• H. Sako, M. Ikegami, A. Miura, G.H. Wei
  JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken

Transverse emittance growth was observed in J-PARC Drift Tube Linac (DTL). In order to suppress the growth, we searched for optimum parameters at MEBT1, by measuring transverse emittance using four wire scanner monitors at the exit of DTL. At 15 mA peak beam current in Dec 2009, horizontal and vertical rms emittance was reduced by 12 % and 10 %, respectively, by setting the amplitudes of the first and second bunchers to 120 % and 90 % with respect to the designed settings. The resulting normalized horizontal and vertical emittance was 0.230 and 0.205 pi mm mrad. At 20 mA in Jan 2010, horizontal and vertical rms emittance was reduced by 17 % and 10 %, respectively, by setting the amplitudes of the first and second bunchers to 110 % and 80 % with respect to the designed settings. The resulting normalized horizontal and vertical emittance was 0.273 and 0.253 pi mm mrad. At 15 mA, we further reduced the horizontal and vertical emittance to 0.171 and 0.200 pi mm mrad by increasing the eighth quadruple magnet field at MEBT1 by 20 % to the designed value. The measured transverse emittance dependence on buncher electric field and quadruple magnetic field will be compared with simulation.

• A. Miura, M. Ikegami
  JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
• H. Sako
  JAEA, Ibaraki-ken
• G.H. Wei
  KEK/JAEA, Ibaraki-Ken

Residual gas in beam transport line essentially affects the beam loss and residual radiation on the accelerator. J-PARC linac is usually operated under 1.0 ·10^-6 to 1.0 ·10^-5 Pa in SDTL and A0BT sections. In this situation, no serious beam loss was observed during the beam operation. In future development of J-PARC linac, because the peak beam energy and output will be increased, it is getting more serious problem. Before the development, it is important to understand a cause of beam loss and relation between beam loss and residual gas pressure. We measured beam loss at the normal and worse vacuum condition in both SDTL and A0BT sections. The result indicates that the beam loss depends on the residual gas pressure and position where the beam loss occurs is about 20 to 30 meter downstream. This suggests the optimum position for installation of vacuum system to minimize the beam loss. In this paper, we describe the experimental result and its discussions. In addition, the cause of the beam loss is considered to be a stripping from negative hydrogen ions to neutral hydrogen atoms. This mechanism is also discussed in this paper.

• G.H. Wei
  KEK/JAEA, Ibaraki-Ken
• M. Ikegami
  KEK, Ibaraki

On the beam line after linac in high power proton accelerators, like J-PARC, debuncher system plays an important role for beam injection to the succeeding ring. The debuncher system usually gives two functions, namely, to correct the center energy jitter and to minimize momentum spread and adjust beam energy at the injection. To mitigate the nonlinear effects of RF field, a debuncher system with two debuncher cavities was designed for the 181-MeV operation of J-PARC linac. In this design, the first debuncher is expected to deal with center energy jitter. Then, the second debuncher is utilized to control the injection momentum spread according to the requirements from the ring. Although the debuncher system was originally designed to minimize the momentum spread, beam-commissioning results show a different requirement for the injection momentum spread to minimize the beam loss in the ring. Based on the original design and the experimental findings with 181-MeV operation, we have designed a debuncher system for the energy upgrade of J-PARC linac to 400 MeV. In this paper, the beam dynamics design of the new debuncher system is presented together with some particle simulation results.