• 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.

• A. Miura, N. Kikuzawa, T. Maruta, K. Yamamoto
  JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
• Z. Igarashi, T. Miyao
  KEK, Ibaraki
• M. Ikegami
  J-PARC, KEK & JAEA, Ibaraki-ken
• H. Sako
  JAEA, Ibaraki-ken
• S. Sato
  JAEA/LINAC, Ibaraki-ken

Since November, 2007, J-PARC Linac has been operated at 7.2kW beam power. During the operation, beam losses possibly caused by the H0 particles generated by the interaction between H^- beam and residual gas in the transport line were observed in the SDTL (Separated-type Drift-Tube Linac) section. In the linac operation, Ar-CO2 gas proportional counters are employed for the measurement of beam loss, but they are also sensitive to background noise of X-ray emitted from RF cavities. In this section, protons, secondary hadrons and gamma rays would be mainly generated as a beam loss, but it is not easy to estimate real beam loss using the proportional counter. The plastic scintillation counters with less X-ray sensitivity and 3He proportional counters with high thermal neutron sensitivity will be also employed to measure the beam loss. The combination of these detectors would bring more accurate beam loss measurements with suppression of X-ray noise. A measurement of emission position and angle distributions of protons due to H^- beam loss is being planed. This result would lead to clarify the source of beam loss. This paper reports status of beam loss evaluation using these detectors.

• T. Kobayashi
  JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
• M. Ikegami
  KEK, Ibaraki

The function of the chopped beam loading compensation was implemented into the digital feedback/feed-forward control system of the J-PARC Linac LLRF system to stabilize the ACS cavity fields for the 400-MeV upgrade. The beam test of the chopped beam loading compensation was performed with the present 324-MHz cavity sysmte. Consequently the chopped beam loading was successfully compensated and that this system is valid.

• Y. Kondo
  JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
• M. Ikegami, F. Naito
  KEK, Ibaraki
• J. Qiang
  LBNL, Berkeley, California

For the beam current upgrade of the J-PARC linac, a new RFQ (RFQ III)is developing. The peak beam current of RFQ III is 50mA. To increase the peak current from the existing RFQ (RFQ I), the longitudinal and/or transverse emittances are expected to be increased. However, the increase of the longitudinal emittance will affect the performance of the RF chopper system. In this paper, detailed simulations of the RF chopper system are described and the requirement for the longitudinal emittance of the RFQ is clarified.

• 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.