Carbon Stripper Foils Research Articles

Preparation of different carbon stripper foils and application in beam diagnostics

The production of carbon stripper foils that are mounted at different locations of the heavy-ion accelerator is an important task of the target laboratory, and the process for the preparation of carbon foils is constantly improved. Recently, we tested additional heat treatment of the carbon stripping foils following our standard procedure to reduce stress in the foils, since the performance and the durability of these stripping foils is a crucial factor for effective operation. We investigated the properties of foils produced some time ago and of foils freshly produced with the standard procedure, and foils treated thermally after the standard procedure. For the beam diagnostic department at GSI radiation resistant target materials for transverse ion beam profile measurement is an ongoing research topic. In that framework, the feasibility of Optical Transition Radiation (OTR) generation due to ion beam interaction with metal target for beam profile determination has been investigated. Here, carbon stripper foils are applied to enhance the OTR signal. The differently prepared foils were applied in many different experiments of the beam diagnostic department. We describe the performance of the foils during irradiation and compare the features of foils before and after irradiation.

Installation and commissioning of the stripping foil system in the new CERN PS booster 160 MeV H− injection region

During Long Shutdown 2 (LS2) at CERN, the new Linac4 (L4) accelerator was connected to the PS Booster (PSB) to inject 160 MeV H− beam into the 4 superposed PSB rings. In order to achieve this, a completely new H− charge-exchange injection chicane system, with 200 µg/cm2 carbon stripping foil units, to convert the negative hydrogen ions into protons, by stripping off the electrons, has been installed. Beam commissioning of this system started in December 2020 and different types of foils have been used for this purpose. In parallel, stripping efficiency measurements, of different foil types, continued to take place with the stripping foil test stand in the L4 Transfer Line. This paper briefly recalls the final design, and installation of, the main components in the PSB injection region, before reporting on the important results obtained during measurements and commissioning with beam.

Measurements with the stripping foil test stand in the Linac4 Transfer Line

In 2020, after the CERN accelerators complex Long Shutdown 2 (LS2), a novel Linac4 (L4)-to-PS Booster (PSB) charge-exchange injection system will allow to transform the L4 160 MeV H- beam into H+, which will be injected into the four PSB superposed rings. For this, a 200 µg/cm2 carbon stripping foil will convert negative hydrogen ions (H-) into protons by stripping off the electrons. L4 is now performing operational reliability runs, which include a stripping foil test stand installed in the L4 transfer line. These tests will permit to gain experience on the fragile foils, test different foil materials and thicknesses, measure the efficiency and lifetime of the foils, and evaluate the foil changing mechanism as well as the interlocking functions. This paper briefly describes the stripping foil test stand setup, before reporting on the obtained important test results.

Progress status in fabrication of HBC stripper foil for 3-GeV RCS at J-PARC in Tokai site

In the 3-GeV Rapid Cycling Synchrotron (RCS) at Japan Proton Accelerator Research Complex (J-PARC), we adopted thick Hybrid type Boron-doped Carbon (HBC) stripper foil for multi-turn H- charge-exchange injection. The HBC stripper foil developed at KEK has demonstrably improved the foil lifetime significantly. In addition, Graphite Thin Films (GTF) developed by KANEKA corp. was attempted to use for user operation. The GTF is a multilayer graphene sheet and it is expected to be a new candidate for highly durable charge stripping foil. HBC and GTF foils can be both employed the J-PARC RCS for long-term user operation. Early manufacturing of the stripper foil for the J-PARC was carried out according to the following two steps: foil fabrication at the KEK Tsukuba site and foil preparation at the JAEA Tokai site. Recently, the HBC foil deposition apparatus was relocated from KEK to JAEA, and new HBC foils were fabricated at the Tokai site. The performance of the new HBC foil was verified to be equivalent to the original HBC foil with offline beam irradiation tests. Furthermore, the use of the new HBC foils was successfully demonstrated during RCS user beam operation for one week.

The irradiation study of carbon stripper foils by a 12 MeV Ar beam

Irradiation performance of multilayer HBC foils which layers are more than 20 were conducted for the first time, and different types of carbon stripper foils were dosed under a series of irradiation conditions systematically. Eight (8) types of carbon stripper foils were striked by Ar4+ beam under different intensities and fluences. Multilayer foils performed generally better than the monolayer ones. The existence of boron layer helped to increase the resistance of the foils to irradiation damage, elongating their lifetimes.

Imaging Neutral Particle Analyzer (INPA) measurements of confined fast ions in DIII-D

A novel imaging neutral particle analyzer (INPA) which provides energy-resolved radial profiles of confined fast ions on the DIII-D tokamak is discussed. The INPA measures charge-exchanged energetic neutrals by viewing an “active” neutral beam through a 1D pinhole camera with a rear collimating slit that defines the neutral particle collection sightlines. The incident neutrals are ionized by 10 nm thick carbon stripping foils and the local tokamak magnetic field acts as a magnetic spectrometer to disperse ions onto a phosphor scintillator. A fast (160 Hz) CCD camera provides 2D images of the escaping neutrals mapped to energy and radial position in the plasma. The INPA typically probes passing orbits with an energy resolution of ≈7.5 keV (E= 20−80 keV) and a spatial resolution that ranges from 4 cm half width at half max (HWHM) in the core to 3 cm at the plasma edge. The INPA clearly resolves fast ion transport in localized regions of phase space due to individual sawteeth and a replenishing before each event. Extension to proton and triton DD fusion product measurements in high-performance DIII-D plasmas is analyzed and simulations show peak signals which are 10−6 lower than that from neutral beam ions for the same configuration. Possible modifications to increase fusion product signals are discussed along with upgrades to improve the overall diagnostic performance.

Measurement of the differential cross sections of natLi(d,d0) for EBS purposes

In the present work, the 6,7Li(d,d0)6,7Li elastic scattering differential cross sections were measured in the energy range Elab = 900 – 2000 keV for EBS purposes, using thin lithium targets, made by evaporating natLiF and isotopically enriched 6LiF powder on self-supporting carbon stripping foils, with an ultra-thin Au layer on top for charge normalization purposes. The experiment was carried out in deuteron beam energy steps of 5 to 30 keV, depending on the fine structure of each studied cross section, and for the laboratory scattering angles of 125˚, 140˚, 150˚, 160˚, and 170˚.

Development and verification of a novel scintillator-based, imaging neutral particle analyzer in DIII-D tokamak

A compact scintillator-based imaging neutral particle analyzer (INPA) that provides energy and radially resolved measurements of confined fast ions has been designed and built in the DIII-D tokamak. The system measures charge-exchanged energetic neutrals by viewing an active neutral beam source through a 1D pinhole camera with a rear collimating slit that defines the neutral particle collection sightline and radial positions probed in the plasma. The incident neutrals are ionized by ultra-thin carbon stripping foils of 10 nm thickness with the local tokamak magnetic field acting as a magnetic spectrometer to disperse the ions onto a scintillator. The strike position on the phosphor is determined by the fast ion energy and sightline while the intensity of emitted light from the phosphor is proportional to the ion flux. Fast camera measurements of the scintillator provide 2D images of the escaping neutrals mapped to energy and radial position in the plasma. The INPA system images a broad radial range from the plasma core to edge and deuterium energies up to 80 keV, with energy resolution of ∼7.5 keV and pitch angle resolution of ∼5°. The first data obtained from the INPA demonstrates that the system has excellent signal to noise ratio and provides unprecedented details of phase space dynamics.

Evaluation of induced activity in various components of a PET-cyclotron

For decommissioning a cyclotron facility, it is important to evaluate the induced activity of the various components of the cyclotron; however, activation of the metal components has been rarely investigated. In this study, two types of cyclotrons were examined; one is a proton acceleration type using a deflector, and another is a hydride ion (H-) acceleration type using a carbon stripper foil for beam extraction to the target port. The samples were obtained from various metal components such as the yoke, sector magnet, coil, and vacuum chamber by the core boring method, and the depth distribution of the radioactivity was determined via a germanium semiconductor detector. The activities of 54Mn and 60Co were detected from the surface to a deeper site of the yoke and sector magnet. Most of the observed activities of the cyclotron components were higher than the clearance levels, suggesting that a clearance system should not be applied to the yoke and sector magnet. In the case of a high-activity sample, we have to wait for 30 years to reach the clearance level.

Graphitization of amorphous carbon by swift heavy ion impacts: Molecular dynamics simulation

Stable C-C bonds existing in several sp hybridizations place carbon thin films of different structural compositions among the materials most tolerant to radiation damage, for applications in extreme environments. One of such applications, solid state electron stripper foils for heavy-ion accelerators, requires the understanding of the structural changes induced by high-energy ion irradiation. Tolerance of carbon structure to radiation damage, thermal effects and stress waves due to swift heavy ion impacts defines the lifetime and operational efficiency of the foils. In this work, we analyze the consequences of a single swift heavy ion impact on two different amorphous carbon structures by means of molecular dynamic simulations. The structures are constructed by using two different recipes to exclude the correlation of the evolution of sp2-to-sp3 hybridization with the initial condition. Both initial structures contain approximately 60% of sp2-bonded carbon atoms, however, with different degree of clustering of atoms with sp3 hybridization. We simulate the swift heavy ion impact employing an instantaneous inelastic thermal spike model. The analysis of changes in density, bonding content and the number and size of carbon primitive rings reveals graphitization of the material within the ion track, with higher degree of disorder in the core and more order in the outer shell. Simulated track dimensions are comparable to those observed in small angle x-ray scattering measurements of evaporation-deposited amorphous carbon stripper foils irradiated by 1.14 GeV U ions.

Electron cyclotron resonance plasma reactor for production of carbon stripper foil

A graphite antenna for the production of carbon-containing hydrogen plasmas is being developed to prepare impurity-free charge exchange foils for high-energy synchrotrons. Microwave power at 2.45 GHz frequency drives a coaxial structure antenna with a 12-mm-diameter central graphite cylinder and a tapered surrounding cylinder serving as the ground electrode. The antenna was placed in a linear magnetic field to investigate how it performs under an electron cyclotron resonance (ECR) condition. A clear resonance phenomenon was observed in plasma luminosity, microwave power absorption, and microwave power reflection when the induction current used to produce a linear magnetic field was changed. The antenna realized the best microwave coupling to the plasma with the ECR zone formed 5 mm from the end of the center electrode. The antenna realized stable operation for more than 5 h with 100 W input microwave power and with operating hydrogen pressure from 0.5 to 50 Pa.

Lifetime dependence of nitrided carbon stripper foils on sputter angle during N+ ion beam sputtering

We fabricated high-lifetime thin nitride carbon stripper (NCS) foils with high nitrogen contents using ion-beam sputtering with reactive nitrogen gas and investigated the dependence of their lifetimes on the sputter angle. The nitrogen in carbon foils plays a critical role in determining their lifetime. Therefore, in order to investigate the effects of the nitrogen level in NCS foils on foil lifetime, we measured the sputtering yield for different sputter angles at a sputtering voltage of 10kV while using carbon-based targets. We also measured the nitrogen-to-carbon thickness ratios of the foils using Rutherford backscattering spectrometry. The foils made at a sputter angle of 15° using a glassy amorphous carbon target exhibited an average increase of 200-fold in lifetime when compared to commercially available foils.

Intense heavy ion beam-induced temperature effects in carbon-based stripper foils

At the future FAIR facility, reliably working solid carbon stripper foils are desired for providing intermediate charge states to SIS18. With the expected high beam intensities, the foils experience enhanced degradation and limited lifetime due to severe radiation damage, stress waves, and thermal effects. This work presents systematic measurements of the temperature of different carbon-based stripper foils (amorphous, diamond-like, and carbon-nanotube based) exposed to 4.8 MeV/u U, Bi, and Au beams of different pulse intensities. Thermal and spectroscopic analyses were performed by means of infrared thermography and Fourier transform infrared spectroscopy. The resulting temperature depends on the foil thickness and strongly increases with increasing pulse intensity and repetition rate.

Measurement of continuous degradation of a stripper foil during ths operation with 300 kW beam power in the 3-GeV RCS of J-PARC

In the 3-GeV rapid cycling synchrotron of Japan Proton Accelerator Research Complex, we have clearly measured a continuous degradation of a stripper foil during more than 6 months operation with a beam power of 300 kW. A hybrid-type boron doped carbon stripper foil of 200 μg/cm2 was used for 181 MeV H− stripping injection. In order to know an absolute change of the foil thickness as well as information on the pinhole formation, we precisely measured an absolute change of the partially-stripped H0 and the un-stripped H− waste beams. Two absolutely independent monitor systems were used and the results were found to be very consistent with each other. The foil thickness at the injected beam spot was measured to be gradually thickening and it was more than 10 % thicker at the end compared to the initial thickness, where the measurement accuracy was obtained to be better than 2 %. The missing un-stripped H− were also measured to be gradually increasing due to foil deformation and curling but there was however no clear indication of foil thinning or pinhole formation.

Measurement of 181 MeV H− ions stripping cross-sections by carbon stripper foil

The stripping cross-sections of 181MeV H− (negative hydrogen) ions by the carbon stripper foil are measured with good accuracy. The present experiment was carried out at the 3-GeV RCS (Rapid Cycling Synchrotron) of J-PARC (Japan Proton Accelerator Research Complex). The stripping cross-sections for different charge states, also known as electron loss cross-sections of H− ion, are denoted as σ−11, σ−10 and σ01, for both electrons stripping (H−→H+), one-electron stripping (H−→H0) and the 2nd-electron stripping (H0→H+) proceeding σ−10, respectively. We have established very unique and precise techniques for such measurements so as also to determine a foil stripping efficiency very accurately. The cross-sections σ−11, σ−10 and σ01 are obtained to be (0.002±0.001)×10−18cm2, (1.580±0.034)×10−18cm2 and (0.648±0.014)×10−18cm2, respectively. The presently given cross-sections are newly available experimental results for an incident H− energy below 200MeV and they are also shown to be consistent with recently proposed energy (1/β2) scaled cross-sections calculated from the previously measured data at 200 and 800MeV. The present results have a great importance not only at J-PARC for the upgraded H− beam energy of 400MeV but also for many new and upgrading similar accelerators, where H− beam energies in most cases are considered to be lower than 200MeV.

Stripping accumulation and optimization of HIMM synchrotron

The design of an injection system for a dedicated carbon-ion therapy synchrotron is presented. Improvements to the particle tracking code ACCSIM are developed to support interactions between heavy ions and stripping foil. The three main improvements are concerned with (a) charge exchange between carbon ions and stripping foil; (b) multiple Coulomb scattering of incident heavy ions on stripping foil; and (c) energy loss in particles passing through the foil, and energy-loss straggling. The accumulation process is then simulated, and the parameters related to the injection system are optimized. A gain of about 140 in the number of particles is achieved, and the emittance and momentum spread growth is obtained, together with a central momentum shift. Finally, the superacceptance of the synchrotron, ε−Δp/p, is tracked. According to the simulation, 1.35×109 actual particles are stored successfully in the synchrotron.

Performance characteristics of HBC stripper foils irradiated by 650 keV H− and high intensity DC ion beams

Newly developed Hybrid type Boron mixed Carbon (HBC) stripper foils are extensively used not only for the RCS of J-PARC and PSR of LANL, but also for other low energy, high intensity proton accelerators in medical applications. We had before tested HBC stripper foils with 3.2MeV Ne+ and DC heavy ion beams.In order to further understand characteristics of HBC stripper foils, we measured the following parameters using the KEK-650keV H− and light ion Cockcroft Walton DC accelerator: foil lifetime, thickness reduction, uniformity before and after beam irradiation, and foil deformation. Energy deposition in the present experiment was adjusted to a similar level to that of the HBC foil used in the RCS of J-PARC’. In addition, to understand the reason why the HBC stripper foils have high durability against high intensity beam irradiation, we investigated various physical properties, and compared them between the HBC foils and other tested carbon stripper foils.The sizes of the carbon particles in the HBC foil were found to play a vital role in the lifetime.

Multi-layer stripper foils containing boron, PLD and CVD produced diamond-like carbon

The Applied Technology Group at TRIUMF operates three accelerators, TR30-1, TR30-2 and CP42 that use 30 MeV proton beams in the generation of commercial radioisotopes. These cyclotrons typically use diamond-like carbon (DLC) stripper foils produced in house of between 2.0 and 3.0 lm to extract particle beams from these cyclotrons. Micromatter (formerly the TRIUMF Carbon Foil Laboratory) manufactures DLC foils in a range of thicknesses from 10 nm to *10 lm using pulsed laser deposition. Continuing our previous work '(Zeisler and Jaggi Nucl Instrum Methods Phys Res A 613:434, 2010)' we further investigated the presence of boron in multilayer DLC foils, specifically DLC/boron/DLC hybrid foils. In this work, we investigated the mechanical properties and lifetime of these stripper foils in cyclotrons with varying thicknesses and boron content as well as a special DLC layer deposited by chemical vapor deposition and find an improvement in flexibility, mechanical strength and lifetime.

Carbon stripper foils for high current heavy ion operation

For the proposed new heavy ion linac@GSI the installation of a carbon foil stripper section is under discussion. High duty factor as well as high current (but low duty factor) heavy ion beams were used for machine experiments. Long term tests were performed to check the carbon foil durability. Relevant beam parameters have been measured in three measurement campaigns. After beam testing stripper foils were analyzed with different offline methods. Additionally promising results of high current beam irradiation of rotating target wheels will be presented. In the transfer line to the SIS 18 the heavy ion beam is stripped to higher charge states in a thick carbon foil. The stripper foil is loaded with 3 % of the beam power. To avoid evaporation in a single beam pulse, the beam is rapidly swept over its width. Experiences collected during the last decade of foil stripper operation at GSI will be presented.

Sputtering angle effects by Kr mixing in N+ ion beam on the lifetime of nitrided carbon stripper foils

Nitrided carbon stripper (NCS) foils with high nitrogen content were produced by ion beam sputtering of reactive nitrogen gas. Such foils seem to be very useful as strippers in high-intensity heavy ion accelerators. We have conducted comprehensive research, development, and production of such foils with high reproducibility. Krypton, a heavy noble gas, was mixed with the reaction nitrogen gas (IBSRN) in the ion beam sputtering process to produce stripper foils with short sputtering deposition time and long lifetimes in the high intensity ion beam irradiation. With the (N2 + Kr) mixed gas ions we investigated the influence of different sputtering angles (α) on the lifetime of the NCS-foils in case of poly-graphite material. The lifetime measurement of these foils of 20 ± 5 μg/cm2 was performed with a 3.2 MeV Ne+ ion beam as usual. The foils made at a sputtering angle of 15° showed a maximum of 11.0 and 7.5 C/cm2 average charge density, respectively, which corresponded to about 275 and 187 times the lifetime of the best commercially available foils.