Upgrade of the medium energy dump geometry for the SPIRAL2 single bunch selector

The medium energy beam transport of the SPIRAL2 superconducting linac contains a single bunch selection system equipped with a 7.5 kW beam dump. This device, originally designed with a long flat slope to decrease the power density so that the maximum operating temperature was 170 °C, was impacted by Coulomb scattering generating two side effects: heating of the downstream beam transport components and degrading of the beam current measurement uncertainty. The paper relates the way these two problems were solved.

The corresponding bunch separation of ~11.3 ns being too short for experiments based on time-offlight measurement, a single bunch selection (SBS) system [2] is inserted in the medium energy beam transport (MEBT) to suppress most of the RFQ bunches and leave a lag ranging from 1 to 500 µs between the selected ones.The beam dump (SBS dump) is a key component of this system as it must stop more than 99% of the beam.
Beam power can be up to 7.4 kW in the MEBT and 200 kW after the SC linac.Using the SBS changes these values of a factor 100 at least.Very sensitive and large dynamic range diagnostics are then used by the machine and safety protection systems to monitor beam intensities, transmission and integrated intensity on the targets [3].Threshold settings must take into account any measurement uncertainty, which reduces the possibility to operate the accelerator or the experimental target at nominal performances.

Beam current measure offset
Beam intensities and transmission are continuously monitored by AC and DC current transformers (ACCT, DCCT), used in peak, average, differential and integral modes [3,4].These measurements are required for both accelerator tuning and machine protection A chain of four current transformers is located along the accelerator: a single DCCT is placed before the RFQ, while blocks containing a DCCT and an ACCT (CTB) are placed in the MEBT, at the end of the SC linac, and just before the main beam dump.Figure1 shows one of these blocks with its associated stabilizing and monitoring electronics.Being a protection device, the current is set and read by two different systems.The DCCTs readout suffers from a strong thermal drift.Their temperature is then stabilized around 40°C, to grant a measurement uncertainty around 60 µA.This requires some 45 W that are provided by the control unit power supply.The obtained uncertainty being insufficient when the experimental setup uses the SBS and the average current drops below 50 µA, ACCTs are also installed aside.These devices have faster response time and much better uncertainty: 6 µA at least with continuous beams out of the RFQ, and up to 1 µA, with beam pulsed at 100 Hz by the LEBT chopper.Below these intensities, we consider that there is no thermal risk anymore.
The MEBT CTB is installed immediately after the SBS dump.Since the proton beam commissioning in 2019, we had observed significant offset (up to 100 µA) in the measured currents, and we were forced to take it into account in the machine protection system.Thresholds values were set respecting the following criterion, and the offset was calculated for each beam, in order to take the smallest possible value.In 2020, dedicated machine studies let identify that the offset was correlated to the use of the SBS.This correlation was confirmed during the deuteron beam commissioning when, in addition to the offset, the CTB temperature could not be stabilized and reached some 44°C, as ~100 W were coming from elsewhere, contributing to warm up the device.
Temperature probes were then installed around the beam transport tube at the exit (probes 1 to 4) and at the entrance (probes 5 to 8) of the CTB. Figure 2   The temperature rise was quite fast, while the drift of the control unit operating point was much slower (unlocking the temperature took few hours).This abnormal behaviour confirmed that the deviated beam was not totally stopped by the SBS dump and that some particles were somehow reflected and stopped on the bottom of the tube crossing the CTB, as shown in Fig. 3.The picture also shows that the slit below the dump shields the position of T06, which explains that T02 is higher.

Coulomb scattering
This effect is due to Coulomb scattering of the incident particles on the copper nuclei of the SBS dump.These reactions occur because of the low relative angle of the dump surface.In fact, the scattering cross section is very large at low angle  as indicated by the Rutherford formula: Scattering takes place in the first microns of the dump surface: for a particle energy of 0.73 MeV/u, the maximal penetration distance R (the range) is around 9.4 µm for deuterons and 4.7 µm for protons.The energy spectrum of scattered particles outside the dump is rather wide (from 0 to 0.73 MeV/u) depending on the depth of interaction point and the energy loss in the copper.Most of the scattered particles are lost in between the dump chamber and the downstream rebuncher.
A simulation of the beam scattering, on a dump layer depth of R/2, with an average deflection angle of 7.5° toward the exit of the SBS (representing a solid angle of 0.028 sr) has been undertaken using the Rutherford cross section.The simulation justifies almost 1% and 2% of scattered current for deuterons and protons respectively, which roughly fit to the current transformer offsets, even if observed percentages for both beam kinds were invertedOther paragraphs are indented.

SBS dump first and new geometries
The initial design of the beam dump aimed at keeping a low temperature on the hottest spot to avoid strong outgassing.For this reason, the 7.3 kW beam power was distributed over a long slightly sloped surface, leading to maximum power density and corresponding temperature around 150 W/cm 2 and 165 °C, respectively.Considering the beam and the surface tilts, the angle of incidence was of some 3.5°.The surface tilt being variable in a range of ±1°, we could observe that the scattering phenomenon was increasing when the glancing angle was reduced To avoid the scattering, the linear profile was modified by the staircase [5] one.We took the idea from an application to emittance meters [4] but similar geometries are also used in high energy colliders or photon beams [5,6].The new beam dump geometry is described in Fig. 4 and 5, where the longitudinal direction z is reversed.The steps are not horizontal but have a slightly angle (3°) higher than the beam (~2°) tilt, so that all particles are stopped on the risers only.The dump counts 23 risers of almost 0.5 mm and a higher nose, and was machined at the laboratory workshop, on a 4-axis machine.The corresponding amount of beam power intercepted by each riser is shown in Fig. 6. been simulated and is shown in Fig. 7, where it is compared to the measured values.Maximum temperature expected on the central probe (T2) is ~160° C but the measured value is slightly lower.Temperature T1 is much lower than expected and than T3, which could be due to a real beam distribution different from the simulated one or to a not perfect longitudinal centring of the beam.
The hottest spots are located on the step edges, reach over 300 °C in the simulation presented in Fig. 8, but are strongly dependent on the beam distribution.High temperatures are nevertheless concentrated on very small surfaces, and do not degrade the vacuum pressure that is still around 210 -7 mbar.We consider that temperature is low enough and will not deteriorate the step hedges, but this will take some time to be demonstrated.We expect the beam current offset to show-up again in case of edge smoothing.
Finally, it is interesting to notice that a first trial with perfectly horizontal steps had managed to reduce the loss heating and the offset, but only of a factor 4, due to some residual interaction between the beam and the steps.
reports the values reached with a 3.5 mA deuteron beam.The two of them located on the tube bottom line are also plotted to show the time constant of the temperature rise.The tube was reaching 130 °C, providing enough thermal energy to prevent proper control of the CTB temperature.

Figure 2 :
Figure 2: Temperature values around the tube and plots of the ones on the bottom : T02 and T06.

Figure 3 :
Figure 3: Possible impact area of scattered particlesOther paragraphs are indented