`DISCHARGES
`
`Frank Leipold, AbdeI—Aleann H. Mohamed, and Karl H. Schoenbach
`Physical Electronics Research Institute, Old Dominion University, Norfolk, VA 23529
`
`Abstract
`
`The pulsed electron heating effect has been studied on an
`atmospheric pressure air glow discharge. Application of a
`high voltage pulse causes a shill in the electron energy
`distribution function to higher energies. This causes a
`temporary increase of the ionization rate and consequently
`an increase of the ‘electron density. The electron density
`afier a 10 ns pulse application to a direct current glow
`discharge increased fiom its dc value of 2 10” cm'3 to 2.8
`10” cm’. The average power density,
`required for
`sustaining the high pressure plasma with a given
`minimum electron density, was found to be lowered when
`thedischarge was operated in a repetitive pulsecl mode
`compared to a dc mode. For an atmospheric pressure air
`plasma, an average power density of 1.5 kW/cm’ and 50
`W/cm’ is ret}uil’ed for an average electron density of 10"
`cm'3 and 10 2 cm’, respectively. This value is less by a
`factor of three than that required to sustain a dc plasma
`with the same base electron density.
`'
`
`required for reflection of microwave radiation of up to 30
`GHz [1]. The MCSG plasma was found to be scalable in
`size by extending the electrode gap and by placing the
`discharges
`in parallel
`[2]. However,
`at equilibrium
`conditions,
`the power density required to sustain an
`atmospheric pressure air plasma of 10" cm“ electron
`density is approximately 5 kW/cm’ [3], a value which
`makes these equilibrium plasmas difiicult to scale to large
`volumes.
`
`Pulsed electron heating has been shown to allow
`reduction of the electrical power, while keeping the
`average electron density at
`the
`required level
`for
`microwave reflection [4]. In order to explore the elfect of
`pulsed electron heating on the temporal development of
`single discharges and discharge arrays we have measured
`the electrical and the optical response to pulsed electron
`heating with a temporal resolution on the order of 10 ns.
`Laser interlerometry, electrical conductivity and optical
`spectroscopy was used to determine
`the temporal
`development of electron density, and gas temperature.
`
`I. INTRODUCTION
`
`II. EXPERIIVIENTAL SETUP
`
`‘Weakly ionized plasmas, generated in high pressure air
`glow discharges,
`reflect or
`absorb electromagnetic
`radiation in the microwave range and consequently act as
`temporally controllable barriers for this radiation: as
`plasma ramparts. Direct current microhollow cathode
`sustained glow discharges (MCSG) have been shown to
`provide plasmas with an electron density of 10“ cms,
`
` Delay
`
`f""'l
`
`Line Type
`Pulse Generator
`_l"L
`
`l. The
`shown in Fig.
`setup is
`experimental
`The
`atmospheric pressure air discharge(s) were operated in a
`direct current mode, with a 10 ns to 12 ns voltage pulse
`superimposed. The gap was set at 0.6 cm, the distance
`between
`discharge
`axes,
`for
`a
`three
`discharge
`arrangement, was 0.4 cm (Fig. 2).
`
` Electrical Field:
`
`5 kVlcm to 10 kV/cm
`Gap Distance:
`2 mm to 10 mm
`
`Figure 1. Experimental setup
`
`serve as
`(MHCD)
`Microhollow cathode discharges
`plasma cathodes. In order to increase the size of the
`0-7803-7540-81021‘-517.00©2002 IEEE
`
`plasma, three discharges were operated in parallel [2]. The
`discharges
`can
`be
`operated
`either
`in DC with
`130
`
`INTEL 1018
`
`INTEL 1018
`
`
`
`decay of the temperature could not be obtained with this
`method.
`
`2400
`
`__
`5‘ 2300
`E 200
`
`E 3
`
`2100
`-
`
`1800
`
`in pulsed mode only. Two
`superimposed pulses or
`line
`type
`pulse generators
`independent
`triggerable
`provided 10 ns pulses. The applied diagnostics are
`emission spectroscopy for gas temperature measurements,
`interferometry [1], and conductivity measurements for
`electron density measurements. High-speed photography
`was used to obtain the spatial plasma distribution.
`
`
`
`at
`parallel
`2. Three MCSGS operated in
`Figure
`atmospheric pressure in air. Electrode gap: 6 mm,
`distance between two discharges: 4 mm.
`
`III. EXPERHVIENTAL RESULTS
`
`A. Gas temperature
`The gas temperature is obtained by comparison of a
`measured and a simulated spectrum of the 2"‘ positive
`system of nitrogen. For a DC glow discharge, the gas
`temperature was found to be 2200 K close to the cathode.
`The temperature in the plasma column reaches with
`increasing gap length a constant level of 2000 K (Fig. 3).
`
`2400
`
`N‘-cde1 M0652 M0693
`
`[K] ss 1600
`GasTemperature
`
`
`F38
`
`1400
`
`1200
`
`o
`
`2
`
`4
`
`e
`
`s
`
`10
`
`Distance from MHCD [mm]
`
`Figure 3. Gas temperature on the axis of a DC MCSG for
`various electrode distances. The discharge current was 13
`nm.
`
`For a DC operated MCSG with a superimposed pulse, an
`increase of the gas temperature by 300 K was measured
`10 ns afler pulse application (Fig. 4). Due to the low light
`intensity 25 ns afier pulse application, information on the
`
`Exposure Time: 1 no
`O O 0
`
`0
`
`o 0
`
`
`
`Dc Temperature (2000 K)
`
`E
`'6
`(9 1900 10 ns)
`
`0
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`Delay Time [ns]
`
`Figure 4. Temporal Development of the gas temperature
`in the center of a MCSG discharge operated DC with
`superimposed pulse. Electrode distance: 2 mm, IMcs(; Dc =
`10 mA.
`
`B. Electron density
`and
`spatially
`The
`electron
`density was measured
`temporally resolved by means of infrared heterodyne
`interferometry. The radial profile was found to be time
`independent. It can be fit by a gaussian profile with a
`width of <3 = 0.056 mm. Due to the limited temporal
`resolution, the electron density cannot be measured during
`the pulse. However, a measurement at 22 ns after pulse
`application provides an electron density of 2.8 10” cm“
`for a discharge with a gap distance of 2 mm and an
`applied pulsed electrical field of 8 kVlcm. This indicates,
`that the electron density during the pulse is at least 2.8
`10" cm". The radial electron density distribution in the
`center plane of the discharge for difi‘erent
`times alter
`pulse application is shown in Fig. 5.
`
`H
`
` ‘EIn
`
`'2 2
`
`1
`
`-§
`
`5O
`
`§ 0
`
`-0.1
`
`0.0
`
`0.1
`
`Distance y from Center [mm]
`
`Figure 5. Radially resolved electron density in the center
`plane of the discharge for different
`times after pulse
`application. Electrode Distance: 2 mm, applied electrical
`field: 8 kV/crn.
`
`131
`
`
`
`also be
`The electron density afier pulse application
`obtained from measurements of the electrical field _and
`current density. The relation-between current density;
`electron density arld_electric'a1 field is given by the .'
`equation:_
`<1)‘
`_
`5'”
`J=n.ev(E/n)
`The electron density was calculated using average values
`. of electric fields (E = V/d)‘ and_-current density J. The
`current density is given by _the measured. current and
`spatial distribution profile of the electron density, which is
`assumed to be represented by the optical emission profile.
`Photographs of the discharge plasma for difierent times
`after pulse application are shown in Fig. 6. The exposure
`time is 5 us. The drift velocity v,>which _depends on the
`mls and 1<';’_
`reduced electrical field, varies between 2 10
`
`Td [5].
`
`,
`
`100
`
`-4- Modeling
`l:l‘ Pulsed, no oil
`(Conductivity)
`Q‘ Pulsed,DC on
`(Conducllvlty)
`A 'Pulsed.DCon
`(Interferometry)
`
`A
`
`1019
`
`101: 9,‘
`E
`1°" ‘*3
`I;
`=‘*
`10“ 8
`=
`1015 9
`2%
`1014 Q
`E‘in
`1013 gD
`
`1012 5
`ii
`10“ 2
`l.l.|
`
`200
`
`.
`
`191.0
`300
`
`Reduced Electric Field, EIN (Td)
`
`m/s for reduced electrical fields between 10 Td and 209
`\||("l)
`
`Figure 7. Electron density difference afier a 10 ns pulse
`application ve_rsus the applied electrical field. The solid
`line represents modeling results [4].
`
`Tot high pulsed electrical field, the dc contribution to the
`electron density can be neglected. For investigation of the
`electron heating efiect for low applied electrical fields, the
`current of the discharge has to be turned off before the
`pulseis applied. A typical temporal development of the
`~ current is shown in Fig. 8.
`
`|’l:1llcul'
`\lc.l.»llIu:Illcl'll
`
`\lt;li-Lll1.‘Ill'\ lll
`
`lxll ll»
`
`\ll|('|)
`
`-lllll H5
`
`Figure 6. Photographs of the MCSG for dilferent times ,
`after pulse application. ‘Electrode gap: Zmrn, applied
`electrical field: 10 lcVIcm [6].
`-
`'
`
`A
`
`field’ of 10 kV/cm and an
`applied electrical
`For
`electrode distance of 2 njlm,
`the FWHM of the radial
`profile was found togbe 0.16 mm. The profile is
`independent. The electron density afier pulse application‘
`versus the reduced electrical field is shown in Fig. 7.
`
`Current
`
`[mA}NA,.-or_
`
`0.0
`
`A 0.5
`
`1.0
`
`1.5
`
`2.0
`
`Time_[ms]
`
`Figure 8. Temporal development of the discharge current.
`Before the two voltage pulses were applied, the direct
`current was turned off.
`’
`
`132
`
`
`
`C. Power Density
`The power density, P, for repetitive pulsed mode is:
`
`‘P=EJtp“|u/tkq,
`
`All factors in this equation can be expressed in terms of
`the electrical
`field intensity. The expression for
`the’
`current density is given in equ. I. The drift velocity in this
`equation depends on the reduced electrical field [5]. The
`repetition time, tg.,,, is the time, required for the electron
`density to decay fi'om the peak value to a minimum value.
`In our case, the major electron loss process is dissociative
`recombination. Attachment
`can
`be
`neglected
`-in
`atmospheric pressure air plasmas with a gas temperature
`exceeding 1500 K [7]. The repetition time tR,,, is therefore
`given by
`
`For an electron density of 10” cm“, the minimum power
`density is 0.85 kW/cm}. For an electron density of 10”’
`crn'3,
`the power consumption can be reduced to 18
`W/cma. The theoretical values are confirmed by the
`experimental results.
`
`IV. SUIVINIARY
`
`Atmospheric pressure air plasmas could be generated with
`characteristic
`dimensions
`of
`centimeters
`at
`gas
`temperatures of 2000 K. A reduction in the power
`consumption compared to the DC glow discharge could
`be achieved by operating the discharges in pulsed mode.
`Nlinimum power densities required to sustain atmospherita:
`pressure air plasmas with electron densities of 10‘
`cm‘
`and 10'’ cm'3 are 850 Wlcms and 18 W/cm’, respectively.
`
`tan '-‘ (Hp-no)/(np*no B)
`
`(3)
`
`V. ACKNOWLEDGEMENT
`
`the peak electron density alter pulse
`is
`where n,
`application, no is the minimum electron density, and B is
`the recombination coefiicient. As shown in Fig. 7,
`the
`peak electron density is a function of the applied reduced
`electrical field.
`
`.
`
`The total power consumption for atmospheric pressure
`plasmas with minimum electron densities versus the
`applied electrical field is shown in Fig. 9. The solid lines
`represents the modeling results and the squares and circles
`the experimental results. If the applied electrical field is
`too low, a high repetition rate is required, a mode of
`operation which approaches direct current operation.
`Consequently the power increases towards the dc value.
`With increasing electrical field, most of the energy is used
`to generate electron densities far exceeding the desired
`minimum value. This density decreases rapidly due to
`recombination and as shown in equ. 3, contributes only
`minimally to the repetition time. Consequently, there is an
`optimum electrical
`field
`for minimum power
`consumption, as shown in Fig. 9.
`
`10‘
`
`§
`
`.-n9
`
`3%
`
`[kWIcrn3] "5.
`PowerDensity
`
`
`This work was supported by the US Air Force Otlice of
`Scientific Research, and the National Science Foundation
`(Award # INT-0001438).
`
`VI. REFERENCES
`
`[1] Frank Leipold, Robert H. Stark, Ahmed El-Habachi,
`and Karl H. Schoenbach, “Electron Density
`Measurements in an Atmospheric Pressure Air
`Plasma by Means of IR Heterodyne Interferometry”
`J. Phys. D: Appl. Phys. 12;, 2268 (2000).
`[2] Abdel-Aleam H. Mohamed, Rolf Block, and Karl H.
`Schoenbach “Direct Current Glow Discharges in
`Atmospheric Air,” IEEE Trans. Plasma Science. 10,
`182 (2002).
`[3] Robert H. Stark and Karl H. Schoenbach, “pircct
`Current Glow Discharges in Atmospheric Air,” Appl.
`Phys. Lett. 15, 37700999).
`[4] Robert H. Stark and Karl I-I. Schoenbach, “Electron
`Hearing in Pulsed Atmospheric Pressure Glow
`Discharges,” J. Appl. Phys. Q9, 3568 (2001).
`{5} A. V. Phelps, “Excitation and Ionisation
`Coeflicients”, in L. G.
`Christophourou, and D. W. Bouldin, eds, Pergamon
`Press, 1987, p. 1
`[6] Hisham Merhi, Masters Thesis, Old Dominion
`University, Norfolk, VA, August 2001.
`[7] Laux, C.0., Yu, L., Packan, D.M., Gessman, R.I.,
`Pierrot, L., Kruger, C.H., and Zare, R.N.,
`30th AIAA Plssmadynamics and Lasers Conference,
`Norfolk, VA, June 28-July 1, 1999
`
`Electrical Field [kVIcmj
`
`Figure 9. Power density versus the applied electrical field
`for different electron densities The solid lines represent
`the modeling results,
`circles and squares
`represent
`measured values.
`
`133