Martin P. Stockli

Nanometer-size surface features produced by single, low energy, highly charged ions

We have studied the interaction of ions with solid surfaces in the limit of slow, highly charged ions (Xe44+ 0.1–20 keV/q). Using atomic force microscopy we are able to see individual ion impact sites on mica, approximately one site per incident ion. Such features are typically circular with ∼20 nm diameter. This damage may be caused by local charge depletion and conversion of the 50 keV Coulomb potential energy of the ions. Contact mode microscopy shows the features to be either pits or blisters depending on scanning direction and force between tip and substrate. By using lateral force microscopy we see the features to be regions of increased friction. The observed surface damage is independent of the ion kinetic energy. Repeated scanning over the same region causes erasure of the observed features.

Non-kinetic Damage on Insulating Materials by Highly Charged Ion Bombardment

Abstract We have measured the damage caused by the impact of low velocity, highly charged ions on insulating surfaces. Atomic force microscopy allows us to observe directly the surface topography with nanometer resolution. Using constant velocity (100 keV) Xe q + ions (25 ⩽ q ⩽ 50) impinging on mica, we observe damage caused by single ion impacts. Impact sites typically are circular hillocks. Within the range and accuracy of the data, the height and volume of the damaged regions are well approximated by a linear function of ion potential energy.

Analog gain of microchannel plates for 1.5–154 keV/q Arq+(3⩽q⩽16)

The gain of microchannel plates operated with low bias voltages in the analog mode has been measured for Arq+ ions (3⩽q⩽16) with energies in the range from 1.5 to 154 keV/q. The results show that the gain, most likely due to the varying number of secondary electrons emitted upon impact of the detected ions, depends substantially on the charge as well as the energy of the ions. The measured gain is shown as a function of the charge state for five different ion energies per charge to assist in the interpretation of the results from the ion sources. The measured gain is also shown as a function of ion impact velocity for all measured charge states, which indicates a rather complex dependence on the ion impact velocity. The interpolated gain is also shown as a function of charge states for four different ion impact velocities. For the lowest ion impact velocity, the gain seems to increase linearly with the ions’s potential energy with the gain measured for Ar16+ being roughly twice as large as the gain measur...

Method of processing ion energy distributions using a Thomson parabola ion spectrograph with a microchannelplate image converter camera

A Thomson parabola ion spectrograph (TP) is a very useful tool for the investigation of pulsed laser ablation. Measurements performed with the TP give useful information about physical processes, ion species and their energy distributions, as well as charge states. For ions with the lower charge states, q<20, complete information about energy distributions of all ionization states of ions can be obtained from a single laser shot. For ions with higher ionization states, parabolas generated in the TP interfere and it is impossible to get energy distributions for all the ion species. In this situation, the registered ions are composed of a few groups with different charge states and different energies. The TP enables the charge states and energetic ranges of different ion groups to be estimated. This presentation describes a method of processing experimental results, obtained from a TP, using a microchannelplate (MCP) image converter. Ion energy distributions for C1+–C6+ and Ta1+–Ta12+ are shown, and the eff...

Recent performance of the SNS H(-) ion source and low-energy beam transport system.

Recent measurements of the H(-) beam current show that SNS is injecting about 55 mA into the RFQ compared to ∼45 mA in 2010. Since 2010, the H(-) beam exiting the RFQ dropped from ∼40 mA to ∼34 mA, which is sufficient for 1 MW of beam power. To minimize the impact of the RFQ degradation, the service cycle of the best performing source was extended to 6 weeks. The only degradation is fluctuations in the electron dump voltage towards the end of some service cycles, a problem that is being investigated. Very recently, the RFQ was retuned, which partly restored its transmission. In addition, the electrostatic low-energy beam transport system was reengineered to double its heat sinking and equipped with a thermocouple that monitors the temperature of the ground electrode between the two Einzel lenses. The recorded data show that emissions from the source at high voltage dominate the heat load. Emissions from the partly Cs-covered first lens cause the temperature to peak several hours after starting up. On rare occasions, the temperature can also peak due to corona discharges between the center ground electrode and one of the lenses.

Producing persistent, high-current, high-duty-factor H− beams for routine 1 MW operation of Spallation Neutron Source (invited)a)

Since 2009, the Spallation Neutron Source (SNS) has been producing neutrons with ion beam powers near 1 MW, which requires the extraction of ∼50 mA H(-) ions from the ion source with a ∼5% duty factor. The 50 mA are achieved after an initial dose of ∼3 mg of Cs and heating the Cs collar to ∼170 °C. The 50 mA normally persist for the entire 4-week source service cycles. Fundamental processes are reviewed to elucidate the persistence of the SNS H(-) beams without a steady feed of Cs and why the Cs collar temperature may have to be kept near 170 °C.

ELECTRONIC EXCITATION EFFECT IN THE SPUTTERING OF CONDUCTIVE MATERIALS BOMBARDED BY HIGHLY CHARGED HEAVY IONS

Abstract We have measured the yield of the sputtered ions from the surface of solid targets of conductive materials (Al, Ni, Cu, Ag, Si) bombarded by Xe q + ( q =15–44) at 1.0 MeV ( v p =0.55 a.u). The experiment was performed using micro-bunched highly charged Xe ion beams from CRYEBIS at Kansas State University. The secondary ions were detected and mass analyzed by a TOF spectrometer. The secondary ion yields showed a significant increase with increasing projectile charge state q , demonstrating the electronic sputtering in the interaction of slow highly charged ions with conductive materials. The increase rate became larger above q =26 compared to lower charge states, and showed a more prominent increase above q =35. These strong onsets of the increase rate at particular incident charge states can be attributed to the opening up of new inner shell holes in the incident highly charged ions. This is the first systematic observation of electronic sputtering of conductive materials using slow highly charged heavy ions bearing high potential energy.

The KSU-CRYEBIS: A unique ion source for low-energy highly charged ions

The CRYogenic electron beam ion source at Kansas State University has been in operation since spring 1989. It produces up to Ar18+, Kr34+, and Xe44+ on a regular basis. Acceleration tubes and a beamline system were added during 1990. The ion source is mounted on a high voltage platform, which can be used to vary the ion energies between 2 and 200 kV per charge. The extraction‐, analyzing‐, and beamline systems were designed to accept a wide range of ion masses and charge states with the mass energy product (M⋅E/q2) stretching over four orders of magnitude. The system is used to study the low‐energy highly charged ions interacting with electrons, atoms, thin foils, and surfaces.

Developments on the KSU-CRYEBIS, a user facility for low energy, highly charged ions

The KSU-CRYEBIS, a CRYogenic electron beam ion source, supplies experiments with low-energy, highly charged ions of numerous species. The supplied charge states cover the range from 1+ to 52+, with typical beam currents of a nA for low charge states and a few pA for the highest charge states. The ion energies cover the range from 0.3 to 165 keV per charge. This is an unusually broad range of final ion energies and hence requires an unusual dynamic ion transport system. This paper presents advances made with respect to the CRYEBIS ion beam transport, diagnostics, and identification. In addition, an update on the developments of ion beams with very high duty cycles is given.

Multiply charged ion-induced secondary electron emission from metals relevant for laser ion source beam diagnostics

The number of secondary electrons, γ, emitted when multiply charged ions impact on metallic probe surface was measured to make the quantitative ion diagnostics based on this process more precise. The electron yield γ(q,Ei) was measured for Taq+ and Xeq+ ions (q=6–41) in the region of ion kinetic energy per atomic mass up to Ei/A∼34 keV/amu. For highly charged Xeq+ ions (q>16), a minimum of the electron yield, γMIN, was observed in its dependence on Ei. With increasing q, the γMIN shifts to higher energies. The comparison of available data for Nq+, Neq+, Arq+, Xeq+, and Auq+ ions shows that one can create a similarity law describing the dependence of γMIN for those elements in the Ei/A representation. The value of γ/q evaluated from compiled data ranges from ≈0.3 to ≈3.5 in dependence on q and Ei.