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We present the design of ATJ graphite [1] rods developed for ablation experiments under high heat flux (up to 50 MW/m2) in the lower divertor of the DIII-D tokamak [2], a magnetic plasma confinement device. This work is motivated by the need to test ablation models relevant to carbon-based thermal shields used in high-speed spacecraft atmospheric entries, where the heat fluxes encountered can be comparable to those achieved in the DIII-D divertor plasma. Several different designs for the flow-facing side of the rod are analyzed, including “sharp nose,” “blunt,” and “concave”. The last shape is studied for its potential to lower heat fluxes at the rod surface by increased radiation from trapped neutrals and reduced parallel plasma pressure. We also analyze the possibility of applying a thin (approximately 30 microns) layer of silicon carbide (SiC) to the exposed part of several carbon ablation rods to benchmark its erosion calculations and lifetime predictions. Such calculations are of interest as SiC represents a promising material for both thermal protection systems (TPS) and a fusion plasma-facing material (PFM). [3,4] Preliminary results from the DIII-D rod ablation experiments are also discussed.

Physical review accelerators and beams 24(5), 050101 (2021). doi:10.1103/PhysRevAccelBeams.24.050101, Published by American Physical Society, College Park, MD

An electrostatic cryogenic storage ring, CSR, for beams of anions and cations with up to 300 keV kinetic energy per unit charge has been designed, constructed and put into operation. With a circumference of 35 m, the ion-beam vacuum chambers and all beam optics are in a cryostat and cooled by a closed-cycle liquid helium system. At temperatures as low as (5.5 $\pm$ 1) K inside the ring, storage time constants of several minutes up to almost an hour were observed for atomic and molecular, anion and cation beams at an energy of 60 keV. The ion-beam intensity, energy-dependent closed-orbit shifts (dispersion) and the focusing properties of the machine were studied by a system of capacitive pickups. The Schottky-noise spectrum of the stored ions revealed a broadening of the momentum distribution on a time scale of 1000 s. Photodetachment of stored anions was used in the beam lifetime measurements. The detachment rate by anion collisions with residual-gas molecules was found to be extremely low. A residual-gas density below 140 cm$^{-3}$ is derived, equivalent to a room-temperature pressure below 10$^{-14}$ mbar. Fast atomic, molecular and cluster ion beams stored for long periods of time in a cryogenic environment will allow experiments on collision- and radiation-induced fragmentation processes of ions in known internal quantum states with merged and crossed photon and particle beams., PDFLaTeX with 17 pages, 16 figures, 2 tables. This article has been accepted by Review of Scientific Instruments. After it is published, it will be found at http://scitation.aip.org/content/aip/journal/rsi

The photorecombination rate coefficients of potassium-like Fe viii ions forming calcium-like Fe vii and of argon-like Fe ix forming potassium-like Fe viii were measured by employing the merged electron-ion beams method at the Heidelberg heavy-ion storagering TSR. New theoretical calculations with the AUTOSTRUCTURE code were carried out for dielectronic recombination (DR) and trielectronic recombination (TR) for both ions. We compare these experimental and theoretical results and also compare with previously recommended rate coefficients. The DR and TR resonances were experimentally investigated in the electron-ion collision energy ranges 0–120 eV and 0–151 eV for Fe viii and Fe ix. Experimentally derived Fe viii and Fe ix DR + TR plasma rate coefficients are provided in the temperature range kBT = 0.2 to 1000 eV. Their uncertainties amount to

The photorecombination spectrum of 48Ti18+ was measured employing the merged electron-ion beams technique at a heavy-ion storage ring. The experimental electron-ion collision energy range 0-80 eV comprises all dielectronic recombination (DR) resonances associated with 2s->2p (Delta N =0) core excitations as well as trielectronic recombination (TR) resonances that involve 2s2->2p2 core double-excitations. At low collision energies DR resonances are observed that are associated with the excitation of metastable 2s2p 3P0 primary ions with nearly infinite lifetime for the isotope 48Ti with zero nuclear spin. For the isotope 47Ti with nonzero nuclear spin hyperfine quenching of these resonances occurs. The procedure for obtaining the associated time constant from a recombination measurement is outlined., 4 pages, 2 figures, procceedings of the 13th International Conference on the Physics of Highly Charged ions, 28.8-1.9.2006, Belfast, Northern Ireland, submited to J. Phys. Conf. Ser

Recent spectroscopic models of active galactic nuclei have indicated that the recommended electron-ion recombination rate coeficients for iron ions with partially filled Mshells are incorrect in the temperature range where these ions form in photoionized plasmas. We have investigated this experimentally for Fe7+ forming Fe6+, Fe8+ forming Fe7+, and Fe13+ forming Fe12+. The recombination rate coeficient was measured employing the electron-ion merged beams method at the Heidelberg heavy-ion storage-ring TSR. The measured energy range encompassed at least all dielectronic recombination (DR) resonances associated with core excitations within the M-shell of the parent ions. Already in our first measurement, that is for Fe13+, we find unusually strong DR resonances at low electron-ion collision energies leading to low temperature plasma DR rate coeficients orders of magnitude larger than the recommended rate coeficient.

Measurements on the energetic structure of the dissociative recombination rate coefficient in the millielectronvolt range are described for ions produced in the lowest rotational levels by collisional cooling and stored as a fast beam in the magnetic storage ring TSR (Test Storage Ring). The observed resonant structure is consistent with that found previously at the storage ring facility CRYRING in Stockholm, Sweden; theoretical predictions yield good agreement on the overall size of the rate coefficient, but do not reproduce the detailed structure. First studies on the nuclear spin symmetry influencing the lowest level populations show a small effect different from the theoretical predictions. Heating processes in the residual gas and by collisions with energetic electrons, as well as cooling owing to interaction with cold electrons, were observed in long-time storage experiments, using the low-energy dissociative recombination rate coefficient as a probe, and their consistency with the recent cold measurements is discussed.

To study electron–ion interactions at the Heidelberg heavy-ion storage ring, electron beams with low-energy spreads and dc-currents of milliamperes are desired. Measurements of the photoelectron energy distribution showed that electron beams with energy spreads of 5–8 meV can be obtained from GaAs photocathodes, cooled to about LN 2 -temperature. However, in order to get milliamperes beam currents, the laser illumination has to be increased up to 1 W, causing substantial cathode heating. The presented new electron gun design based on sapphire-substrate transmission-mode photocathodes, cooled by LN 2 , stabilizes the GaAs bulk temperature under 1 W laser illumination at about 95 K and thereby provides the prerequisites for an electron gun being operated at milliampere-currents with low-energy spreads.

An ultra cold electron source was developed for the storage ring TSR in Heidelberg to study electron-ion interactions with high energy resolution. The heart of the source is a GaAs photocathode which emits electrons with energy spreads below 10 meV. Photoemitted electrons are extracted in the space charge mode and then undergo adiabatic magnetic expansion and adiabatic acceleration to obtain an ultra cold electron beam which is overlapped with the stored ion beam in a straight section of the ring. In the first recombination measurements on HD+ unprecedented energy resolution for low-energy resonances was found, demonstrating a transverse and longitudinal temperature of the electron beam of about 0.5 meV and 0.02 meV, respectively.

A novel cryogenic electrostatic storage ring is planned to be built at the Max-Planck Institute for Nuclear Physics in Heidelberg. The machine is expected to operate at low temperatures (∼ 2K) and to store beams with kinetic energies between 20 to 300 keV. An electron target based on cooled photocathode technology will serve as a major tool for the study of reactions between molecular ions and electrons. Moreover, atomic beams can be merged and crossed with the stored ion beams allowing for atom molecularion collision studies at very low up to high relative energies. The proposed experimental program, centered around the physics of cold molecular ions, is shortly outlined.

A source of ultra-cold magnetically guided DC electron beams with a transmission mode GaAs-photocathode has been developed. The mA currents and the required stability are obtained by using state-of-the-art technology of photocathode fabrication and cleaning in UHV. To optimize energy spreads from photocathodes, studies of the complete (longitudinal and transverse) energy distribution of the magnetized electron beam were performed. They allowed us to get a detailed look on the energy loss and momentum scattering of the electrons at the interface and finally, to obtain narrow electron energy spreads (below 10 meV ) from the photocathode.

For the investigation of electron-ion collisions at the Test Storage Ring with a much enhanced energy resolution a cold electron source with cryogenic GaAs(Cs,O) is being developed. To ensure optimal performance of GaAs photocathodes the two-dimensional energy distributions of emitted electrons are studied at room and low (90 K) temperature. The photocathode performance data promise electron beams of several 10 mA/cm2 with an energy spread ⩽ 10 meV.

We used an ion-selective electrode to measure potassium release from bacteria treated with antimicrobial peptides. This broadly applicable and simple technique for assessing membrane integrity deserves greater use.

Rhesus monkey bone marrow expresses a cathelicidin whose C-terminal domain comprises a 37-residue alpha-helical peptide (RL-37) that resembles human LL-37. Like its human counterpart, RL-37 rapidly permeabilized the membranes of Escherichia coli ML-35p and lysed liposomes that simulated bacterial membranes. When tested in media whose NaCl concentrations approximated those of extracellular fluids, RL-37 was considerably more active than LL-37 against staphylococci. Whereas human LL-37 contains five acidic residues and has a net charge of +6, rhesus RL-37 has only two acidic residues and a net charge of +8. Speculating that the multiple acidic residues of human LL-37 reduced its efficacy against staphylococci, we made a peptide (LL-37 pentamide) in which each aspartic acid of LL-37 was replaced by an asparagine and each glutamic acid was replaced by a glutamine. LL-37 pentamide's antistaphylococcal activity was substantially greater than that of LL-37. Thus, although the precursor of LL-37 is induced in human skin keratinocytes by injury or inflammation, its insufficiently cationic antimicrobial domain may contribute to the success of staphylococci in colonizing and infecting human skin.

Photoemission from GaAs cathodes with negative electron affinity (NEA) is applied for producing electron beams with very low longitudinal and transverse velocity spread. GaAs transmission-mode cathodes were activated with Cs and either O2 or NF3 in an extremely high vacuum setup (base pressure below 10−12 mbar). Quantum efficiencies of 20%–25% (at 670 nm) and long dark lifetimes (about 1000 h) could be achieved for both types of activation in a reproducible way. Using a method based on the adiabatic transverse expansion of an electron beam in a spatially decreasing magnetic field, the mean transverse energy (MTE) of the photoemitted electrons was measured systematically, recording longitudinal energy distribution curves. Both the MTE and the longitudinal energy spread strongly depend on the value of NEA and the position of the extracted distribution relative to the bulk conduction band minimum. Electrons with energies above the conduction band minimum are thermalized with the lattice temperature of the cathode, while electrons with energies below this level show a non-Maxwellian distribution with enhanced transverse energies. Thus, when extracting all electrons in a current limited emission mode, the MTE increases with the absolute value of NEA and reaches values up to ≈100 meV. By cutting off the low energy electrons with an external potential barrier, the longitudinal as well as transverse energy spread of the extracted electron ensemble are reduced. The MTE could be reduced down to about 28 meV at room temperature and to about 14 meV at liquid nitrogen temperature. The behavior of the MTE was found equivalent for (Cs, O) and for (Cs, F) activation layers on the same cathode. Conclusions about energy loss and scattering in the emission of photoelectrons from NEA GaAs cathodes are discussed.

We have measured electron-ion recombination for C-like Mg6 + forming Mg5 +, and for B-like Mg7 + forming Mg6 +. These studies were performed using a merged electron-ion beam arrangement at the TSR heavy ion storage ring located in Heidelberg, Germany. Both primary ions have metastable levels with significant lifetimes. Using a simple cascade model we estimate the population fractions in these metastable levels. For the Mg6 + results, we find that the majority of the stored ions are in a metastable level, while for Mg7 + the metastable fraction is insignificant. We present the Mg6 + merged beams recombination rate coefficient for DR via N = 2 → N' = 2 core electron excitations (ΔN = 0 DR) and for Mg7 + via 2 → 2 and 2 → 3 core excitations. Taking the estimated metastable populations into account, we compare our results to state-of-the-art multiconfiguration Breit-Pauli theoretical calculations. Significant differences are found at low energies where theory is known to be unreliable. Moreover, for both ions we observe a discrepancy between experiment and theory for ΔN = 0 DR involving capture into high-n Rydberg levels and where the stabilization is primarily due to a radiative transition of the excited core electron. This is consistent with previous DR experiments on M-shell iron ions which were performed at TSR. The large metastable content of the Mg6 + ion beam precludes generating a plasma recombination rate coefficient (PRRC). However, this is not an issue for Mg7 + and we present an experimentally derived Mg7 + PRRC for plasma temperatures from 400 K to 107 K with an estimated uncertainty of less than 27% at a 90% confidence level. We also provide a fit to our experimentally derived PRRC for use in plasma modeling codes.

A new and technologically challenging project, the electrostatic Cryogenic Storage Ring CSR, is presently under construction at the Max-Planck-Institute for Nuclear Physics in Heidelberg. Applying liquid helium cooling, the CSR, with 35 m circumference, will provide a low temperature environment of only a few Kelvin and an extremely high vacuum of better than 10 −13 mbar. To realize these conditions the mechanical design has been completed and now the first quarter section is in the construction phase. For the onion skin structure of the cryogenic system we have at the outer shell the cryostat chambers, realized by welded rectangular stainless steel frames with aluminum plates. The next two shells are fabricated as aluminum shields kept at 80 and 40 K. The inner vacuum chambers for the experimental vacuum consist of stainless steel chambers cladded with external copper sheets connected to the LHe lines for optimized thermal equilibration and cryopumping. Additional large surface 2 K units are installed for cryogenic pumping of H 2 . The mechanical concepts and the realization will be presented in detail.

The decay of excited aluminum-cluster anions (Al{sub n}{sup -}, n=4 and 5) has been investigated in a cryogenic linear ion-beam trap. A power-law is found to accurately reproduce the time dependence of the observed decay rates at early storage times, although the exponents are significantly larger than the typically observed 1/t decay. It is shown that the power-law exponent is, at most, weakly dependent on the cluster electron affinity and heat capacity. A previous power-law exponent model for small clusters is also shown to be in disagreement with both investigated species. The attribution of a drop in the decay rate at later times to radiative cooling as observed in larger molecules also does not appear justified in our case. A strong dependence of the power-law exponent on the ambient temperature was observed.

A method to map out the energy distribution N(E∥,E⊥) of an electron beam as a function of the longitudinal (E∥) and transverse (E⊥) energy has been developed and applied to study the photoemission process from GaAs(Cs, O) at 90 K. The method proceeds by “marking” electrons with fixed longitudinal energy E∥b and a subsequent measurement of the associated differential transverse energy distribution N⊥(E∥b,E⊥), applying an adiabatic magnetic compression technique. The complete energy distribution N(E∥,E⊥) of electrons from a GaAs(Cs, O) photocathode obtained by a stepwise variation of E∥b provides details about the transfer of electrons through the GaAs(Cs, O)–vacuum interface and demonstrates that not only electron energy loss, but also elastic electron scattering is of crucial importance in the escape process.

We report on the realization and operation of a fast ion beam trap of the linear electrostatic type employing liquid helium cooling to reach extremely low blackbody radiation temperature and residual gas density and, hence, long storage times of more than 5 min which are unprecedented for keV ion beams. Inside a beam pipe that can be cooled to temperatures15 K, with 1.8 K reached in some locations, an ion beam pulse can be stored at kinetic energies of 2-20 keV between two electrostatic mirrors. Along with an overview of the cryogenic trap design, we present a measurement of the residual gas density inside the trap resulting in only 2 x 10(3) cm(-3), which for a room temperature environment corresponds to a pressure in the 10(-14) mbar range. The device, called the cryogenic trap for fast ion beams, is now being used to investigate molecules and clusters at low temperatures, but has also served as a design prototype for the cryogenic heavy-ion storage ring currently under construction at the Max-Planck Institute for Nuclear Physics.

We report on an energy-sensitive imaging detector for studying the fragmentation of polyatomic molecules in the dissociative recombination of fast molecular ions with electrons. The system is based on a large area (10 cm x 10 cm) position-sensitive, double-sided Si-strip detector with 128 horizontal and 128 vertical strips, whose pulse height information is read out individually. The setup allows to uniquely identify fragment masses and is thus capable of measuring branching ratios between different fragmentation channels, kinetic energy releases, as well as breakup geometries, as a function of the relative ion-electron energy. The properties of the detection system, which has been installed at the TSR storage ring facility of the Max-Planck Institute for Nuclear Physics in Heidelberg, is illustrated by an investigation of the dissociative recombination of the deuterated triatomic hydrogen cation D2H+. A huge isotope effect is observed when comparing the relative branching ratio between the D2+H and the HD+D channel; the ratio 2B(D2+H)/B(HD+D), which is measured to be 1.27 +/- 0.05 at relative electron-ion energies around 0 eV, is found to increase to 3.7 +/- 0.5 at ~5 eV., 11 pages, 12 figures, submitted to Physical Review A

Compact proximity focused vacuum tubes with GaAs(Cs,O) photocathodes are used for experimental studying spindependent phenomena. Firstly, spin-dependent emission of optically oriented electrons from p-GaAs(Cs,O) into vacuum in a magnetic field normal to the surface was observed in a nonmagnetic vacuum diode. This phenomenon is explained by the jump in the electron g-factor at the semiconductor-vacuum interface. Due to this jump, the effective electron affinity on the semiconductor surface depends on the mutual direction of optically oriented electron spins and the magnetic field, resulting in the spin-dependent photoemission. It is demonstrated that the observed effect can be used for the determination of spin diffusion length in semiconductors. Secondly, we developed a prototype of a new spin filter, which consists of a vacuum tube with GaAs(Cs,O) photocathode and a nickel-covered venetian blind dynode. Preliminary results on spin-dependent reflection of electrons from the oxidized polycrystal nickel layer are presented.

The dissociative recombination of the lowest rotational states of H3+ has been investigated at the storage ring TSR using a cryogenic 22-pole radiofrequency ion trap as injector. The H3+ was cooled with buffer gas at ~15 K to the lowest rotational levels, (J,G)=(1,0) and (1,1), which belong to the ortho and para proton-spin symmetry, respectively. The rate coefficients and dissociation dynamics of H3+(J,G) populations produced with normal- and para-H2 were measured and compared to the rate and dynamics of a hot H3+ beam from a Penning source. The production of cold H3+ rotational populations was separately studied by rovibrational laser spectroscopy using chemical probing with argon around 55 K. First results indicate a ~20% relative increase of the para contribution when using para-H2 as parent gas. The H3+ rate coefficient observed for the para-H2 source gas, however, is quite similar to the H3+ rate for the normal-H2 source gas. The recombination dynamics confirm that for both source gases, only small populations of rotationally excited levels are present. The distribution of 3-body fragmentation geometries displays a broad part of various triangular shapes with an enhancement of ~12% for events with symmetric near-linear configurations. No large dependences on internal state or collision energy are found., 10 pages, 9 figures, to be published in Journal of Physics: Conference Proceedings

A spin-dependent emission of optically oriented electrons from $p\text{-GaAs}(\text{Cs},\text{O})$ into vacuum was experimentally observed in a magnetic field normal to the surface. This phenomenon is explained within the model which takes into account the jump in the electron $g$ factor at the semiconductor-vacuum interface. Due to this jump, the effective electron affinity on the semiconductor surface depends on the mutual direction of optically oriented electron spins and the magnetic field, resulting in the spin-dependent photoemission. It is demonstrated that the observed effect can be used for the determination of spin diffusion length in semiconductors.

Term energies for dielectronic-recombination Rydberg resonances below 0.07 eV are determined for Sc18+ with absolute accuracies below 0.0002 eV by electron collision spectroscopy in an ion storage ring, using the twin-electron-beam technique and a cryogenic photocathode. The lithiumlike 2s_{1/2}-2p_{3/2} transition energy for Z=21 is determined to 4.6 ppm, less than 1% of the few-body effects on radiative corrections. Features from the hyperfine structure of the 2s state could be resolved in the dielectronic-recombination spectrum.

On a dense energy grid reaching up to 75 meV electron collision energy the fragmentation angle and the kinetic energy release of neutral dissociative recombination fragments have been studied in a twin merged beam experiment. The anisotropy described by Legendre polynomials and the extracted rotational state contributions were found to vary on a likewise narrow energy scale as the rotationally averaged rate coefficient. For the first time angular dependences higher than 2$^{nd}$ order could be deduced. Moreover, a slight anisotropy at zero collision energy was observed which is caused by the flattened velocity distribution of the electron beam., Comment: 8 pages, 4 figures; The Article will be published in the proceedings of DR 2007, a symposium on Dissociative Recombination held in Ameland, The Netherlands (18.-23. July 2008); Reference 19 has been published meanwhile in S. Novotny, PRL 100, 193201 (2008)

Angular fragment distributions from the dissociative recombination (DR) of HDwere measured with well directed monochromatic low-energy electrons over a dense grid of collision energies from 7 to 35 meV, where pronounced rovibrational Feshbach resonances occur. Significant higher-order anisotro- pies are found in the distributions, whose size varies along energy in a partial correlation with the relative DR rate from fast-rotating molecules. This may indicate a breakdown of the nonrotation assumption so far applied to predict angular DR fragment distributions. DOI: 10.1103/PhysRevLett.100.193201 PACS numbers: 34.80.Lx, 34.80.Ht Positively charged molecules can be fragmented effi- ciently by low-energy electron impact at collision energies down to the sub-eV range in the process of dissociative recombination (DR) (1). This reaction attracts strong in- terest in the modeling of the chemical networks in cold and dilute ionized media such as interstellar clouds (2), atmos- pheric layers (3), and industrial plasmas (4). It is driven by the formation of intermediate neutral compound states, in which the incident electron is captured following resonant energy exchange with the cation, and which then dissociate into neutral product channels reached via excited potential surfaces. Rovibrational Feshbach resonances were recognized to play an important role at low electron energies in the DR (5) as well as in the related process of dissociative electron attachment (DEA), an efficient destruction channel of neu- tral molecules by low-energy electron impact (6,7). They reflect the energy exchange of the incident electron with the nuclear motion, in contrast to the one within the elec- tronic system alone. Thus, in the direct mechanism (5 )o f DR purely electronic energy exchange forms a neutral, doubly excited potential surface unstable against dissocia- tion. In the indirect mechanism (5) a rovibrationally ex- cited neutral Rydberg state is formed with the initial bound electronic state of the ion as a core and then predissociated by often the same doubly excited surface. These mecha

The electron-ion recombination rate coefficient for $\mathrm{Si}\phantom{\rule{0.3em}{0ex}}\mathrm{IV}$ forming $\mathrm{Si}\phantom{\rule{0.3em}{0ex}}\mathrm{III}$ was measured at the heavy-ion storage-ring TSR. The experimental electron-ion collision energy range of $0$--$186\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ encompassed the $2{p}^{6}nl{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ dielectronic recombination (DR) resonances associated with $3s\ensuremath{\rightarrow}nl$ core excitations, $2s2{p}^{6}3snl{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ resonances associated with $2s\ensuremath{\rightarrow}nl$ $(n=3,4)$ core excitations, and $2{p}^{5}3snl{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ resonances associated with $2p\ensuremath{\rightarrow}nl$ $(n=3,\dots{},\ensuremath{\infty})$ core excitations. The experimental DR results are compared with theoretical calculations using the multiconfiguration Dirac-Fock (MCDF) method for DR via the $3s\ensuremath{\rightarrow}3p{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ and $3s\ensuremath{\rightarrow}3d{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$(both ${n}^{\ensuremath{'}}=3,\dots{},6$) and $2{p}^{5}3s3l{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ $({n}^{\ensuremath{'}}=3,4)$ capture channels. Finally, the experimental and theoretical plasma DR rate coefficients for $\mathrm{Si}\phantom{\rule{0.3em}{0ex}}\mathrm{IV}$ forming $\mathrm{Si}\phantom{\rule{0.3em}{0ex}}\mathrm{III}$ are derived and compared with previously available results.

A cold electron target with a cryogenic GaAs‐photocathode electron source was developed for the Heidelberg Test Storage Ring. Two independent electron facilities (cooler and target) allow to separate cooling of the ion beam from target operation improving the quality of electron and ion beams. In addition a strong gain in the resolution was achieved with a help of a cryogenic photoelecron source providing dc electron currents up to 0.5 mA with an emission energy spread of about 10 meV. In first recombination measurements at the target, performed on HD +, H3+ and Sc18+, low energy resonant structures at milli‐eV collision energies revealed unprecedented low transverse and longitudinal electron temperatures of about 0.5 meV and 0.025 meV, respectively. The photocathode source will be also used to provide cold beams for electron cooling of low‐energy ions stored at the electrostatic Cryogenic Storage Ring which will be built at MPIK. The perspectives of photocathode‐driven electron coolers operating at very ...

At the Max‐Planck‐Institut fur Kernphysik in Heidelberg a next generation electrostatic storage ring for cryogenic temperatures is under development. The main focus of this unique machine is the research on ions, molecules and clusters up to bio molecules in the energy range of 20–300 keV at low temperatures down to 2 Kelvin. The achievement of this low temperature for all material walls seen by the ions in the storage ring will allow novel experiments to be performed, such as rotational and vibrational state control of molecular ions and their interaction with ultra‐low energy electrons and laser radiation. The low temperature of the storage ring not only causes a strong reduction of black body radiation incident onto the stored particles, but also acts as a large cryopump, expected to lead to a vacuum in the 10–15 mbar range. In this paper the cryogenic concept of the storage ring and the related vacuum design will be presented.

The energy-resolved rate coefficient for the dissociative recombination (DR) of $\mathrm{H}_{3}{}^{+}$ with slow electrons has been measured by the storage-ring method using an ion beam produced from a radiofrequency multipole ion trap, employing buffer-gas cooling at 13 K. The electron energy spread of the merged-beams measurement is reduced to $500\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$ by using a cryogenic GaAs photocathode. This and a previous cold-$\mathrm{H}_{3}{}^{+}$ measurement jointly confirm the capability of ion storage rings, with suitable ion sources, to store and investigate $\mathrm{H}_{3}{}^{+}$ in the two lowest, $(J,G)=(1,1)$ and $(1,0)$ rotational states prevailing also in cold interstellar matter. The use of para-${\mathrm{H}}_{2}$ in the ion source, expected to enhance para-$\mathrm{H}_{3}{}^{+}$ in the stored ion beam, is found to increase the DR rate coefficient at meV electron energies.

A new electron beam facility at the storage ring TSR is presented which makes electron–ion collision experiments possible without using the existing electron cooler device as an electron target. A long adiabatic acceleration section combined with magnetic adiabatic expansion (factor up to ≈90) is used to reduce the longitudinal and transverse electron temperatures. The facility can be operated with either a thermionic or a photocathode. Results from two-dimensional electron beam profile measurements and longitudinal electron energy distribution measurements are discussed.

Longitudinal and differential transverse energy distributions of electrons emitted from GaAs-photocathodes are studied at room (295 K) and low (90 K) temperatures. The obtained data prove that electron energy loss as well as elastic electron scattering are of crucial importance in the electron transfer through the GaAs(Cs,O)-vacuum interface.

The comparative study of (Cs,NF3) and (Cs,O2) activation procedures and the measurements of the vacuum level position versus activation layer thickness show that the activation layer for optimally activated GaAs photocathodes with QE of about 20–30% should be treated as a dipole layer. For thick activation layers, the saturation of the vacuum level at the lowest position is observed. The saturation proves, that the activated surface should be treated as a heterojunction.

The longitudinal component of the photoelectron energy distribtuion from a transmission mode GaAs photocathode with quantum efficiency up to 18% is accurately measaured by using Generation III image intensifier as a parallel plates retarding field electron energy analyzer. The measurements are performed within the temperature range of 77-300 K with different photon energies for photoelectron excitation above or below the band gap of semiconductor. It is shown that conduction band tails in heavily doped GaAs influence photoelectron transport near the surface during the process of emission into vacuum. The transmission coefficient of a residual surface barrier is measured as a function of electron kinetic energy above the conduction band minima and is found to be much less than unity. The obtained results demonstrate the usefulness of this technique for the investigation of the physical factors which control the quantum efficiency and other parameters of photocathodes with neagitive electron affinity.

Storage ring studies of low energy electron collisions with molecular ions have been carried out for dissociative recombination (DR) of fluorine-bearing molecules. Here we report on work aiming to improve the understanding of astrochemistry involving HF, a possible spectroscopic tracer of interstellar H2. For CF+ the rate coefficient was obtained for temperatures down to 10 K. For D2F+ the DR fragmentation branching ratios were determined to be 66(3)%, 24(2)%, and 10(2)% for the F+D+D, DF+D, and D2+F channels, respectively. The molecular DR products of this reaction, DF and D2, display an unusually high level of internal excitation, close to their dissociation limit.

Dissociative recombination (DR) is an exothermic process leading to neutralisation and chemical fragmentation in many plasma environments, down to the lowest temperatures. We present results of complex investigations on DR of HF+ and CF+ performed at the TSR storage ring. Beyond the rate coefficient down to meV collision energies also dissociation pathways and geometries have been investigated using full 3D fragment imaging technique. Fragment imaging also provides HF+ rotational structure and fine-structure as well as the HF+ dissociation energy with sub-meV precision.

Dielectronic recombination of highly charged atomic ions and dissociative recombination of molecular ions have been investigated using fast ion beams merged with cold intense electron beams (down to

We present results from our recent studies of the dissociative recombination of the CF+ cation. On one hand, dissociative recombination was measured with 3 MeV CF+ ions in the heavy-ion Test Storage Ring in Heidelberg, using the twin electron beam configuration with an electron cooler and a separated electron target for collision measurements. In this experiment, the low temperatures of the electron beam provided by a photocathode (temperature in co-moving frame below 1 meV) account for a fast kinetic cooling of the heavy-ion beam and a high resolution in the measured rate coefficients. Fragment imaging measurements show a complete switching of the dissociation route by only a small change of the collision energy and the disappearance of neutral Rydberg product states on crossing the DE threshold. On the other hand, extensive calculations of energy positions and autoionization widths for the doubly excited states of CF between the first and second ionization thresholds have been obtained from electron scattering calculations using the complex Kohn variational method, followed by calculations of the dissociative recombination process with the multichannel quantum defect theory. In preliminary computations, only the first dissociative state in each molecular symmetry, which lies closest in energy to the ion potential at its equilibrium internuclear separation, and thus is dominant for the low-energy dissociative recombination, was included. Although only the direct mechanism of dissociative recombination reaction has been considered in this step, the size and the shape of the DR rate coefficient are already well reproduced.

Atomic hydrogen, produced by thermal dissociation of H2 molecules inside a hot tungsten capillary, is shown to be an efficient tool for multiple recleaning of degraded surfaces of high quantum efficiency transmission-mode GaAs photocathodes within an ultrahigh vacuum (UHV) multichamber photoelectron gun. Ultraviolet quantum yield photoemission spectroscopy has been used to study the removal of surface pollutants and the degraded (Cs,O)-activation layer during the cleaning procedure. For photocathodes grown by the liquid-phase epitaxy technique, the quantum efficiency is found to be stable at about 20% over a large number of atomic hydrogen cleaning cycles. A slow degradation of the quantum efficiency is observed for photocathodes grown by metal-organic chemical vapor deposition, although they reached a higher initial quantum efficiency of about 30%–35%. Study of the spatial distributions of photoluminescence intensity on these photocathodes proved that this overall degradation is likely due to insertion o...