Nuclear Instruments and Methods in Physics Research B13 (1986) 658-662
`North-Holland, Amsterdam
`
`HIGH KINETIC ENERGY (1-10 eV) LASER SUSTAINED NEUTRAL ATOM BEAM SOURCE
`
`Jon B. CROSS and David A. CREMERS
`
`Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545. USA
`
`Development of high energy (1-10 eV) neutral atom beams has relied primarily on the use of charge exchange or DC/RF
`discharge techniques. A new source is described which uses a laser sustained plasma technique for producing high intensity
`(>1O15/cmz s) and high translational energy (>1 eV) atomic beam species for use in gas—surface scattering experiments. Laser
`sustained plasmas have demonstrated temperatures of 10 000K in xenon. 18000K in argon, and a predicted temperature of
`30 000 K in helium. Combining this plasma with hydrodynamic expansion techniques should produce atomic beam velocities greater
`than 10 km/ s for many species. Initial experiments with xenon using 70W of CO2 laser power have demonstrated beam kinetic
`temperatures of 8-9000 K with Mach numbers of 4-6 resulting in peak velocities for xenon of 1.5 km/s (1.5 eV). Extrapolation of these
`results to helium predicts that velocities in excess of 10 km/ s are possible but will require laser powers in excess of 1.3 kW.
`
`1. Introduction
`
`The production of intense (1019 /ssr) high energy
`(1-10 eV) beams of heavy (>100 amu) molecules
`and/or atoms is straightforward using heated nozzle
`helium seeding techniques [1]. The production of in-
`tense high energy low mass (<40 amu) beams, how-
`ever, is a difficult challenge. The two main techniques
`which use charge exchange [1] and DC arcs [2] have a
`number of disadvantages. Charge exchange suffers
`from intensity limitations at low energies (<100eV)
`because of space charge defocusing. Although attempts
`to overcome this problem using electron neutralization
`of the beam have been attempted, this source is used
`primarily for producing large beam fluxes at energies
`greater than 100 eV. Direct current arcs [2] have been
`used extensively in gas phase scattering experiments
`but exhibit instabilities due to electrode erosion and
`
`have large power (10-20 kW) and water cooling re-
`quirements. This source also produces reduced vel-
`ocities for reactive species (4 km/ s for O-atoms) which
`have to be added to the lower
`temperature flow
`downstream of the arc in order to protect the electrodes
`from catastrophic erosion. Light
`species
`such as
`O-atoms produced by radio frequency [3a] and high
`pressure microwave sources [3b] are limited to about 1 eV
`translational energy.
`
`2. Background
`
`It has been demonstrated (see ref. [4] for an exten-
`sive listing of laser sustained discharge references) that
`a free standing discharge of high temperature can be
`produced by focusing the beam from a sufficiently
`
`0168-583X/ 86/ $03.50 © Elsevier Science Publishers B.V.
`(North—Holland Physics Publishing Division)
`
`powerful cw-CO2 laser (wavelength = 10.6 pm) in inert
`and/ or molecular gases at atmospheric pressures or
`greater. The discharge is located near the focal point of
`the laser beam and does not require electrodes or
`coupling coils for production. Since the focused power
`density of the cw-laser beam needed to sustain the
`discharge is 106-107 W/cmz, a factor of about 10 lower
`than typical gas breakdown power densities, a suitable
`spark is required for initiation. This can be provided
`with a Tesla-coil-electrode arrangement or a pulsed
`laser which produces a spark within the focal volume of
`the cw-laser beam. The pulsed plasma contains a high
`density (>10“‘ cm”) of electrons which act as an
`absorption center for the cw-laser beam. Once started,
`the plasma operates continuously as long as sufficient
`power is supplied to the focal volume. At laser frequen-
`cies (30 THZ) which are an order of magnitude above
`the plasma frequency [5], absorption occurs primarily
`by free—free transitions (inverse bremsstrahlung) as-
`sociated with electron—ion collisions. Power loss results
`from electron-ion recombination, radiation, and ther-
`mal conduction. The very high temperature of the laser
`sustained plasma is due to its higher power density as
`compared to conventional RF sources. At RF frequen-
`cies, which are below the plasma frequency, mainly the
`outer layer of the plasma is heated because of the very
`large cross section for direct EM wave plasma interac-
`tion with the interior being heated by conduction. A
`typical inductively coupled 1.5 kW plasma with a vol-
`ume of 8 cm} has a power density of 188 W cm”
`compared to 5 X 103 W cm” for a 1 mm diameter
`plasma sustained by a 45 W laser beam. Since the laser
`sustained plasma is highly dependent on power density,
`the mode quality of the laser beam and the optical
`properties of the laser beam focusing system determine
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`ASML 1129
`
`ASML 1129