A scheme to extract the beam in advance and transverse beam distribution measurement for the CSNS RCS

The China Spallation Neutron Source (CSNS) is a large scientific facility for frontier research by using the medium energy protons bombardment of tungsten target to produce a large number of scattered neutrons. The rapid cycling synchrotron (RCS) adopts the single-turn fast extraction scheme to extract the proton beam with the energy of 1.6 GeV, and the extraction repetition frequency is 25 Hz. In order to optimize the painting distribution and reduce the beam loss in the beam commissioning, it is necessary to measure the transverse beam distribution during the injection and acceleration processes. In this paper, a scheme to extract the beam in advance is proposed which is performed by adjusting the extraction timing and extraction mode. By using this extraction mode, the beam can be extracted at different time and the transverse beam distribution can be measured by a multi-wire scanner located on the beam transport line from the RCS to the target. Then, the beam distribution at the desired position on the RCS can be deduced by using the beam transfer matrix.


Introduction
The China Spallation Neutron Source (CSNS) accelerator [1,2] consists of an 80 MeV negative hydrogen Linac and a 1.6 GeV rapid cycling synchrotron (RCS) with a repetition rate of 25 Hz.It accumulates an 80 MeV injection beam, accelerates the beam to the design energy of 1.6 GeV and extracts the high energy proton beam to the tungsten target [3,4].Figure 1 shows the layout of the CSNS.
The extraction system is the core component of the RCS which consists of eight kickers and a lambertson magnet.The kickers are designed for vertical deflection and the lambertson magnet for horizontal deflection.Figure 2 shows the layout of the RCS extraction system.
In the normal operation of the CSNS accelerator, the repetition frequency of the beam extraction is 25 Hz.The extraction time point is about 20 ms after the beam injection [5].Since there is no detector to measure the transverse beam distribution in the CSNS RCS, it is hoped to extract the beam in advance at different time and then measure the transverse beam distribution by a multi-wire scanner located on the beam transport line from the RCS to the  target (RTBT).Therefore, a scheme to extract the beam in advance is proposed which has been performed by adjusting the extraction timing and extraction mode.By using this extraction mode, the beam can be extracted at different time, and then the transverse beam distribution can be measured.

A scheme to extract the beam in advance
For the design scheme of the RCS beam extraction, the beam can only be extracted after the energy reaches 1.6 GeV at 20 ms.If it is hoped to extract the beam at any time during the 20 ms, a special timing structure and a special extraction mode are needed.The commissioning of extracting the beam in advance consists of multiple steps.Firstly, the timing of the kickers should be modified from the original 20 ms to the desired moment T .Secondly, the mode of the extraction and RTBT magnets should be modified to match the energy corresponding to the desired moment T .Then, the timing of the beam diagnostic elements (multi-wire scanners, current transformers, beam position monitors, beam loss monitors) on the RTBT should be modified accordingly to match the extraction time T .Finally, the new extraction mode should be adjusted to ensure the quality of the extraction beam.Figure 3 shows the timing layout to extract the beam in advance.
In the beam commissioning of CSNS accelerator [6], the scheme to extract the RCS beam in advance has been optimized carefully.The commissioning results show that the method is feasible and can measure the transverse beam distribution during the painting and acceleration processes.Figure 4 shows the RCS DCCT diagram of the beam extraction immediately after injection.Figure 5 shows the beam distribution measured by a multi-wire scanner located on the RTBT when the beam extraction immediately after injection.As can be seen from the figures,  the mode to extract the beam in advance works well and meets the design requirements.Figure 6 shows the repeatability of the beam distribution obtained by the beam extraction in advance.The results show that the reproducibility of the scheme to extract the beam in advance is very good.

Method for measuring transverse beam distribution
With the mode to extract the beam in advance, the transverse beam distribution during the painting and acceleration processes can be measured by using a multi-wire scanner located on the RTBT.In the beam commissioning, the transverse beam distribution for different painting ranges, different beam powers, single and double bunch modes, and different time can be measured.Then, the painting range, working mode and beam loss can be optimized.
In the beam commissioning, in order to study the variation of beam size with the painting  range, the mode to extract the beam in advance can be used to measure the transverse distribution for different painting ranges.Figure 7 shows the effect of horizontal painting range on the transverse beam sizes and figure 8 shows the effect of vertical painting range on the transverse beam sizes.It can be found that the variation of horizontal painting range not only causes the variation of horizontal beam distribution, but also causes the variation of vertical beam distribution.This phenomenon shows that there is a serious transverse coupling.
In order to study the growth trend of transverse beam size with the increase of beam power, we compare the transverse beam sizes under different beam powers.Figure 9 shows the comparison of transverse beam sizes for different beam powers on the target.As can be seen from the figure, when the beam power increases from 100 kW to 130 kW, the transverse beam sizes increase by about 15%, which is consistent with the simulation results.
In the beam commissioning, it can be found that the machine states (RCS beam loss, coherent oscillation and so on) of single-bunch and double-bunches are very different.Compared to the  single-bunch mode, the RCS beam loss and coherent oscillation for the double-bunches mode are much larger.Therefore, the transverse beam distributions for the single-bunch and doublebunches modes need to be compared, as shown in figure 10.It can be seen from the figure that there is little difference between the two states.As a result, the difference between the machine  states of single-bunch and double-bunches is not caused by the beam distribution.

Conclusion
For the CSNS RCS, in order to extract the beam at different time, a new scheme to extract the beam in advance has been performed by using a special timing structure and extraction mode.
The feasibility and repeatability of the beam extraction mode have been verified in the beam commissioning.With this extraction mode, the beam distribution for different painting ranges, different beam powers, single and double bunch modes, and different time can be measured by a multi-wire scanner located on the RTBT.Then, the painting range, working mode and beam loss can be optimized.

Figure 2 .
Figure 2. Layout of the RCS extraction system.

Figure 3 .
Figure 3.The timing layout to extract the beam in advance.

Figure 4 .
Figure 4. RCS DCCT diagram of the beam extraction immediately after injection.

Figure 5 .
Figure 5.The beam distribution measured by a multi-wire scanner located on the RTBT when the beam extraction immediately after injection.

Figure 6 .
Figure 6.Repeatability of the beam distribution obtained by the beam extraction in advance.

14thFigure 7 .
Figure 7. Effect of horizontal painting range on the transverse beam sizes when the vertical painting range is fixed.

Figure 8 .
Figure 8.Effect of vertical painting range on the transverse beam sizes when the horizontal painting range is fixed.

Figure 9 .
Figure 9.Comparison of transverse beam sizes for different beam powers on the target.

Figure 10 .
Figure 10.Comparison of transverse beam distributions for the machine states of single-bunch and double-bunches.