Ion Beam Handling by an Einzel Lens Chopper for the KEK Digital Accelerator

The KEK-Digital Accelerator (DA) has been constructed, and it is in commission now. This accelerator is a rapid cycle synchrotron. Its acceleration principle is based on the induction synchrotron (IS) concept which was demonstrated in 2006 using the existing KEK 12 GeV proton synchrotron. Even before this demonstration, the small IS not employing any large scale injector had been proposed as All Ion Accelerator, which has the capability to accelerate all kinds of ions with their possible charge-state. Similarly, there is no large-scale injector in the KEK-DA. For KEK- DA, an ion beam is extracted from an ion source, chopped, post-accelerated and immediately guided to be injected to the DA ring. Presently, a permanent magnet type of Electron Resonance Ion Source (ECRIS) is utilized as the ion source for the KEK-DA. In order to mitigate the space charge effect and closed orbit distortion caused by a remanent field in the ring, the ion beam must be accelerated after extraction. Therefore, the KEK-DA ECRIS is embedded inside a 200 kV High Voltage Platform (HVP).In order to accelerate an ion beam in a synchrotron, a beam pulse length must be less than revolution time period in the ring. Since the typical revolution time is ~10 µs in our case, a ms ion beam which is produced from an ECRIS must be chopped before injection into the ring. For such purpose, a novel chopper device has been developed after our examination and consideration of various chopping systems. The novel chopper, so-called Einzel lens chopper, has been developed and demonstrated. Its principle is rather simple: by modulating a voltage applied to the middle electrode of the Einzel lens, it is worked as a longitudinal gating device as well as the focusing device. This performance can be realized by introducing a Marx generator to provide a pulse voltage in a desired duration time. The Marx generator used in the present studies has the fast response of rising and falling times with the solid-state switching device.For this implementation, the functionality and performance of the Marx generator have been confirmed by measurement and simulation. At first, in order to obtain a voltage for beam blocking and another voltage for beam optics matching of the Einzle lens, a beam blocking experiment was carried out by using the helium and nitrogen ion beams. In addition, a simulation was also performed by using the IGUN code. The chopping performances with different chopping timings and time durations have been investigated by observing time profiles of the chopped beam at a Faraday cup. The results were further confirmed and explained by using the circuit model of the Faraday cup. Reconstruction of a beam profile from the observed signal, which is modified by the Faraday cup response, is important to investigate a chopper performance. By solving the inverse problem, the beam profile was reconstructed successfullyA chopped beam is transported through a transport line and injected into the ring. In order to avoid a beam loss during transportation, it is important to know a beam emittance. Therefore, a beam emittance measurement was carried out by using a pepper pot emittance monitor. Through this experiment, the measurement and analysis procedures have been established. The optimization of the LEBT parameter was performed to reduce the beam losses.For further investigation of the transient beam behavior from the Einzel lens to the DA ring, a simple code was developed. By using the simulation result, we can identify the intrinsic characteristic of the Einzel lens chopper. Such an intrinsic nature is that the bunch head is retarded and bunch tail is moving forward. This source is originated from the transient time region of the Einzel lens chopper voltage, which modulates a momentum at the bunch head and tail. This phenomenon, which is called as “drift compression”, has been confirmed by comparing the bunch profile at the entrance and exit of transportation line. For the beam motion in the ring, beam diffusion or spread is also observed in the experimental and simulation results. For a long distance beam motion in the ring, the space charge effect is clearly seen: the particles are diffused from the bunch in time. The experimental results for the longitudinal motion can be quantitatively reproduced by using our simulation code.Presently, the Einzel lens chopper has been operating successfully with its stable and reliable performances and without causing any trouble during the KEK-DA beam commissioning

Compact hadron driver for cancer therapies using continuous energy sweep scanning

A design of a compact hadron driver for future cancer therapies based on the induction synchrotron concept is presented. To realize a slow extraction technique in a fast-cycling synchrotron, which allows energy sweep beam scanning, a zero momentum-dispersion D(s) region and a high flat D(s) region are necessary. The proposed design meets both requirements. The lattice has two-fold symmetry with a circumference of 52.8 m, a 2-m dispersion-free straight section, and a 3-m-long large flat dispersion straight section. Assuming a 1.5-T bending magnet, the ring can deliver heavy ions (200  MeV/u) at 10 Hz. A beam fraction is dropped from the barrier bucket at the desired timing, and the increasing negative momentum deviation of this beam fraction becomes large enough for the fraction to fall in the electrostatic septum extraction gap, which is placed at the large D(s) region. The programmed energy sweep extraction enables scanning beam irradiation on a cancer site in depth without an energy degrader, avoiding the production of secondary particles and the degradation of emittance. Details of the lattice parameters and computer simulations for slow extraction are discussed. An example extraction scenario is presented. Qualities of the spilled beam such as emittance and momentum spread are discussed, as well as necessary functions and parameters required for the extraction system

Induction acceleration of heavy ions in the KEK digital accelerator: Demonstration of a fast-cycling induction synchrotron

A fast-cycling induction synchrotron was demonstrated. Ions with extremely low energies and mass-to-charge ratios (A/Q) in the range from 2 to 10 were injected, captured by barrier voltages, and accelerated to the end of the acceleration cycle of 50 ms by flat pulse voltages generated by pulse transformers referred to as induction cells. Induction acceleration in a wide dynamic frequency range of 56 kHz to 1 MHz was also demonstrated. This accelerator is expected as the next generation of a heavy ion driver for cancer therapy, where a large scale injector is not required. A wide variety of ions for ion energy implantation experiments needing novel materials will be delivered from this compact circular accelerator