Measurement of lifetimes of thin carbon stripper foils produced by ion-beam sputtering

Thin carbon stripper foils used in high-intensity proton accelerators and heavy-ion accelerators must have long lifetimes. Thin carbon foils were fabricated by ion-beam sputtering using reactive and inert gas ions. The lifetime of the foils was measured using a KEK 650-keV high-intensity DC H⁻ (negative hydrogen ion) beam; changes in the foil thickness and surface deformations during irradiation were investigated. The lifetime of a typical stripper foil fabricated by heavy-ion-beam (Ar and Kr) sputtering was 60–70 times longer than that of the best commercially available foils. This paper reports a fabrication method for carbon stripper foils, along with an investigation of their lifetimes and changes in foil thickness during beam irradiation.     

Measurement of carbon sputtering yield by N ion beam and lifetime dependence of resulting foils on thickness

Carbon stripper foils with a higher nitrogen content were made by ion beam sputtering with reactive nitrogen gas. Such foils seem to be very useful as strippers for high-intensity heavy ion accelerators. To know further characteristics of the lifetime of such carbon foils, we have measured the sputtering yield of the carbon source material at a sputtering voltage of 4–15 kV and the lifetime dependence of such foils on thickness. Lifetime measurement was performed with a 3.2 MeV Ne⁺ ion beam. The sputtering yield on average showed 0.75 atoms/ion at over 9 kV sputtering voltage. The lifetime of the foils noticeably depends on the foil thickness, and the thickness range as practical stripper foil is to be around 15 to 33 μg/cm². Two foils made at 13 kV showed extremely long lifetimes of 6800 and 6000 mC/cm² at maximum and the foils made above 10 kV lived longer than about 900 mC/cm², which correspond to about 270 and 40 times longer than commercially available best foils. We measured the thickness ratio of nitrogen to carbon in each foil made at the different sputtering voltages and at the different irradiation stages (mC/cm²) by RBS method. We also inspected the structure of a nitrided carbon foil by transmission electron microscopy.     

Effect of impurities in charge-exchange carbon foil on foil thickness reduction

Carbon thin foils are commonly used as a charge stripping material in particle accelerators. Depending on the original foil thickness, changes in thickness during beam irradiation vary: thin foils (∼10 μg/cm²) thicken by build-up, medium thickness foils (∼15 μg/cm²) remain unchanged, and thick foils (∼20 μg/cm²) become thinner. The thickness reduction differs even under identical manufacturing processes and conditions.The factor causing foil thinning is unknown. On the basis of the low sputtering rate of carbon, it can be said that impurities contained in the foil cause foil thinning.Carbon foils contain impurities such as water. These impurities dissociate and combine with carbon and then evaporate. Taking this into consideration, we examined the gas composition during beam irradiation, to determine which impurity causes foil thinning. As a result, we found that oxygen contained in the foil plays a role in foil thinning.     

Optimum thickness of carbon stripper foils in tandem accelerator in view of transmission and lifetime

Optimum thickness of charge stripper foils installed at the terminal of a tandem accelerator has been investigated from the view of (1) charge stripping effect, (2) transmission of ions through accelerator, (3) lifetime of foils for the irradiation of ions. For this purpose, measurements have been done for (a) transmission of H, Li, O, Br and Au ions, passing through 12 UD Pelletron tandem accelerator for carbon stripper foils of 1.8–19.5 μg/cm² thickness, at terminal voltages of 5 and 10 MV, and (b) lifetime of 2–15 μg/cm² thick Tanashi foils developed by Sugai by irradiating Au ions at the terminal voltage of 10 MV. The results obtained are as follows: (a) From the view of above items (1) and (2), the optimum thickness of foils is 10 μg/cm² for ions of Z=1, several μg/cm² for Z=8, and less than a few μg/cm² for heavier ions. (b) From the view of item (3), the lifetime of Tanashi foils by means of new arc-discharge method is demonstrated to be much longer than that of commercial foils for foils thicker than about 5 μg/cm² thick. This superiority rapidly decreases with decreasing foil thickness, and at around 2 μg/cm², the lifetime of Tanashi foils is at the most 2.4 times longer than that of commercial foils.     

Mounting stripper foils on forks for maximum lifetime

While research and development continue to produce forms of carbon for longer lasting stripper foils, relatively little attention has been paid to other factors that affect their survival in use. It becomes apparent that the form of carbon is only part of the issue. Specific mounting methods increase the lifetimes of carbon stripper foils. These methods are determined in part by the specific use and carbon type for a foil. With careful handling, appropriate adhesive, and slack mounting, premature breakage can be avoided. Foil lifetimes are then primarily affected by less easily controlled factors such as high-temperature expansion, shrinkage and evaporation.     

Polymer coating method developed for carbon stripper foils

The problem of handling the fragile carbon foils (mounting on the frame, placing in the stripper changer) that easily break when self-supporting has been solved by coating carbon foils with poly-monochloro-para-xylylene. It was found that the polymer-coating method could also be used to produce carbon foils thicker than 100 μg/cm² by alternated deposition of carbon and poly-monochloro-para-xylylene layers. Carbon foil of 500 μg/cm² thick and 10 cm in diameter was produced by this method and mounted to a foil holder. Results of lifetime measurement for singly coated foils are also presented.     

Nucleation and initial growth processes of diamond thin films for stripper foils

Carbon ion beam stripper foils were fabricated from diamond films synthesized on silicon via chemical vapor deposition. Fine-grained polycrystal diamond foils with decent surface flatness were obtained using a nucleation enhancement pretreatment process. Freestanding diamond foils were formed by etching a portion of the silicon substrate on which the diamond films well-adhered. In preliminary lifetime evaluations, the 1–3 μm-thick diamond foils lasted between 20 and 420 min for 3.2 MeV Ne⁺ion-beam charge stripping.     

Comparison of carbon and corrugated diamond stripper foils under operational conditions at the Los Alamos PSR

To accumulate high-intensity proton pulses, the Los Alamos Proton Storage Ring (PSR) uses the charge-exchange injection method. H⁻ ions merge with already circulating protons in a bending magnet, and then are stripped off their two electrons in a carbon stripper foil. The circulating protons continue to interact with the foil. Despite efforts to minimize the number of these foil hits, like “painting” of the vertical phase space, they cannot totally be eliminated. As a result, foil heating and probably also radiation damage limit the lifetime of these foils. In recent years, LANL has collaborated with KEK to improve the carbon foils in use at PSR, and these foils now last typically for about 2 months. Recently, an alternative in the form of corrugated diamond foils has been proposed for use at SNS. These foils have now been tested in PSR production for a year, and have already shown to be at least as enduring as the LANL/KEK carbon foils. Advantages of the diamond foil concept, as well as some noteworthy differences that we observed with respect to the LANL carbon foils, will be discussed here.     

Lifetime improvement of slackened thin cluster carbon stripper foils by suppression of carbon buildup

We control the amount of carbon buildup on slackened thin cluster carbon stripper foils (less than 3.5 μg/cm²) by heating with a high-power infrared lamp during beam bombardment. Foil lifetime measurements were performed using 2.0±0.5 μA beams of 3.2 MeV Ne⁺ ions and quantified as the total charge/area before breakage. Lifetimes were obtained up to 1286 mC/cm² at maximum and 1139 mC/cm² on the average; these values are, respectively, approximately 51 times at maximum and 46 times on average greater than the best commercially available foils, when used unheated and unslackened.     

Stripping efficiency and lifetime of carbon foils

Charge-exchange injection by means of carbon foils is a widely used method in accelerators. This paper discusses two critical issues concerning the use of carbon foils: efficiency and lifetime. An energy scaling of stripping efficiency was suggested and compared with measurements. Several factors that determine the foil lifetime—energy deposition, heating, stress and buckling—were studied by using the simulation codes MARS and ANSYS.     

Structure studies of carbon foils with the aim to improve the ability for heavy-ion stripping

Slackened carbon stripper foils of 3 to 5 μg/cm² produced by vacuum evaporation-condensation were graphitized by annealing at about 3300 K with a pulsed laser beam (1.06 μm). An average crystal size of 10 nm was measured by electron diffraction. The texture of the 002-plane is parallel to the surface of the foil as known from slightly annealed carbon foils. Radiation damages caused by high doses of heavy ions dramatically change the crystal structure. Electron diffraction patterns reveal newly formed graphite crystals with their 002-plane perpendicular to the texture.Compared to carbon foils of the same kind, but not graphitized, a lifetime prolongation of a factor of two was achieved for such foils tested in the terminal of the Munich MP tandem. The slope of the curve for heavy-ion transmission through carbon stripper foils varies for foils of the same kind and for different ion-beam conditions.     

New methods for testing cyclotron carbon stripper foils

Carbon foils are used in cyclotrons and other accelerators to strip electrons from fast ions, enabling the extraction or further acceleration of those ions. Lifetimes of stripper foils (extractor foils) need to be as long as possible to maximize the efficiency of accelerator use and minimize radiation exposure of maintenance personnel. A foil's useful lifetime is determined in part by operating temperatures, manufacturing method, and mounting method. We used two methods to test foils in a simulated accelerator environment. These methods involve the heating in vacuum to temperature ranges 500–1750 and 500–4000 K. Either kind of testing can be done in a few minutes instead of the days or weeks typically needed in a cyclotron. Such tests show clearly that foils manufactured by different methods have different responses to elevated temperatures. Because of this fact, optimum mounting techniques depend on the foil type. Results to date are summarized.     

A model of carbon stripper target failure under ion beam bombardment

A physical model of carbon stripper foil failure is presented. Failure of foils as a result of radiation induced stresses exceeding the ultimate strength is considered. These stresses are ascribed to lattice distortion caused by point defect accumulation inferred from kinetic rate equations with defect sinks being considered. The linear changes account for the lattice anisotropy. The advantage of this model as compared with others is that it uses no fitting parameters. On the basis of the model, the experimental data of carbon stripper foil lifetimes in ion beams are analyzed. The calculated foil lifetimes agree well with the experimental data.     

Multilayer carbon foils for cyclotron beam extraction

The TRIUMF Applied Technology Group operates high-power industrial cyclotrons for commercial radioisotope production. Two of these cyclotrons, TR30-1 and TR30-2, are capable of accelerating H⁻ ions to an energy of 30 MeV and beam currents in excess of 1000 μA. For many years, amorphous carbon foils of approximately 2.0 μm thickness have been utilized to extract proton beams from these accelerators.Novel multilayer foils consisting of layers of amorphous and diamond-like carbon (DLC) of 2.0±0.2 μm thickness were manufactured in-house by carbon arc and pulsed laser deposition, respectively. In the TR30 cyclotrons, the new composite foils with 25% DLC content show a three times longer lifetime than the purely amorphous foils, while maintaining their excellent physical and mechanical characteristics during irradiation.     

SNS stripper foil development program

When the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL) becomes fully operational it will be the world's highest intensity neutron spallation source. The charge-exchange injection section in the accumulator ring, which strips the injected H⁻ beam to H⁺, requires a stripper foil 260 μg/cm² thick, 12 mm wide, a height of at least 20 mm, and support from just the top edge. The foil will get very hot due to the 1.4 MW, 1 GeV, 60 Hz H⁻ beam that passes through the foil, in addition to the 7–10 foil traversals each circulating proton makes through the foil. The planned upgrade to 3 MW beam power presents an even greater challenge. To meet this challenge a diamond foil development program has been underway at ORNL since 2001. Both microcrystalline and nanocrystalline foils have been developed and tested. In this paper we will discuss the SNS injection process, stripper foil requirements, and results from the diamond foil development and testing program.     

Development of flexible long lived carbon stripper foils

We have developed flexible long-lived carbon stripper foils of about 10 μg/cm² in thickness with an enhanced carbon arc-discharge method. The foils are not only mechanically strong, but also long-lived against the bombardment of a 3.2-Mev Ne ion beam of 1 μA in intensity with a beam spot of 3.5 mm in diameter. The average lifetime is 15 h. This value is about 30 times longer than those of commercially available foils. We have found the areas of the foils under the heavy ion bombardment to change their crystal structure. An inspection by means of an electron microscope shows that the nature of the area has transformed to a crystallographic characteristic from amorphous crystals.     

Exchange and observation systems for charge stripper foils at the J-PARC 3GeV-RCS

The Japan Proton Accelerator Research Complex (J-PARC) has been under construction in Tokai-mura, Ibaraki, Japan. Three independent charge stripper devices are set up at the injection line of the 3 GeV Rapid Cycling Synchrotron (RCS). The H⁻ beam accelerated by a 181 MeV Linac is charge-exchanged to a H⁺ beam by the first stripper foil, and then injected into the RCS. The H⁰ and H⁻ fractions of the beam, which are not stripped by the first stripper foil, are converted into a H⁺ beam by the second and the third stripper foils.We have designed the charge exchange devices by adopting the transfer-rod system for moving the foils in a vacuum. We have fabricated a new type of transfer-rod, which can move over a distance of 1500 mm.We have also developed a new telescope system to observe possible wrinkles and pinholes of the foil. The system withstands more than 1 MGy of radiation dose and has a resolution of 250 μm at a distance of 10 m from an object.     

Carbon stripper foils held in place with carbon fibers

The Spallation Neutron Source (SNS) currently under construction at Oak Ridge National Laboratory, Oak Ridge, Tennessee, is planned to initially utilize carbon stripper foils having areal densities approximately 260 μg/cm². The projected design requires that each foil be supported by only one fixed edge. For stability of the foil, additional support is to be provided by carbon fibers. The feasibility of manufacturing and shipping such mounted carbon foils produced by arc evaporation was studied using two prototypes. Production of the foils is described. Fibers were chosen for satisfactory mechanical strength consistent with minimal interference with the SNS beam. Mounting of the fibers, and packaging of the assemblies for shipping are described. Ten completed assemblies were shipped to SNS for further testing. Preliminary evaluation of the survivability of the foils in the SNS foil changer is described.     

Charge-stripping foil changer with energy adjuster function

A charge-stripping foil changer that has an energy adjuster function was constructed. One of the 30 foils in the changer is placed at the beam position without opening the vacuum chamber. The energy of the beam behind the charge stripper is adjusted by changing the effective thickness of the foil, which is changed by tilting the foil at an angle from 0° to 60°. A uranium beam was accelerated to 345 MeV/nucleon using two charge-stripping foil changers with an energy adjuster function.