ISSN 0020-4412, Instruments and Experimental Techniques, 2009, Vol. 52, No. 3, pp. 394–399. © Pleiades Publishing, Ltd., 2009. Original Russian Text © V.M. Akimov, G. Witte, L.I. Kolesnikova, L.Yu. Rusin, J.P. Toennies, 2009, published in Pribory i Tekhnika Eksperimenta, 2009, No. 3, pp. 91–97. 394 The set of experimental methods used in studies with molecular beams includes a wide circle of instru- ments intended for generating various beams of various atoms and molecules in a very wide range of kinetic energies. One of the most urgent problems of this important component of the molecular-beam method is production of beams of free atoms and radicals possess- ing a high chemical activity, for example, hydrogen and oxygen atoms. The most general requirement imposed on beam sources is to reach a maximal intensity with as narrow as possible velocity distribution of beam particles and the possibility of changing their energies. To one or another extent, this requirement can be satisfied using a supersonic beam with production of atoms in the high- frequency or microwave discharge plasma [1–3]. It is evident that the higher the gas pressure in the discharge region and the higher the efficient degree of dissocia- tion, the more efficient these methods are. The latter value is determined by the ratio of the dissociation and atomic-recombination rates in the discharge region and the gas expansion from the nozzle. Paper [3] presents an atomic oxygen beam source with a degree of é 2 dissociation of ~70% at a 350-Torr pressure for a 5- to 10% mixture of oxygen and argon, when the power in the discharge varies from 140 to 195 W. The generation of hydrogen atomic beams has a substantial problem, and the working pressures in the discharge at sufficiently high dissociation degrees are, as a rule, no more than 1 Torr [4–8]. To initiate a stable discharge for generating an atomic particle beam at increased pressures is more dif- ficult than to do it at low pressures. These difficulties are, in particular, related to the problem of matching the impedances of the plasma and HF or microwave source when the gas pressure and, hence, the plasma density increases. In turn, the plasma characteristics substan- tially depend on the gas composition, the reachable plasma temperature, and another conditions. To increase the efficiency of plasma sources of beams, it is necessary to support the discharge directly near the nozzle to decrease the degree of atomic recombination before the gas-dynamic beam expansion occurs. In this case, the temperature of the discharge-tube walls near the nozzle should be in the optimal range, taking into account the kinetics of reactions in the plasma and the risk of melting of the nozzle orifice. This work describes the results obtained in the pro- cess of designing the source for generating and support- ing stable microwave discharges in various gases and their mixtures with noble gases in quartz tubes with a 2-mm inner diameter and a 0.15-mm-diameter nozzle. The design is based on a surfatron [9, 10], in which the discharge is initiated by a surface electromagnetic wave. The advantage of such devices is that it is possi- ble to produce a stable plasma in them at pressures of up to several atmospheres, since, in this case, draw- backs related to problems of sustaining an HF or micro- wave discharge at elevated pressures in resonance sources are eliminated. Characteristics of the discharge initiated by the surface wave were intensively studied for plasma of Ar [11]; He [12]; O 2 [13, 14]; N 2 [15]; He, Ne, and Ar [16]; H 2 [17, 18]; and é 2 –N 2 [19]; and Ar–N 2 mixtures [20, 21]. GENERAL EXPERIMENTAL TECHNIQUES A Microwave Discharge Source Operating at Pressures of Several Atmospheres V. M. Akimov a , G. Witte b , L. I. Kolesnikova a , L. Yu. Rusin a , and J. P. Toennies c a Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences, Leninskii pr. 38, korp. 2, Moscow, 119334 Russia b Ruhr-Universität Bochum, Universitätsstraße 150, NBCF03/296, Bochum, D-44780 Germany c Max-Planck-Institut für Dynamik und Selbstorganisation, Bunsenstrasse 10, Gottingen, D-37073 Germany Received July 11, 2008 Abstract—The design of a microwave source in which a discharge is initiated by an electromagnetic surface wave at 2.45 GHz is described. A stable discharge was supported at a gas pressure p 0 exceeding the atmospheric pressure in He, N 2 , and in H 2 –Ar, H 2 –He, and O 2 –He mixtures in a 2-mm inner diameter quartz tube with a 0.15-mm diameter nozzle at a 50- to 115-W microwave power. A degree of dissociation of up to 80% was reached for pure H 2 at p 0 = 6 Torr and a 6% mixture of H 2 and He at p 0 = 50 Torr. When p 0 increases to 19 Torr for H 2 and to 300 Torr for the mixture, the hydrogen-atom beam intensity, in spite of a decrease in the degree of dissociation, increases due to narrowing of the beam particle velocity distribution. PACS numbers: 52.50.Dg, 07.77.Gx, 39.10.+j DOI: 10.1134/S0020441209030166