H. P. Winter

Statistics of ion‐induced electron emission from a clean metal surface

With a novel combination of experimental approaches, ion‐induced electron emission from clean polycrystalline gold bombarded with (1–5 keV/amu) H+, H+2, and H+3 has been studied by measuring total electron yields, ejected‐electron energy distributions, and, in particular, electron emission statistics (ES). Evaluation of ES included corrections for backscattering of electrons from the surface of the applied solid‐state electron detector. By comparing ES for atomic and molecular ions of equal impact velocities, a negative molecular effect could be clearly demonstrated. This effect was further investigated by comparing measured ES for molecular ion impact with ES synthesized from measured ES for H+ and H+2 ion impact, respectively. The quality of approximating measured ES by Poisson and Polya statistical distributions was investigated in detail and found to be generally unsatisfactory.

On the measurement of statistics for particle-induced electron emission from a clean metal surface

Statistics of electron emission from clean gold under impact of slow (< 1 au) heavy particles have been measured by means of a surface barrier detector for collecting the ejected and subsequently accelerated electrons. Backscattering of these electrons from the detector surface causes a characteristic background between individual peaks of the pulse-height spectra, which must quantitatively be taken into account before evaluation of the measured statistics. From the latter, among other pieces of information the mean electron emission yield γ can be calculated. For γ values below and of the order of unity such determined emission statistics deviate clearly from a Poisson distribution, the reason of which is finally discussed.

Reconstruction of plasma edge density profiles from Li I (2s-2p) emission profiles

The injection of 10-100 keV Li0 diagnostic beams into magnetically confined fusion plasmas causes collisionally induced Li I emission at 670.8 nm, in close relation to the edge plasma electron density. A numerical method for quantitative reconstruction of the plasma density exclusively from relative Li I 670.8 nm emission profiles as measured along the diagnostic beam has been developed, involving all relevant collisional interactions of the Li atoms with plasma constituents. The applicability of the described algorithm is illustrated by experimental results obtained for the TEXTOR Tokamak edge plasma at KFA Julich.

Single‐stage 5 GHz ECR‐multicharged ion source with high magnetic mirror ratio and biased disk

A low‐cost, single‐stage 5 GHz electron cyclotron resonance (ECR) multicharged ion source (MCIS) has been constructed for various atomic collision experiments. It features an axial magnetic field with a mirror ratio of up to five, and a magnetic hexapole field produced by a simple Nd–Fe–B permanent‐magnet assembly. A disk probe axially mounted near the ECR resonance zone opposite to the ion extraction, and negatively biased with respect to the ECR plasma potential, permits reduction of the appropriate neutral feeding gas pressure by an order of magnitude, resulting in greatly improved ion charge state distributions, as normally offered by two‐stage ECR–MCIS only. We present performance data for multicharged ion production from Ar and N2, including measured ion current emittances.

Auger neutralization of multiply charged noble gas ions at a tungsten surface

Abstract Neutralization processes for slow doubly and triply charged noble gas ions impinging on clean polycrystalline tungsten are studied by observation of the resulting ejected electron energy distributions. A sequence of Auger de-excitation and Auger neutralization processes is demonstrated to occur and therefore strong evidence for participation of one-electron like transitions is presented. In the special case of Ne 2+ impact this may yield straightforward information on the density of states of the uppermost surface layer.

A highly sensitive quartz-crystal microbalance for sputtering investigations in slow ion–surface collisions

A quartz-crystal microbalance technique for measuring total sputter yields in ion–surface collisions is described. The electronic circuit to drive the quartz crystal ensures low noise and high frequency stability. By measuring total sputter yields for impact of singly charged ions on LiF target films a sensitivity limit of 0.5% of a monolayer per minute could be achieved.

Fast lithium‐beam spectroscopy of tokamak edge plasmas

Plasma‐wall interaction and impurity transport processes in the outermost region of magnetically confined hot plasmas (the so‐called plasma edge) must be well understood for successful development of future thermonuclear fusion reactors. To this goal, sufficiently detailed edge plasma diagnostics are in great demand. By injecting a fast Li beam into the edge plasma region, a great number of information can be obtained with excellent space and time resolution. This so‐called Li‐beam plasma spectroscopy gives access not only to edge plasma density profiles from the collisionally excited Li atoms, but also to the impurity concentration and temperature profiles via line emission induced by electron capture from the injected Li atoms by the impurity ions. Full utilization of all capabilities requires a reliable data base for the atomic collision processes involving injected Li atoms and plasma constituents (i.e., electrons, hydrogen ions, and relevant impurities in their various charge states), since a precise...

Electron Emission from Polycrystalline Lithium Fluoride Induced by Slow Multicharged Ions

Total electron yields have been determined from electron emission statistics measured for impact of H+, Nq+ (q = 1, 5, 6) and Arq+ (q = 1, 3, 6, 9) on clean, polycrystalline lithium fluoride, by varying the impact energy from almost zero up to (10 × q) keV. Dependences of the electron emission statistics and yields on projectile charge and impact energy deviate strongly from the corresponding properties for clean metal surfaces, because of grossly different contributions of potential and kinetic emission and, in particular, a more efficient secondary electron emission if fast electrons are ejected from neutralising projectiles inside the LiF bulk.

Recent advances in understanding particle-induced electron emission from metal surfaces

Abstract We present a collection of experimental results on electron emission from a clean gold surface bombarded by slow neutral atoms and singly or multiply charged ions, which have been obtained by measuring the corresponding emission statistics. Apart from an intrinsic interest in the emission statistics themselves,our investigations provide a physically sound distinction between potential and kinetic emission processes, and at sufficiently low projectile energies they permit an unambiguous separation of both contributions. The threshold impact energy for kinetic emission can now be correctly determined and dependencies of the kinetic emission process on charge state and molecular composition of the projectiles can clearly be demonstrated. In their entirety our results provide new information on the relevant mechanisms for slow particle-induced electron emission.

Electron emission from metal surfaces bombarded by slow neutral and ionized particles

Abstract Slow ( E ≤5 keV/amu) particle-induced electron emission from clean polycrystalline gold has been investigated based on measurements of electron statistics. The latter were found to deviate considerably from commonly assumed simple statistical functions, in particular Poisson distributions. Bombardment with respectively hydrogen atoms, protons and molecular hydrogen ions ( E ≤ 5 keV/amu) showed a clear correlation of the electron emission yield with the number of electrons per proton in the projectile, from which observed “molecular effects” can be explained. Furthermore, bombardment with respectively neutral and singly and multiply charged noble gas ions of He, Ne and Ar was applied to separate contributions by potential and kinetic emission processes. This can only be achieved below certain impact energy limits, which have been determined in the present study.