxation to thesteady-state RF discharge.

Fig.3. Space-time diagram of a dark phase
According to the simulation the bare cloud of
positive ions, formed in the discharge gap after theremoval of the high voltage from the electrodes,
will create a strong electric field (positive potential
in the kV range), which lives long enough to
produce excessive ionization in the discharge gap.
This is confirmed by the experimental observationof a strong overshoot of the emission intensity,
starting right after the pulse is over (Fig.2). An
important role of photoelectron emission due to thevacuum UV, radiated by excited argon atoms, in theformation of this overshoot was demonstrated. Roleof the bremsstrahlung, radiated by the energetic
electrons, hitting the surface of the electrode, wason the contrary shown to be negligible.
The dark phase (Fig.3) was revealed not to beconnected with the redistribution of RF voltage inthe external circuit of the discharge. In a standard
situation the reduction of the RF voltage on thedischarge gap during the relaxation of the plasma is
about 15%. We intentionally added additional
impedance into the external circuit so that thereduction of the RF voltage became 50%. This
resulted, however, in a very small change in thedepth of the dark phase. In the simulations the darkphase is obtained without any variation of the RF
voltage at all. The mechanism of the dark phaseformation is therefore provided by the screening of
the RF field by the high density plasma, created inthe overshoot.
References
1. S.M. Starikovskaya: J. Phys. D 39 (2006) R265
2. H. Ayan, G. Fridman, A.F. Gustol, V.N. Vasiletz,
A. Fridman, G. Friedman: IEEE Trans. Plasma Sci.
36 (2008) 504
3. L.M. Vasilyak, S.P. Vetchinin, D.N. Polyakov andV.E. Fortov: JETP 94 (2002) 521
4. M. Pustylnik, A.V. Ivlev, H.M. Thomas, G.E.
Morfill, L.M. Vasilyak, S.P. Vetchinin, D.N.
Polyakov, V.E. Fortov: Phys. Plasmas 16 (2009)
113705
5. Verboncoeur, J.P., M.V. Alves, V. Vahedi, andC.K. Birdsall: J. Comp. Phys. 104 (1993) 321