Residual Gas Ionization Research Articles

Investigation on activation characterization, secondary electron yield, and surface resistance of novel quinary alloy Ti–Zr–V–Hf–Cu non-evaporable getters

The performance of next-generation particle accelerators has been adversely affected by the occurrence of electron multipacting and vacuum instabilities. Particularly, minimization of secondary electron emission (SEE) and reduction of surface resistance are two critical issues to prevent some of the phenomena such as beam instability, reduction of beam lifetime, and residual gas ionization, all of which occur as a result of these adverse effects in next-generation particle accelerators. For the first time, novel quinary alloy Ti–Zr–V–Hf–Cu non-evaporable getter (NEG) films were prepared on stainless steel substrates by using the direct current magnetron sputtering technique to reduce surface resistance and SEE yield with an efficient pumping performance. Based on the experimental findings, the surface resistance of the quinary Ti–Zr–V–Hf–Cu NEG films was established to be 6.6 × 10−7 Ω m for sample no. 1, 6.4 × 10−7 Ω m for sample no. 2, and 6.2 × 10−7 Ω m for sample no. 3. The δmax measurements recorded for Ti–Zr–V–Hf–Cu NEG films are 1.33 for sample no. 1, 1.34 for sample no. 2, and 1.35 for sample no. 3. Upon heating the Ti–Zr–V–Hf–Cu NEG film to 150 °C, the XPS spectra results indicated that there are significant changes in the chemical states of its constituent metals, Ti, Zr, V, Hf, and Cu, and these chemical state changes continued with heating at 180 °C. This implies that upon heating at 150 °C, the Ti–Zr–V–Hf–Cu NEG film becomes activated, showing that novel quinary NEG films can be effectively employed as getter pumps for generating ultra-high vacuum conditions.

Longitudinal ion-clearing mechanism in space-charge neutralization of low energy beams

In electron guns, generating high-intensity beams comes with the cost of increasing the number of secondary ions produced by the residual gas ionization which can be a limiting factor in the cathode lifetime. However, it is often overlooked that the longitudinal potential generated by the beam can be the main factor influencing secondary ions to drift back toward the source and limit the maximum beam intensity. Using 3D simulations, this paper shows how the flux of trapped ions depends on the beam parameters. The results provide a practical framework that can be used as a guide in the design of the electron gun focal point to mitigate the ion back bombardment. Published by the American Physical Society 2024

Trivelpiece–Gould modes and low-frequency electron–ion instability of non-neutral plasma

The frequency spectra of the Trivelpiece–Gould modes of a waveguide partially filled with non-neutral plasma are determined numerically by solving the dispersion equation. The modes having azimuthal number $m = 1$ are considered. The results are presented for the entire acceptable range of electron densities, magnetic field strengths, for different values of the charge neutralization coefficient. The Cherenkov resonance condition of an ion with a diocotron mode having a finite value of the longitudinal wave vector was studied. The characteristics of resonant low-frequency electron–ion instability caused by relative azimuth motion of electrons and ions in crossed fields and by the anisotropy of the distribution function of ions are discussed. Ions are created by ionization of residual gas in the plasma volume. Due to the anisotropy, instability occurs not only in the vicinity of the resonance, but also outside it. For typical values of plasma parameters in experiments, estimations of the frequency growth rate are given. A conclusion is drawn that this instability can be the cause of the low-frequency oscillations observed in linear devices with non-neutral plasma produced in an electron beam channel.

Delayed electron emission in photomultiplier tubes

The time distributions of pulses generated by noise or external light sources in the ХР2020, ХР2232В, ХР1021, FEU-85, FEU-87, FEU-93, FEU-130, and R7600U-200 photomultiplier tubes (PMTs) were investigated using an autocorrelation delayed-coincidence time spectrometer. The observed pulse time correlations were of two types: non-exponential, related to the detection of the effects of feedback ions; and exponential, caused by the interaction of secondary-emission electrons with the PMT dynodes. Detection of multi-electron events arising from the action of external radiators leads to intensification of residual-gas ionization processes in the PMT. However, no analogy is observed in the shapes of the feedback ion spectra for different types of PMT despite the identical composition of photocathodes or dynodes. This might be related to differences in PMT manufacturing processes and to possible different restructuring of molecular electron shells in the photocathode material constituents. The detection of single-electron events (noise pulses) in some of the PMTs studied was not always characterized by nanosecond-scale emissions that might be expected to result from metastable states related to properties of their photocathodes and dynodes, and on the material processing technology used in PMT manufacture. It is difficult to describe exactly the mechanisms leading to delayed electron emission. The source of such events remains open to attempts at explanation and further research.

Origins of the hydrogen signal in atom probe tomography: case studies of alkali and noble metals

Atom probe tomography (APT) analysis is being actively used to provide near-atomic-scale information on the composition of complex materials in three-dimensions. In recent years, there has been a surge of interest in the technique to investigate the distribution of hydrogen in metals. However, the presence of hydrogen in the analysis of almost all specimens from nearly all material systems has caused numerous debates as to its origins and impact on the quantitativeness of the measurement. It is often perceived that most H arises from residual gas ionization, therefore affecting primarily materials with a relatively low evaporation field. In this work, we perform systematic investigations to identify the origin of H residuals in APT experiments by combining density-functional theory (DFT) calculations and APT measurements on an alkali and a noble metal, namely Na and Pt, respectively. We report that no H residual is found in Na metal samples, but in Pt, which has a higher evaporation field, a relatively high signal of H is detected. These results contradict the hypothesis of the H signal being due to direct ionization of residual H2 without much interaction with the specimen’s surface. Based on DFT, we demonstrate that alkali metals are thermodynamically less likely to be subject to H contamination under APT-operating conditions compared to transition or noble metals. These insights indicate that the detected H-signal is not only from ionization of residual gaseous H2 alone, but is strongly influenced by material-specific physical properties. The origin of H residuals is elucidated by considering different conditions encountered during APT experiments, specifically, specimen-preparation, transportation, and APT-operating conditions by taking thermodynamic and kinetic aspects into account.

Carbon-coated electron field emitters

The properties of field electron emitters made of iridium and rhenium, whose surface is coated with carbon by decomposition of benzene molecules on the surface, are studied. The stability of the emission characteristics and the stability of the obtained emitters to the adsorption of residual gas molecules and ion bombardment are shown. The coating is obtained either in the form of a monoatomic graphene film on the flat faces of the emitter, or in the form of graphite growths. The field electron emission is localized on the edges and corners of the faceted graphite outgrowth. After adsorption of alkaline and alkaline earth metals onto the carbon coating, an abnormally large increase in emissions from flat faces and localization of emissions on graphene-coated faces occurs. The emission amplification occurs in the presence of alkali metal atoms, both on the surface and only under the graphene film. Keywords: electron emission, carbon, iridium, rhenium, alkali metals, barium, intercalation.

Полевые эмиттеры электронов с углеродным покрытием

The properties of field electron emitters made of iridium and rhenium, whose surface is coated with carbon by decomposition of benzene molecules on the surface, are studied. The stability of the emission characteristics and the stability of the obtained emitters to the adsorption of residual gas molecules and ion bombardment are shown. The coating is obtained either in the form of a monoatomic graphene film on the flat faces of the emitter, or in the form of graphite growths. The field electron emission is localized on the edges and corners of the faceted graphite outgrowth. After adsorption of alkaline and alkaline earth metals onto the carbon coating, an abnormally large increase in emissions from flat faces and localization of emissions on graphene-coated faces occurs. The emission amplification occurs in the presence of alkali metal atoms, both on the surface and only under the graphene film.

Long-term reproducibility of emission characteristics of high-current diamond-graphite sources of electrons with field emission under non-stationary vacuum operating conditions

The results of a study of the long-term reproducibility of the emission characteristics of electron sources with field emission density of up to 1000 A/cm2 of based on composite nanocarbon films are presented. It was found that high-current field emission is accompanied by the sputtering of residual gas ions from atoms of the cathode material and its sedimentation on the anode. The results can be used to predict the durability of high-current diamond-graphite cathodes with field emission when operating in unsteady temperature-vacuum conditions. Keywords: diamond-graphite nanocomposite, high-current field emission, reproducibility of field emission parameters, service life of cathode with field emission.

Progress in space charge compensation using electron columns

Space charge compensation allows for increased beam current in linacs and RFQs. A novel application of space charge compensation, the electron column, offers the opportunity to realize more intense beams for high energy physics in circular accelerators. The concept relies on ionization of residual gas by the primary beam, with electromagnetic fields used to confine and shape the space charge neutralizing plasma. Prior experimental efforts and simulation studies are reviewed. They indicate that electron columns could be successfully deployed in accelerator rings. The experimental demonstration of an electron column is underway at the Integrable Optics Test Accelerator at Fermilab. The experiment, instrumentation, and physics program are discussed.

Долговременная воспроизводимость эмиссионных характеристик алмазографитовых полевых источников электронов в нестационарных вакуумных условиях эксплуатации

The results of a study of the long-term reproducibility of the emission characteristics of electron sources with field emission density of up to 1000 A/cm2 of based on composite nanocarbon films are presented. It was found that high-current field emission is accompanied by the sputtering of residual gas ions from atoms of the cathode material and its sedimentation on the anode. The results can be used to predict the durability of high-current diamond-graphite cathodes with field emission when operating in unsteady temperature-vacuum conditions.

Upgrading the Scanning Two-Dimensional Ionization Profile Monitor in Beam Transport Lines

The scanning two-dimensional ionization profile monitor (IPM) has been upgraded for the beam diagnostics in beam transport lines. The main parts of the monitor are an extractor, a scanner, and two analyzers with a slit at which residual-gas ionization products collected from a beam path as long as 50 mm arrive. The current amplifier based on microchannel plates and a current collector are placed in sequence behind the slit in order to increase the IPM sensitivity under high-vacuum conditions (n × 10−6 Pa). Two-dimensional beam profiles are obtained by scanning. A special electronic unit containing high-voltage supplies and collector-current meters has been created for the scanning and IPM control. Three upgraded IPMs have been placed between the accelerator and the physical facilities in each beamline of the DC-280 FLNR JINR cyclotron. Data processing and simultaneous display of experimental results from several IPMs are carried out by a program developed in the LabVIEW design environment. The minimum current of the 40Ar ion beam with the energy of 5 MeV/nucleon at which the monitor is still operable is estimated to be several tens of picoamperes. The reliability of the upgraded IPM has been demonstrated by its operation.

Design of a Novel Pixelated Residual Gas Ionization Profile Monitor for the 320 kV high-voltage platform at IMPCAS

In this paper, a Pixelated Residual Gas Ionization Profile Monitor (RGIPM) has been proposed for the 320 kV High-Voltage Platform at the Institute of Modern Physics, Chinese Academy of Sciences. Beam profile monitoring in both the horizontal and vertical directions is realized in one-meter length. A compact microchannel plate (MCP) detector with an array of the Topmetal-II− pixel ASIC as the anode is proposed for the device. The spatial resolution of the detector is expected to be comparable to the intrinsic pixel size of the anode. Compensation electric and magnetic fields have been integrated into the RGIPM to minimize the influence of the measurement fields on the transportation of low energy ion beams. This paper presents the test results of a prototype of the MCP detector and the study on the electric and magnetic fields.

Electrodes of a Vacuum Chamber under the Action of an Arc in the Explosive Melting Zone

After repeated exposure to an arc vacuum discharge, Cu (60%)—Cr (40%) alloyed systems show a change in the atomic concentration of copper (from 53.7 to 71.8 at %) and, correspondingly, of chromium. Mass spectrometry showed the presence in the residual atmosphere of diffusion devices of fragments of Cx $${\text{H}}_{y}^{ + }$$ -type molecules and molecules desorbed from all parts of the chamber—residual gas ions ( $${\text{CO}}_{2}^{ + }$$ , H2O+, $${\text{N}}_{2}^{ + }$$ , N+ and their polyatomic compositions). The inheritance of technological impurities from previous operations in the mass spectrum was found.

Dynamic pressure in the LHC: influence of ions induced by ionization of residual gas by both the proton beam and the electron cloud

For the future HL-LHC or FCC study, the understanding of the beam interactions with the vacuum chamber is fundamental to provide solutions to mitigate the pressure rises induced by electronic, photonic and ionic molecular desorption. The proton beam circulating in the LHC vacuum chamber ionizes the residual gas producing electrons as well as positive ions. In-situ measurements were carried out, on the LHC Vacuum Pilot Sector during the LHC RUN II, to monitor the dynamic pressure, and to collect the electrical signals due to the electron cloud and to the ions interacting with the vacuum chamber walls. Experimental measurements of electrical signals recorded by copper electrodes were compared to calculations taking into account both the Secondary Electron Yield of copper and electron energy distribution. Finally, it seems that copper electrodes were not fully conditioned and an ion current could be estimated.

Fast beam–beam collisions monitor for experiments at NICA

Two compact detector systems are proposed to meet the challenges of the fast beam–beam collisions monitoring, event selection and the precise timing determination in proton–proton and nucleus–nucleus collisions at NICA. Beam Position Monitor (BPM) and Fast Beam–Beam Collision (FBBC) monitor are capable to provide the information for each bunch crossing both on the beams location and on the intensity of collisions, as well as on the azimuthal distribution of the particles in the event. The systems are using fast Micro Channel Plate detectors (MCPs). The ultra-high vacuum (UHV) compatibility and low-mass compact design of the BPM and FBBC components allow one to consider their application inside the vacuum beam line of NICA collider. The BPM is based on the effect of the residual gas ionization and provides the high accuracy, fast, bunch-by-bunch measurements of the beam location. The FBBC uses the concept of the fast isochronous timing for the multi-pad readout of short (∼1ns) MCP signals, produced by the particles in the collisions of the beams. Studies of the polarization phenomena in light- and heavy-ion interactions at NICA beams is another goal of research at NICA, and the FBBC is also considered for the local polarimetry at the SPD. In this report we present the results of model simulations and tests of the new prototypes of the fast MCP readout set-ups that are proposed for NICA.

The Ion-Hose Instability in a High-Current Multipulse Linear Induction Accelerator

The ion-hose instability is considered to be a potential danger for long-pulse, high-current electron linear induction accelerators (LIAs). As is shown in this paper, it is also a concern for multipulse LIAs. This instability is a direct result of beam-electron ionization of residual background gas, and the growth rate is proportional to the ion density. In a typical accelerator vacuum, there is little ion density decay between beam pulses, and the instability will grow throughout the pulse train. We have simulated this effect, taking into account ion density decay due to recombination and channel expansion. These simulations were done for the new 2-kA, 20-MeV Scorpius multipulse LIA presently being developed. The simulations have shown that the magnetic focusing field designed for Scorpius will be strong enough to inhibit ion-hose instability if the background pressure is kept below a value that is readily attainable with the present designs of induction cells and other accelerator components.

All-optical structuring of laser-driven proton beam profiles

Extreme field gradients intrinsic to relativistic laser-interactions with thin solid targets enable compact MeV proton accelerators with unique bunch characteristics. Yet, direct control of the proton beam profile is usually not possible. Here we present a readily applicable all-optical approach to imprint detailed spatial information from the driving laser pulse onto the proton bunch. In a series of experiments, counter-intuitively, the spatial profile of the energetic proton bunch was found to exhibit identical structures as the fraction of the laser pulse passing around a target of limited size. Such information transfer between the laser pulse and the naturally delayed proton bunch is attributed to the formation of quasi-static electric fields in the beam path by ionization of residual gas. Essentially acting as a programmable memory, these fields provide access to a higher level of proton beam manipulation.

Regularities of Plasma-Immersion Formation of Long-Pulse High-Intensity Titanium Ion Beams

The results of experimental investigations on plasma-immersion formation of ballistically focused beams of low-energy titanium ions with the pulse duration up to 30 μs are presented. The processes of neutralization of the space charge of high-intensity beams upon increasing the ion-current density by a few orders of magnitude during their focusing are addressed. The conditions of preliminary injection of the metal plasma, its parameters, the formation of the beam plasma during residual-gas ionization and secondary and thermionic emissions are shown to be of critical importance for an effective transport of these beams in the equi-potential drift space. It is demonstrated that the use of a disc electrode, preventing the macroparticles from penetrating the region of the ballistic beam focusing, increases the efficiency of long-pulse titanium ion beam transport and simultaneously reduces the probability of instability development.

A novel field cage design for the CPS IPM and systematic errors in beam size and emittance

A residual gas ionization profile monitor has been recently installed in the CERN proton synchrotron. We present the design for a novel and simplified field cage structure that suppresses the secondary electrons that are induced by the ionized ions. We discuss the field cage design and the beam size and emittance systematic error considering the non-uniformity of the fields, the space-charge effect of the beam, and the lattice parameter errors.

Axial heating and temperature of RF-excited non-neutral plasmas in Penning-Malmberg traps

Electro-magnetostatic traps have been used for decades to provide long-term storage of charged particle samples or non-neutral plasmas. The dynamics and equilibrium states of these ideally simple systems can be strongly diverted from the usual working conditions (i.e. single-species, quiescent samples) in the presence of oppositely charged particles or external electric field perturbations. Both these conditions occur when the plasma is generated by means of a radio-frequency (RF) excitation continuously applied on a trap electrode. The application of RF drives of some volts over periods larger than typical collisional time scales leads to residual-gas ionization and to the accumulation of an electron plasma, a process that has previously been exploited as an alternative to thermionic or photoemission electron sources. The analysis of the axial energy distribution shows a deviation of the continuously excited final state from maxwellianity dependent on the radial position and the subsequent relaxation to equilibrium after the interruption of the drive. Systematic measurements also indicate the high sensitivity to the residual gas pressure of both the total confined charge and of the attainable densities and plasma profiles. The results are compared to the information obtained from a very simple one-dimensional electron heating model and show the validity of its most basic features together with its shortcomings.