Towards quantitative simulations of high power proton cyclotrons

We describe a large scale simulation effort using Object Oriented Parallel Accelerator Library, that leads to a better quantitative understanding of the existing Paul Scherrer Institut high power proton cyclotron facility. The 1.3 MW of beam power on target poses stringent constraints on the controlled and uncontrolled beam losses. We present initial conditions for the Ring simulation, obtained from the new time-structure measurement and the many profile monitors of the 72 MeV transfer line. A trim coil model is developed, including trim coil TC15, which is needed to avoid the dangerous ${\nu }_r=2{\nu }_z$ resonance. By properly selecting the injection position and angle (eccentric injection), the flattop voltage, and phase, very good agreement between simulations and measurements at the radial probe RRE4 is obtained. We report on $3–4$ orders of magnitude in dynamic range when comparing simulations with measurements. The relation between beam intensity, rms beam size, and accelerating voltage is studied and compared with measurement. The demonstrated capabilities are mandatory in the design and operation of the next generation high power proton drivers. In an outlook we discuss our future plans to include more physics into the model, which eventually leads to an even larger dynamic range in the simulation.

Figure 1

The PSI Injector 2 cyclotron and beam transfer line to the PSI Ring cyclotron.

Figure 2

Envelope of the beam in the 72 MeV transfer line for a 2 mA beam. The error bars show different measurements at the same intensity level.

Figure 3

Bunch length of the beam in the 72 MeV transfer line for a 2 mA beam (in Fig. 1 the positions of the time-structure measurements are shown).

Figure 4

The field (solid line) and field gradient (dashed line) of TC15 in the PSI Ring cyclotron.

Figure 5

Tune diagram with and without TC15.

Figure 6

Schematic representation of the turn pattern in the PSI Ring cyclotron. $I_c$, ${\varphi }_c$, and $I_r$ represent the intensity distribution of a centered beam with 6 mm turn separation, the betatron oscillation phase of an eccentric beam, and the intensity distribution of a real beam with eccentric injection.

Figure 7

Radial beam profile with indicated turn numbers at extraction for a 2 mA beam, for a parabolic initial distribution at MIC.

Figure 8

The comparison of different initial beam distributions at the extraction probe, for a 2 mA beam.

Figure 9

Radial beam size at extraction vs beam current.

Figure 10

Production beam current versus turn number over a span of 32 years of the PSI Ring cyclotron, in double logarithmic scale.