Experimental demonstration of the short bunch extraction by bunch rotation in a high-intensity rapid cycling proton synchrotron

Short bunch proton beams are of great significance for the applications of white neutron beams and muon beams. The accelerator complex of the China Spallation Neutron Source (CSNS) was designed to support the applications mainly based on neutron scattering techniques where the proton pulse length is not very sensitive. Some theoretical and experimental studies have been performed to see if one can extract a short-bunch proton beam by bunch rotation from the rapid cycling synchrotron (RCS) at CSNS. The experimental results at RCS have evidently displayed the bunch lengthening and rotation process, which demonstrates the effectiveness of this method even with a very short available time for the rf gymnastic processes and a high-intensity beam. With a beam power of 50 kW and normal longitudinal emittance at the injection, the proton beam with a bunch length of about 53% with respect to the one in the normal operation mode was obtained and transported to the spallation target. With a reduced longitudinal emittance at injection and the beam power of 30 kW, the shortest extraction bunch length obtained is about 26% of the one in the normal operation mode. Different machine settings have also been tested to show the impact of the desynchronization between the rf and magnetic fields, the influence of the nonadiabatic rise time, and the adiabatic decay time of the rf voltage on the extraction bunch length. The experimental results are well consistent with the theoretical and simulated ones. It is interesting to observe that space charge has a beneficial effect on the bunch lengthening which will result in a shorter bunch at the extraction with the later bunch rotation. The controlled desynchronization method between the rf and magnetic fields in an RCS was also proven successful.

Figure 1

The rf voltage and frequency patterns: (a) the normal operation mode; (b) the short bunch extraction mode. The rf voltage and frequency patterns before extraction are modified to accomplish the bunch compression and the desynchronization between the rf and magnetic fields (see Sec. 4).

Figure 2

(a) The beam signal during the whole acceleration cycle measured by the WCM. (b) The rms bunch length from 15 to 19.8 ms or the extraction moment with the corresponding bunch signal fit by a Gaussian function.

Figure 3

(a) Charge distribution of the two bunches in the CSNS/RCS measured by the WCM from 15 to 19.8 ms. (b) Oscillation of the bunch center measured by the FCT in the same period.

Figure 4

Evolution of the rms bunch length from 10 ms to the extraction from the measurement (green), the theoretical calculation (blue), and the ORBIT simulation (red). The theoretical bunch length is calculated based on Eq. (1) with the rms bunch area of 0.36 eVs.

Figure 5

Beam loss distribution across the whole RCS measured by beam loss monitors (BLM) in the target-shooting mode. The green color indicates that the beam loss is totally negligible, and the orange color gives a warning of beam loss level but still below the threshold to trigger the machine protection system.

Figure 6

Evolution of the charge distribution of the two bunches in the RCS from 19.3 ms to the extraction (a) and the evolution of the rms bunch length from 15 ms to the extraction (b) in the target-shooting mode.

Figure 7

Proton bunch length by measuring the $\gamma$-flash response waveform in the Back-n white neutron beam line: the normal operation mode (red); the target-shooting mode with the bunch compression (blue). It shows that the proton bunch length in this target-shooting mode is approximately half of that in the normal mode.

Figure 8

(a) The changing trend of the extraction bunch length obtained by experiment, simulation, and theory under different chopping factors. (b) The dramatic effect of the quadrupole oscillation caused by the strong space charge under the chopping factor of 75% on the process of the whole bunch compression.

Figure 9

Measured and simulated evolution of bunch length during the bunch stretching and bunch rotation for three rf voltage rise times of 50, 75, and $100\mu \mathrm{s}$.

Figure 10

(a) The rf voltage curves for different decay times: 0.5, 0.6, and 0.7 ms. (b) The evolutions of the bunch length during the bunch stretching process from both the experiments and simulations.

Figure 11

Simulation results about the effect of space charge on the bunch length during the bunch stretching and bunch rotation processes. Simulation conditions: beam power of 30 kW and chopping factor of 70%.

Figure 12

(a) The rf frequency pattern for the normal mode (blue) and the desynchronization mode obtained from the simulation (red). (b) The simulated bunch phase during the CSNS/RCS acceleration cycle for the normal mode (blue) and the desynchronization mode (red).

Figure 13

Experimental verification of the controlled desynchronization from (a) the analysis of the amplitude of the WCM signal; (b) the FCT phase; (c) the BPM beam centroid. “synchronization” denotes that only the rf voltage pattern is applied to extract short bunches, without modification to the rf frequency and phase. Experimental conditions: beam power of 30 kW and chopping factor of 60% for (a) and (b); beam power of 50 kW and chopping factor of 41% for (c).

Figure 14

Comparison between the desynchronization and synchronization on the measured bunch length under the equivalent beam power of 30, 50, and 100 kW. The nonadiabatic rf voltage rise time is $100\mu \mathrm{s}$ for all cases. The extraction moment is 20.2 ms except 19.8 ms for the case of 50 kW with desynchronization.