Solution to Beam Transmission Decline in the CSNS Linac Operation Using Measurements and Simulations

The China Spallation Neutron Source (CSNS) started operation from 2018 and now run at its design power. However, a problem was observed that the beam transmission of the linac was decreasing and the beam loss was increasing during the operation. With simulations and measurements, we found that a long longitudinal tail existed in the beam bunch output from the RFQ. And this tail caused the longitudinal mismatch in the following linac. After inhibition of the longitudinal tail in the beam bunch, the beam transmission in operation can keep stable. INTRODUCTION The layout of the CSNS linac is shown in Fig. 1. It consists of an Hion source, a 3 MeV RFQ, an 80 MeV DTL and several beam lines [1]. Table 1 shows the main parameters of the CSNS linac. The commissioning of the linac started from 2015. In January 2018, the last DTL tank has been commissioned and the Hbeam has been accelerated to the design energy of 80 MeV for the first time. The commissioning was performed with the peak current of 10 mA, the pulse width of 100 μs, and the repetition rate of 1 Hz. Figure 2 shows an overlay of Current Transform signals along the linac. After performing orbit correction, transverse matching, and model optimization, the beam transmission of the RFQ can be about 97% and that of the DTL can be about 100% (with 1% uncertainty). Figure 1: CSNS linac layout. Table 1: Main parameters of the CSNS linac Ion Source RFQ DTL Input Energy (MeV) 0.05 3.0 Output Energy (MeV) 0.05 3.0 80 Pulse Current (mA) 20 15 15 RF frequency (MHz) 324 324 Chop rate (%) 50 50 Duty factor (%) 1.3 1.05 1.05 Repetition rate (Hz) 25 25 25 Figure 2: Current Transform signals along the linac. BEAM TRANSMISSION DECLINE The CSNS facility started operation in September 2018. Now it runs at its design power 100 kW. However, a problem was observed in the operation. The beam transmission of the DTL might drop about 2~4% in the operation. Firstly, we thought the decline may due to the instability of magnet current or RF filed. After monitoring these parameters for a long time, they were found to be stable, but the transmission decline was still observed. Finally, we found out that the reason to this problem is beam instability from the ion source. The RFQ transmission was affected by the beam instability. And the beam parameters output from the RFQ were changed. As a result, the beam was mismatched while transporting in the DTL and then lost in the DTL. In our experiments, we found the DTL transmission decline was synchronous with the RFQ transmission decline, like showing in Table 2. Table 2: Measured Beam Transmission of the Linac Transmission (%) RFQ 83.4 94.04 96.85 DTL 95.88 96.36 98.00 BEAM MISMATCH The beam mismatch contains two aspects: transverse mismatch and longitudinal mismatch. We will analyse them by using measurements and simulations. Transverse Mismatch As shown in Fig. 3, the MEBT is used to match beam output from the RFQ to the DTL. The MEBT includes ten quadrupole magnets (Q1~Q10) for transverse matching, two 324 MHz buncher cavities for longitudinal matching, and various beam diagnostic instrumentation for beam diagnosis [2]. To do matching, it is essential to get the initial beam Twiss parameters output from the RFQ. Two sets of diagnostics are adopted to measure beam Twiss. Firstly, four wire scanners are place in the MEBT to measure beam profile. The beam sizes are calculated from profile data obtained at the measurement stations. Calculating the beam ___________________________________________ * Work supported by National Natural Science Foundation of China (11505201) † pengjun@ihep.ac.cn 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-THPAB185 THPAB185 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I 4134 MC4: Hadron Accelerators A08 Linear Accelerators Twiss parameters is done using beam sizes and an envelope model. The beam Twiss parameters are found numerically by minimizing the RMS error between the measurements and the model predictions, as shown in Fig. 4. Table 3 shows the comparison of the design values (with PARMTEQM) and the measured values [3]. The Twiss parameters in the horizontal plane are agreed well with the simulated values, while those in the vertical plane are obviously deviated from the simulated values. Figure 3: Layout of the CSNS MEBT. Figure 4: Beam RMS size along the MEBT (Lines represent model predictions, and dots represent measurements with wire scanners). Table 3: Twiss Parameters at the MEBT Entrance α β (mm/ π mrad) ε Norm.rms (π mm mrad)