-A cascade arc is used as a source for a hydrogen/argon particle beam. The plasma is produced at thermal plasma conditions in the pressure range of 0.1 to 1 bar. It is expanded with a high velocity into a vacuum chamber. Measurements with a Langmuir double probe and with a mass spectrometer are performed. The resulting parameters of the plasma and the extracted ions are discussed. Molecular processes as charge exchange and dissociative recombination appear to be important. l -IntroductionWith a cascade arc a thermal plasma is produced. By allowing this plasma to expand into a vacuum chamber, a high velocity jet is obtained. This configuration combines an efficient production of reactive particles (radicals, ions) with a large, directed particle flux. The flowing cascade arc has been studied for argon plasmas in relation to carbon deposition /l/. Applications are found in amorphous and crystalline carbon deposition /1,2/, and more recently also in silicon deposition /3/. Hydrogen sources can be used for e.g. proton production, plasma neutralizers /4/ and for production of atomic hydrogen. On a full hydrogen flowing cascade arc so far no experimental experience exists. In the context of plasma neutralization of fast D--beams a numerical model has been developed /4/. In this work we present first experimental results for T , and n, on the expanding beam of a hydrogen-argon plasma. Some discussion on the dominant atomic and molecular processes will also be given. -Ex~erimental setupThe apparatus basically consists of a cascade arc and an expansion chamber ( fig. 1). The cascade arc consists of a cathode head, a stack of 12 insulated copper plates with a central bore and an anode plate. The central bore is in the first four plates 2 mm, then four plates with a 3 mm bore and finally four with a 4 mm bore, so the arc channel is diverging. At the cathode side argon gas is injected, hydrogen can be added halfway downstream the arc channel. Through the anode plate (bore: 4mm) the plasma is allowed to expand freely into vacuum. Typical operation conditions are a discharge current of 50 A, an argon flow of 2.5 slm and a hydrogen flow of 0. At the tip, the wires have a distance of approximately 1 mm. The Debye length in the plasma is about 10 pm at typical plasma parameters n, = 1018 me3 and Te = 0.5 eV, so the distance between the wires is about hundred Debye lengths and small compared to temperature and density gradient lengths. Probe measurements give data on electron density and electron temperature in the expansion. With a mass spectrometer the various positive ion fluxes in the plasma are studied. Particles are extracted at the end of the vessel through a 0.1 mm pinhole in the backwall. The information gained by these measurements gives insight on the occurrence of species and on the fluxes.The probe measurements were done at a position on the plasma axis 7 cm from the anode plate.Results are given on the influence of the pressure and of the gas ratio H2/Ar on n, and Te.According to fig. 2a the electron densit...

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