The PSI meson target facility and its upgrade IMPACT-HIMB

. The high intensity proton accelerator complex (HIPA) at the Paul Scherrer Institute (PSI) delivers a 590 MeV CW proton beam with currents up to 2.4 mA (1.4 MW). Besides two spallation targets for thermal/cold neutrons (SINQ) and for ultracold neutrons (UCN), the beam feeds two meson production targets Target M and Target E. The targets consist of graphite wheels of effective thickness 5 mm (M) and 40/60 mm (E). The target stations M and E are of quite different design; however, both of them rotate at 1 Hz to dissipate the heat (20 kW/mA for the 40 mm target E) efficiently. Recent progress was made by a new type of bearings and a new target geometry able to increase the rate of surface muons by up to 50 %. This is also foreseen for the upgrade of the target station M within the High Intensity Muon Beam (HIMB) initiative aiming to increase the surface muons available for experiment by two orders of magnitude. HIMB is part of IMPACT (Isotope and Muon Production with Advanced Cyclotron and Target Technology), an application for the Swiss Roadmap of Research Infrastructure.


Introduction
The 590 MeV proton beam of HIPA [1] with currents up to 2.4 mA serves four target stations, two spallation targets and two meson production targets. The Ultracold Neutron Source UCN gets the full beam for a few seconds every 5 minutes, whereas the neutron spallation source SINQ is located at the end of the main beam line after the two meson production targets M and E [2]. Therefore, the beam on the SINQ target already lost part of the energy and current on its way through target M and E. While this effect is negligible for target M due to the small effective target thickness of 5 mm graphite, the influence of target E (either 40 or 60 mm thick graphite), on the beam is substantial. E.g. after the 40 mm graphite the beam is reduced in energy by 15 MeV and by 30 % in current, mainly due to multiple scattering in the target. The positive side effect is that the emittance of the beam is already enlarged and therefore the beam can be distributed easily on a large area onto the SINQ target to avoid overheating. On the other hand, this also means that target E must not be missed any time to avoid damage of the SINQ target by a pencil beam.
Both targets are made of graphite, which has a large production rate for pions in the energy range of HIPA for a given beam transmission. Detailed studies [3] revealed that beryllium would be more effective. However, beryllium could not stand currents higher than 120 µA for more than a few weeks in the 590 MeV proton beam at PSI [4]. Most of the experimental stations at the seven beam lines (s. Fig. 1) prefer muons, which originate from pion decay. Muons are produced either by decay-in-flight of pions ('cloud muons') or by stopped pion decay in the target. Muons produced close to the target surface, 'surface muons', leave the graphite, while those deeper inside are stopped. The large polarization of surface muons is used e.g. for investigating the magnetic structure of materials via the µSR technique [5]. With spin rotators, the spin of the surface muons can be turned in the desired direction. At PSI, three spin rotators are in use, the third one in PiE1 is not shown in Fig. 1. At the beam line MuE4 with a rate of the order of 10 8 surface muons per second, up to now the world highest intensity, low energy muons (LEM) of 0.5 to 30 keV are available.

Target E station
To operate a meson production target of 40 mm graphite in a 2.4 mA proton beam of 590 MeV and a size of about 1.5 mm (2-sigma) poses several challenges. At such conditions, a power of 50 kW is lost at the target, which heats up the target and has to be dissipated. The material needs not only to be temperature resistant but also has to tolerate thermal stress as well as the change of physical properties due to radiation damage for a reasonable time period. In addition, not only the target material but also the mechanical parts of the target station have to withstand the radiation.
Natural graphite has a layered structure and therefore is quite anisotropic, i.e. its physical properties are significantly different perpendicular to and within the layer. E.g. when natural graphite is heated, large thermal stress occurs due to the different thermal expansion coefficients. Usually, this leads to cracks, if the thermal stress exceeds the ultimate tensile limit. To mitigate this, polycrystalline graphite is used instead. This graphite consists of very small grains, which have no preferred order and therefore have almost isotropic properties. Nowadays, the grains can be as small as 10 µm or even less. The smaller the grains the higher the density, which is achieved during sintering by isostatic pressing. Typically densities are between 1.8 and 1.85 g/cm 3 , which is still 20% below the natural graphite. The grade used for Target E, R6510 from the SGL group, has a density of 1.84 g/cm 3 and a grain size of 10 µm.
The graphite target is a wheel with a diameter of about 450 mm, which rotates at 1 Hz to distribute the power on a rim with a circumference of about 150 cm. Due to the rotation, direct cooling by contact to e.g. water tubes would be difficult. In addition, since the thermal conductivity of graphite is known to decrease fast due to radiation damage [6], the cooling efficiency would drop soon. Therefore, the graphite is cooled by thermal radiation, which is an efficient way for materials with large emissivity at high temperatures. With an emissivity of 0.75 [4], the temperature of the graphite at the impact location of the beam is predicted to be 1700 K by simulations, since no direct measurement is available. The thermal power lost by radiation is dissipated by blocks of copper, which are cooled by water. One block is mounted directly behind the graphite wheel, others protect the vacuum chamber. Even though the vacuum chamber is cooled by water, it cannot dissipate as much power as the copper blocks, since the chamber is made of stainless steel with smaller thermal conductivity.
The wheel is connected to the hub through six spokes. At 1700 K the graphite wheel expands, which would lead to thermal deformation and cracks. For this reason the spokes can slightly slide on the wheel, such that the wheel is allowed to expand and cracks are avoided. However, with increasing beam current it was observed that the deformation of the wheel increases, which has to be limited to a maximum of 2 mm, since the target wheel is 6 mm wide only. This was significantly improved by choosing a graphite grade with more isotropic properties, i.e. smaller grains. In addition, the thermal stress was released by dividing the rim of the wheel in 12 segments with 1 mm space in between. The highest total proton charge on a target was 39 Ah, which would correspond to 4 to 5 full beam periods. However, the target was not continuously in use. After usage, the parts of the graphite containing a high concentration of desired radionuclides like 10 Be were used to prepare targets for new experiments [7,8].
The wheel is turned by a motor, which is mounted 2.5 m above the beam line to avoid radiation damage. At this location, it can be in operation for about 5 to 8 years. Unfortunately, the three ball bearings are mounted close to the graphite wheel and therefore in a high radiation field. That means that the bearings cannot be lubricated with grease, as an organic material would soon get hard and brittle, preventing a reliable operation. In addition, the first bearing can heat up to 140 o C, due to thermal radiation from the wheel but also due to the connection of the hub to the wheel by the spokes, which are hollow to minimize thermal conductivity. The demand that the bearings have to work under cold conditions as well as during beam operation requires a larger radial clearance in the bearings to compensate for thermal expansion. Both effects reduce the lifetime of the bearings.
When the bearings fail they have to be replaced. For this, the 4 m thick shielding of many concrete blocks has to be removed to access the so called service platform. The water supply is disconnected from the target insert and the remaining water removed. The vacuum in the beam line is broken, however, to minimize tritium release, an underpressure is kept. Due to the high dose rate of the target of about 3 Sv/h mainly due to 7 Be, the target insert has to be taken out by a 45 t exchange flask shielded by up to 40 cm of steel. The flask is remotely operated by a control unit showing the operation status of the flask on a screen. Due to the infrastructure on the service platform above the beam line, there is no space to position the flask. Therefore, the flask is mounted on a frame bridging this area. The frame, the so called bridge, also contains a contamination protection, a door to close the lifting hole as well as sticks for positioning the exchange flask precisely. Inside the flask, a hook couples to the target insert, which pulls it out afterwards. During the beam period of HIPA, the spare target insert is taken out of a shielded parking position. The exchange of the bearings is done in the shielded service cell ATEC at PSI, which is equipped by manipulators and cameras. The exchange of the target insert takes 1.5 to 2 days usually. If possible, the target exchange is moved to a service period, however, the failure of the bearings is difficult to predict. For this, the current needed for the motor to turn the wheel is monitored. Stronger spikes and jumps often indicate that wear debris is blocking the balls in the bearing; however, sometimes it disappears after few days. In this case, the debris could be milled to dust or fell out of the bearing.

Recent Improvements
The bearings from the company GMN (Germany), which consist of ceramic balls (Si3N4) in a stainless steel cage and rings, the latter coated with MoS2 and Ag as lubricants, had to be usually exchanged once in the beam period of HIPA, i.e. they last 4 to 5 months. Since this is not optimal, several types of bearings were tested, first in three test stands without proton beam. One type called full ceramics from Cerobear (Germany) had both balls and rings from Si3N4, i.e. only the cage was out of stainless steel. Under real conditions on the target in the proton beam it failed after a few weeks. The metal cage was strongly twisted in such a way that some balls fell out of the bearing during the exchange in the service cell. The reason could be that the cage got too hot because of the low conductivity of the ceramic ring, which dissipated only a small amount of heat to the axis. Another bearing ordered from the company Koyo, Germany (belonging to JTEKT, Japan), had still a good performance after 420 days in the test stand [9]. This ball bearing is entirely made out of stainless steel with blocks from WS2 in between the balls as lubricant. It is also used in the meson production target at J-PARC with a predicted life time of 22 years [10]. During the HIPA shutdown, before the start of the beam period in April 2021, three Koyo bearings were mounted. No change of the bearings was needed. Moreover, the motor current monitored was almost constant compared to previous experience. No degradation of the bearings at the end of the year was observed. It is very likely that this set of bearings could have been operated longer. To avoid the risk of a failure, a new set of Koyo bearings were used, which also lasted the whole beam period in 2022.
Since 2019, several new target types were tested in the Target E station. These developments had two goals: to use the target itself as beam position monitor and to increase the surface muon rate. In the beginning, these developments were followed up independently, however, finally combined. In the first case, the basic idea is to modulate the beam transmission according to the beam position on target. In the case of the slab type target, where the beam passes through the full rim, this can be performed by additional grooves on the left and right side of the rim. If the beam leaves its nominal position in the region of the target centre, part of beam will pass through the grooves on the outer surface of the target. Since the grooves reduce the effective thickness of the target, this leads to a higher beam transmission. As the grooves are placed in a regular but different pattern for the left and right sides of the target, the amplitude of the modulation extracted by Fast Frequency Analysis (FFT) contains information about the beam position. This is much more sensitive than the usual method of measuring the total transmission through the target by comparing the current behind and before the target. Already a beam deviation of 1 mm from the centre increases the modulation by two [11].
To increase the rate of surface muons, the active target surface has to be enlarged. For this, a slanted target geometry has been developed, where the beam impinges on the target under a small angle keeping the effective thickness the same but increasing the surface [9]. Such a target was first tested at the end of 2019. Up to 50% more surface muons were measured and its dependence on the secondary beam lines was confirmed as predicted by simulations. Due to the larger rim of 85 mm instead of 40 mm as well as the slightly smaller width of 5.6 mm, the target is more prone to deformations. In addition, an important difference to the slab target is that the beam heats just the centre part of the rim whereas the surface temperature on the rim in case of the slab target is nearly homogeneous. Indeed, ANSYS calculations showed that the von Mises stresses are larger but do not exceed the ultimate tensile stress limit. No deformation is observed so far, however, the irradiation time is still too short for a final judgement.
Both functions, monitoring the beam position and increasing the surface muon rate, were combined in the newest target development, a slanted type with grooves and ribs. The ribs shown in Fig. 2 vary in thickness and are located on the right of the nominal beam entering the target. On the backside of the target, there are two more rows of ribs and grooves for the left and centre position of the exiting beam. First operation of the target in beam was in 2022, analysis of its performance is underway.

Target M station
The present target M station dates back to 1985 with a renewal of the target insert in 2012. Except for the fact that it is a graphite target rotating with 1 Hz, its design is quite different from the Target E station. Since the target M plug is inserted horizontally, only one long rotational axis is needed and the bearings are shielded. Therefore, the bearings last several years. Since the effective thickness is 5 mm only, the temperature reaches about 1100 K cooled mainly by thermal conductivity. The two beam lines for secondary particles are positioned in the forward direction under 22.5 o , i.e. optimal for high energetic particles like pions. However, both users of the beam lines, one used for particles physics, the other for material research, are aiming for low energetic surface muons. Therefore, this target station is subject of renewal in the framework of the HIMB initiative. HIMB belongs together with TATTOOS, the Targeted Alpha Tumor Therapy and Other Oncological Solutions, to the IMPACT application for the Swiss Roadmap of Research Infrastructures from PSI, University of Zurich (UZH) and University hospital Zurich (USZ) and aims to keep PSI at the forefront of research in material, particle physics and life science. Details can be found in the Conceptual Design Report (CDR) [12].

HIMB
HIMB aims to increase the muon surface rate by a factor of 100 to 10 10 µ/s by optimising target and secondary beam lines for this purpose. However, first the Target M station has to be dismantled by remote handling since the residual dose rate, after removing collimators and target with the exchange flask, is estimated in the order of 2 Sv/h. The key components of the upgrade are: • A capture solenoid with a large diameter and magnetic field (~0.45 T) in close distance (250 mm) to and on both sides of the target. • Large transmission for low energetic muons in a series of solenoids with large aperture. • Increase target thickness from 5 to 20 mm.
• Slanted target type. The close distance of the capture solenoid to the target requires the compensation of the fringing field at the location of the proton beam, which is otherwise strongly bent upwards. The fringing field is reduced by a mirror plate at the capture solenoid, which has to be cooled due to its close vicinity to the target. Further, an additional steering magnet for the beam is necessary. Due to the four times thicker target, a new collimator system as well as more shielding is needed to cope with the larger losses and radiation. The new target H insert resembles the target E station; however, due to the small amount of space available, it is not possible that the beam passes through the target at the height of the rotational axis, because otherwise it would hit the hub. To save even more space the beam has to pass through the cooling plate behind the graphite wheel (s. Fig. 3). This requires measures to avoid that the beam hits the inside of the copper plate. For this, an aperture loss monitor is mounted in front of a current sensitive collimator out of tungsten in the cooling plate. Both can trigger the interlock system to switch off the beam. The slanted target (Fig. 3) is the favourite version. It is flatter compared to target E and consists of two disks, each containing every second segment with 3.5 mm thickness. ANSYS simulations look promising regarding the expected deformation and thermal stress.

Summary
The operation of a meson production and muon target at 50 kW poses several challenges like cooling, thermal stress and high radiation. With the Koyo bearings from J-PARC an important step forward was achieved to increase the reliability of Target E. This is particularly important for the new Target H station, foreseen in the IMPACT initiative as upgrade of Target M to a 100 times larger surface muon rate. It is planned for 2027 with first beam after the HIPA shutdown in 2028. Target H will also profit from the operational experience with the Target E station as well as from the recent target developments, particularly the slanted target type, which was proven to increase the surface muon rate up to 50%.