F. Aumayr

Interaction of slow multicharged ions with solid surfaces

Abstract The present report deals with the main aspects of the interaction of slow (impact velocity typically below 1 a.u.) multicharged ions (MCI) with atomically clean solid surfaces of metals, semiconductors and insulators. It is based to a large extent on the results obtained by the authors and their affiliates within the Human Capital and Mobility Network of the European Union on “Interaction of Slow Highly Charged Ions with Solid Surfaces”, which has been carried out during the last three years. After briefly reviewing the pertinent historical developments, the experimental and theoretical techniques applied nowadays in the field of MCI-surface interaction studies are explained in detail, discussing especially the transient formation and relaxation of “hollow atoms” formed in such collisions. Further on, the status of the field is exemplified by numerous results from recent studies on MCI-induced emission of slow and fast electrons (yields and energy distributions), projectile soft X-ray spectroscopy, charge-changing and energy loss of scattered and surface-channelled projectiles, MCI-induced sputtering and secondary ion emission, and coincidence measurements involving different signatures from the above processes. The presented theoretical and experimental work has greatly contributed to an improved understanding of the strongly inter-related electronic transitions taking place for MCI above, at and below a solid surface.

Single ion induced surface nanostructures: a comparison between slow highly charged and swift heavy ions

This topical review focuses on recent advances in the understanding of the formation of surface nanostructures, an intriguing phenomenon in ion-surface interaction due to the impact of individual ions. In many solid targets, swift heavy ions produce narrow cylindrical tracks accompanied by the formation of a surface nanostructure. More recently, a similar nanometric surface effect has been revealed for the impact of individual, very slow but highly charged ions. While swift ions transfer their large kinetic energy to the target via ionization and electronic excitation processes (electronic stopping), slow highly charged ions produce surface structures due to potential energy deposited at the top surface layers. Despite the differences in primary excitation, the similarity between the nanostructures is striking and strongly points to a common mechanism related to the energy transfer from the electronic to the lattice system of the target. A comparison of surface structures induced by swift heavy ions and slow highly charged ions provides a valuable insight to better understand the formation mechanisms.

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.

Recent developments for plasma edge diagnostics using atomic beams

Abstract Li-atom beams with velocities of 1.5 × 105 cm/s and 1× 106 cm/s have been used to measure ne profiles in the density range 1011–1013 cm−3 from the Li line emission with a spatial resolution of about 1 mm. Injecting in addition a different type of atoms, the ionization rate of which shows a strong dependance on the electron temperature in the interesting range of 5–100 eV (e.g. carbon and helium), also radial Te profiles are obtained. A high intensity 30 keV Li beam is employed to measure radial profiles of impurity ion concentrations (e.g. He, C, 0) by charge exchange recombination spectroscopy. For the different purposes several types of injectors have been developed: thermal beams using ovens, suprathermal beams using laser ablation techniques, and a high energy beam using a plasma ion source in combination with a charge exchange cell. They are located at different poloidal and toroidal positions. The spatial line emission profiles of the beams are recorded either by Si diode array cameras or by a photomultiplier in combination with a scanning mirror so that a spatial resolution better than 2 mm is achieved. The combination of all these systems delivers information about the structure of the TEXTOR boundary layer in poloidal and toroidal direction.

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.

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.