Thareja, R K

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Indian J. Phys. 66 B  (5 &  6), 5 4 9  ~  554  (1992)Laser induced gas breakdown : a possible fast plasma switchR K TharcjaDepartment of Physics and Centre for Laser I'echnology.Indian Institute of Technology, Kanpuf-208 016, LJtiar Pradesh, IndiaAbstract : A  review of on laser induced gas breakdown using only laser beam, a laser beam and static electric field, the eltect ot laser prsiduced plasma from a biased mclal target in a background gas is reported. We report that switching of gas discharge is controlled by metal plasma or polarity of the targetK e yw o rd s  : Lasers, gas bieakdown, plasma switch P A C S  Nos. : 52.50 Jrn, 52.75 kq1. IntroductionSince the first report of laser induced breakdown 11) of air in 1963, there has been an enormous growth of undersUinding of the phenomenon (2,4|. By focusing a laser beam in gas it is possible to obuiin a high-tempcraiurc, high dcnsiiy plasma. Viu-ious profvrtics of the gaseous plasma created by focusing laser radiation have also been studied. Laser produced plasmas have been u.scd as source for high inicnsiiy X-rays 151 and VUV conlinuum in rare gases [6]. Sensitive techniques to measure UV radiation using gas breakdown have also been developed (7J. Lasers have been used to trigger electrical disetuuge between electrodes m vacuum and in gases in spark gap switches [Kj. Pendleton and Guenther reported on the initiation of the breakdown by a laser at fX)ini in the inter electrode space between a gap or one of the two electrodes. The laser prcxluced plasma con.sequenily elcctneally closes the gap. effect has been utilized for laser trigger switching (LTS).DcUiilcd calculations of the gas breakdown threshold cau.sed by laser irradialion have been carried out by Kroll and Watson [9J. Biischer e t a l  [101 were the lirsi lo study the breakdown threshold intensity of rare gases at wavclengilis 1.06, 0.69, 0.53 and 0.35 pm. They found that the threshold intensity for each rare gas studied, first increases lo a maximum and then dccrca.scs with decreasing wavelength. Alctx'k e t  u l  [111 reported the breakdown ot nitrogen, methane and rare gases using a ruby laser ol pulse width 20 ns and us second harmonic, in agreement with Buscher e t a l  110). Krasyuk e i  a l  [121 studied the breakdown threshold of nitrogen, helium and argon using ps pulses ol ruby laser m the pressure langeCc. 1992 lAGS55 0 R K  Thareja2<p<10‘' Torr. The results show a weak dependence on the pressure for p<10^ Torr in He and Ar and p S3(X) Torr in N2, characlcrisiic of mulli photon absorption. At higher pressure in He and Ar there is a pronounced pressure dependence, indicating the occurrence of collisional ionization by inverse bremsstrahlung absorption, however for N2 the pressure dependence is less pronounced. Gamal and Harith [ 13J and Weyl and Rosen [14] have done theoretical calculations of laser induced breakdown thresholds of atomic molecular gases as a function of pressure of the gas, pulse width of laser, using the equation of growth of electrons, and including the effects of both multi photon and cascade ionization.Simultaneously, there have been reports on the gas breakdown by focusing laser radiation in the presence of transverse static electric field [ 15]. The interaction of laser beam with the target depends on the characteristics of the target material and lasccbeam both. For low irradiation (= 10*’ W/cm^), the absorbed radiation appears as heat which is slowlydistributed through out the material by thermal diffusion. Further increase in irradiation results in vaporization of the Uirgcl material. At still higher power densities = 10* W/cm it is possible to produce plasma directly at the focus of the uirgct without going through the intermediate stepes [16,17]. It has been observed that the presence of a metallic target reduces the threshold intensity for breakdown for gases. Bondarenko ctal 118) reported breakdown of air at the surface of the metals using low power CO2 laser and explained on the basis of thermal procc.ss which accompany optical breakdown. Danshchikov el a/ [19] ob.scrved that the presence of a precursor plasma decreases the threshold intensity for breakdown of inert and molecular ga.scs. Short wavelength radiation emitted form the metal plasma was also found to reduce the threshold intensity for breakdown of the gas [20,21 ].In tlic pre.sent talk, I will review the work done on la.ser induced gas breakdown using only laser beam [22,23], in the presence of static electric field ]24], the effect of metallic target on breakdown [25] of gases and discuss a possible application of laser induced gas breakdown near metal surface to fast switching [26].2 . Experimental setupTo study air breakdown using Nd : YAG laser and its harmonics, a Nd ; YAG laser (DCR-4, Spectra physics) with Gausssian limited mode structure delivering 900 mj in 2.5 ns (FWHM) at fundamental with a repetition rate of 10 pps, was focused with a quartz lens {f=\2 cm) in to the centre of a vacuum chamber. The chamber was evacuated to < lO'  ^Torr with a rotary pump. Laser energy was varied by varying the voltage on the laser amplifier. No significant variation in mode pattern was observed at different energies. The energy of the laser was measured with a laser power meter (Ophir Model No. 30A), by placing the power meter in the path of the main beam. The output power of the laser was stable within ±7%. The beam divergence being less than 0.5 m rad. Threshold energy was measured only when the breakdown .spark, seen visually through the window, was modulated at 10 pps. Breakdown spot size was recorded using a camera.Laser induced gas breakdown : a possible fast plasma switch3 . Results and discussion551Figure 1 exhibits the dependence of threshold intensity on wavelength of laser radiation at different pressures of air.Fi{(ure 1. Dependence of threshold intensity on wavelength of laser radiation at dillerent pressures of air.Figure 2. Variation of bieakdown Itcid lor various focal spots and pulse widths of laserBreakdown threshold intensity of air at l.()6 |jm shows a pronounced pressure dependence, characteristic of collisional ionization by inverse bremsstrahlung absorption. The threshold intensity scales with pressure as However, weak dependence on pressure, is observed at 0.532 and 0.355 pm indicating the occurrence of multi photon absorption. 1’hus breakdown of air at visible and shorter wavelengths may be initiated wiili photon absorption and then proceed due to cascade processes. A systematic study was under taken to investigate the effect of spot size, pulse width on brciikdown of air using 0.226, 0.355, 0.532 and 1.06 pm irradiation. Figure 2 shows the vtirialion of breakdown field for various focal spots pulse widths oflaser. It is observed threshold electric field varies as for 0.532, 0.355 and 0.266 pm irradiation and for 1.06 pm irradiation.In order to look lor practical application of laser induced gas breakdown we kxiked for breakdown of Ne, Xe and Ar gases at various pressures using a low powder laser XcCl (308 nm) in the presence of transverse static electric field. We observe that die number of photons actually involved Xq to ionize the gas depends on bias voltage; with the increase of bias voltage the value of Xq decreases. We continued investigation to look for the effect of a nictallic target, Cu in our case, on the threshold laser energy for breakdown of die argon gas.Experimental arrangement is similar to that of one used earlier. It consists ol a specially designed glass cell having two parallel brass plates (20 x 25 mrn^) separated by 6.5 mm and a copper target at a distance of 15 mm from the edge of the parallel plates. The cell was evacuated to a pressure of < 10‘^  ton* and then filled in with argomgas at a pressure ol 8552 R K  Tharejalorr. A bright green metal plasma is observed by focusing (/=12 cm) an XcCl (308 nm) cxcimcr laser, delivering 3.5 mJ in 25ns at 10 pps, on to the copper target. The best focusing of die laser radiation on to the copper target is defined as jc = 0. The irradiation on to the target and hence-the extent, size of plasma is reduced by moving the lens away from the copper target on the same optic axis. Copper target is biased positive or negative w.r.t. brass plate using an EG Sc G power supply (3.0 kV, 10 mA).In the following we describe the effect of metal plasma on laser induced breakdown of argon gas in various configuration of biased target an plate . Static glow discharge of argon gas was observed when a static voltage of 0.58 k V was applied between the target and plate; however, when laser radiation is focused or dcfocussed, depending on the polarity of the target as shown in Figure 3 ~ 6, pulsed glow discharge modulating at 10 pps was observed with static voltage of 0.32 kV. Breakdown of argon gas was observed visually by the appearance of glow discharge modulating at 10 pps.Case 1:Static voltage is applied across plate (+vc) and target (ground) as shown in Figures 3a and 3b. We observed pulsed discharge in the region between plate and target only in the3a.1— f \ r  5aX ■ 6rr3b r “ i• r r ~ i f - 5b4ar-f4 " Mf i r6a4bP r n r6b' ' ' iV  ~ Argon diachorgo^  > Coppor plasma Figure 3 - 6 .  Vanous configuration.^ for plasma switch.P > Plot#T > Torggt L s LPnsLaser induced gas breakdown : a possible fast plasma switch 553configuration of Figure 3a when laser is defocussed by 8 mm. At this position copper plasma was not visible and only source of initiating breakdown is the photo electron emitted from the copper surface. The emitted electrons gain enough energy in the static field and ionize the argon gas by inelastic collisions leading to quick bridging of the gap. However, when laser radiation is focused on to the copper target a iM-ight green plasma appears and argon discharge disappears as shown in Figure 3b. The electrons are attracted towards the +vc plate and a sheath is formed at a distance of Xp, debyc length (in mks units) given byI _ meterEven if we as.sume minimum electron density  ^n, -  10'* m f* and temperature, 7', -  0.8 cV, debye length is 210 mm. Thus positive potential is screened off beyond 210 mm and argon discharge disappears.Case 2:Static voltage is applied across target (+ve) and plate (ground) as shown in Figures 4a and 4b. Unlike Case 1, here we observe argon di.scharge only in the presence of copper plasma as shown in Figure 4a. The effect of plasma is essentially to shift the position of positive potential by a distance equal to the size of plasma. -  7 mm. This results in decrease m the virtual distance between target and plate and hence increase in electric field resulting in breakdown of argon gas. However, it the laser radiation is defocu.s.sed by 8 mm as shown in Figure 4b. the argon discharge disappears.Case 3:Static voltage is applied across plate (ground) and target (-ve) as shown in Figures 5a and 5b. Argon discharge appears only when laser is defocussed by 8 mm as shown in Figure 5a, Situation is identical to Figure 3a, target being at lower potential than the plate, phoioelectrons emitted from target surface are accelerated towards the plate to gain energy and ionize the argon gas by inelastic collision. However, with focused laser radiation as shown in Figure 5b, we have situation similar to Figure 3b where sheath formation prevents the argon discharge.Case 4:Static voltage is applied across target (ground) and plate (-ve) as shown in Figures 6a and 6b The situation is opposite to Case 3 and similar to Case 2, we observe argon discharge with focussed laser radiation which disappears by defocussing the radiation.In conclusion, we observe that ionization or argon gas can be switched ON or OFF either keeping laser foeusing conditions fixed and by changing the polarity or keeping the polarity fixed and by changing the laser radiation from focused to defocussed state.554 R K TharejaAcknowledgmentsCollaboration with Dr V Kumar, Mr D V S Mulhu and Ms R Tambcy is highly acknowledged. Work is partly supported by Department of Science and Technology, New Delhi.References[1] P D Maker. R W Terhune and C M Savnge P ro(\ T hird  Int. ConJ. Quant. Elect. 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Opt. 31 3460

Laser Induced Gas Breakdown : a Possible Fast Plasma Switch

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Laser Induced Gas Breakdown : a Possible Fast Plasma Switch

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