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Erosion Processes and Micro-Particle Production in Gas Discharge Lasers

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Erosion Processes and Micro-Particle Production in Gas Discharge Lasers ENTE PER LE NUOVE TECNOLOGIE, L’ENERGIA E L’AMBIENTE Dipartimento Innovazione EROSION PROCESSES AND MICRO-PARTICLE PRODUCTION IN GAS DISCHARGE LASERS TOMMASO LETARDI, GUALTIERO GIORDANO ENEA - Dipartimento Innovazione Centro Ricerche Frascati, Roma CHENGEN ZHENG (ENEA GUEST) EL.EN - Via Baldanzese 17, 50041 Calenzano (Fl) RT/1NN/99/14 Manuscript received in final form on January 1999 Printed on July 1999 ... This report has been prepared and distributed by: Servizio Edizioni Scientifiche - ENEA Centro Ricerche Frascati, C.P. 65-00044 Frascati, Rome, Italy The technical and scientific contents of these reports express the opinion of the authors but not necessarily those of ENEA. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. SUMMARY The erosion processes of the cathode for pulsed excimer gas lasers are explained by comparing the initiation conditions of the pulsed excimer gas laser discharge to that of the vacuum discharge breakdown. The numbers of the micro-particles, generated due to the above cathode-processes, are estimated. Several possible influences of the rnicro-par~icles on performances of the gas discharge iasers are analyzed. Two methods for eliminating the micro-particles or reducing their influences are discussed (EXCIMER LASERS, ELECTRODE EROSION, PARTICULATE, DISHCARGE MICRO-PARTICLE, MIRROR PROTECTION) RIASSUNTO Vlene descritto, comparandolo con la scarica in vuoto, il processo di erosione del catodo di un laser ad eccimeri a scarica. Viene stimato il numero delle micro- particelle generate dal processo di scarica. Vengono analizzate le possibili influenze di tali micro-particelle sulle prestazioni dei laser a scarica. Sono presentati e discussi due possibili metodi per la eliminazione delle micro-particelle generate dalla scarica. INDEX 1- INTRODUCTION ............................................................................................................................p. 7 2- SIMILARITY OF THE FORMATION FOR CATHODE SPOTS IN GAS DISCHARGE AND VACUUM BREAKDOWN ...........................................................................................................p. 8 3- DISCHARGE EROSION PROCESS OF THE CATHODE ..........................................................p. 9 4- ESTIMATION FOR GENERATION RATE OF THE MICRO-PARTICLES .............................p. 13 5- INFLUENCE OF THE MICRO-PARTICLES ON PERFORMANCES ON ONE-FILL ............p. 16 GAS-MIXTURE EXCIMER LASERS 5.1. Gas discharge breakdown .........................................................................................................p. 17 5.2 Influence of the particles on mirror quality ...............................................................................p. 18 5.3 Discussion of the laser scattering by micro.paHicles ................................................................P. 25 6- PROTECTION OF THE MI~ORS ...............................................................................................P. 25 7- SUMMARY AND REMARKS .......................................................................................................P. 29 APPENDIX: Movement of a particles in gas chamber .............................................~..........................P. 31 REFERENCES ......................................................................................................................................P. 37 7 EROSION PROCESSES AND MICRO-PARTICLE PRODUCTION IN GAS DISCHARGE LASERS 1- INTRODUCTION The particulate contamination is one of the principal problems which influence the operation life time of the XeCl excimer lasers [1,2]. The particulate, which are composed of many particles with different dimensions, arise primarily from the discharge processes. On one hand, there are always some molten droplets which are emitted from the filament or arcing spots on the electrode surfaces during the discharge either due to large electrostatic forces acting on the electrodes inside the cathode sheath, or due to superheating resulted from joule heat and ion bombardment. On the other hand, some chloride compounds, such as chlorocarbons and so on, can be also generated especially in the discharge or preionizer (for the case using UV-preionization) region, where the reactions of HC1 with metal or dielectric materials are promoted by discharge, and laser beam or UV radiation from the plasma [1]. Both particulate or chloride compounds can deposit or adhere to the interior of the laser window or mirror to form a coating layer, causing degradation of the optics. For the deposition of the chloride compounds, the situation may be more serious, since some of the compounds can be further photo-dissociated to carbon or other compositions by incident laser beam through a single or multi-UV photo effect [1]. Since most of gaseous chloride compounds have liquefying temperatures higher than those for HC1 or other constituents of the laser gas mixture, they can be efficiently removed by a low temperature gas trap, for example, a liquid nitrogen cryogenic purifier. Different from the case for gaseous impurity the solid particles are often eliminated by using some particulate 8 filters. In order to know the requirements for the filters, we need to know something more about these particles, for example, what kind of dimensions they have, where they cc~mefrom, how much their generation rates are, and so on. In this work, we try to discuss these questions. 2- SIMILARITY OF THE FORMATION FOR CATHODE SPOTS :[N GAS DISCHARGE AND VACUUM BREAKDOWN It is observed both in vacuum breakdown and for high pressure gas discharge experiments that after a short interval of the time delay relative to applying a voltage to an electrode gap, there are some spots appearing on the surface of the cathode, which lead to a final short circuit of the gap. For XeCl gas discharge case, the cathode spots, which are called as hot spots, are observed to have a linear density of 4 /cm under the experimental condition reported in [3]. The main discharge energy deposition in the gas mixture is also found to be dominated by these spots. Suppose that at the beginning, there is a very uniform discharge between two electrodes, then the spots emerge first from the cathode and later at the anode, and each of these spots initiates a single filament being growing with time into the main discharge gap [3]. With the development of these filaments, the glow discharge surrounding the filaments becomes weaker and weaker and finally disappears. The cathode spots which appear are not only observed in XeCl discharge lasers, but also for other types of gas discharge devices, for example, pulsed N2 discharges and so on [4,5]. For vacuum breakdown case, it is known that explosive emission of electrons plays an important role for initiating the discharge and maintaining the cathode spots (for example, see [6-9]). Since vacuum breakdown is a very complicated phenomenon, and only some processes are made clear now, here we only mention some results about formation of the spots, which are important to the description of this work: a) There are always micro-protrusions with the height in the order of several Km on the cathode surface, and the local fields can be enhanced by these protrusions with a factor of y=(10-102) [8,10,13-15]. A very high electric field, which may be of the order of 5~106 V/cm at least, can also induce some new micro-protrusions on the surface of the cathode [10- 12]. b) On application of a high voltage to an electrode gap, the micro-protrusions on the cathode surface are heated by field emission current. If the field is high enough, the joule h,eating of the protrusion-tips results in transition of the pure field emission to field-assistecl thermo- ionic emission, leading to a further increase in temperature of the tips, and finally the formation of local plasma burst, i.e. the explosion of the micro-protrusions [6,7.,16]. The 9 critical value of the local field near the micro-tips for vacuum electrical breakdown is - measured to be in the range of (5-11)=107 V/cm for a variety of electrode materials, and this value is related to the explosions of the micro-protrusions on the cathode surface [5,7,16-22]. For the case of pulsed discharges at a high gas pressure, the formation of cathode spots is similar to the above described vacuum breakdown case. In fact, after a pulsed high voltage is applied between two electrodes, an electron-depleted layer, which is often called as the cathode fall region or cathode sheath, is left to be close to the cathode surface due to a drift of free electrons towards the anode inside the electrode gap, leading to a very high electric field strength in this layer. Suppose that the electric field across the electrode gap is finally counteracted by the field produced by the space charge in the sheath, we may approximately estimate the field near the cathode surface as [23] 2e. ne-V0 ECat~ = (1) r E() ‘ where e= 1.602”10-19 C is the electron charge, ~. =8-854-10-14 CN the dielectric coefficient in vacuum, V. the voltage drop on the electrode gap, and ne the free electron number density. For example, Ecath is of the order of 106 V/cm when ne=(1013-1015)/cm3 and VO=30 kV. A detailed study for cathode sheath of a discharge-sustained XeCl laser shows that under normal conditions of operation the cathode electric field can have a value of 4“106 V/cm during the plateau part of the discharge voltage [24]. Taking the field enhancement factor y=(10-100), the local
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