Accelerated Higher Intensity Heavy Ion Beam with New Linac System

　Research Laboratory for Nuclear Reactors in Tokyo Institute of Technology had developed a multi-beam type RFQ (Radio Frequency Quadrupole) linac system accelerating several heavy ion beams in parallel in one cavity. This system has made it possible for generating high intensity heavy ion beams, never before possible such as 100 mA and more, without growing the machine scale.

Acceleration of high intensity heavy ions in low energy region

　One of important subjects in accelerator physics is to generate higher intensity (higher current) beam using accelerators. In accelerating heavy ions especially, the beam is severely affected by space charge effect, refers to Coulomb repulsive force between each ion in a beam, because the traveling velocity is very slower than electrons and protons. A biggest problem to accelerate high intensity heavy ion beams exceeding 10 mA in low energy region, refers to from several kilo to several mega electron volts per nucleon, is therefore how to avoid the beam loss by the strong space charge effect. The heavy ion beam current accelerated by one linac is limited to several dozen milliamperes by the space charge effect. However, generating higher intensity heavy ions has gotten deeper problem because of practical realizations of heavy particle radiotherapy and so on.

Multi-beam type IH-RFQ linac

　In order to suppress the space charge effect, an idea has been proposed to divide a single high intensity beam into several beams, and accelerates these beams in parallel using several sets of a linac system. However, the number of the acceleration cavity and the peripheral devices grows depending on the number of the accelerating beam, and the scale of the system and costs for construction and operation of these accelerators also grow in this conventional method.
　We have studied and developed a multi-beam type RFQ linac that accelerates several beams in one cavity simultaneously because of the background. For instance, figure 1 shows a conceptual diagram of a four-beam type RFQ linac system accelerating four beams in one cavity. This system has as much performance as four sets of the conventional linac system <figure 2> in terms of generating beam current, and the system would achieve downsizing and electric power saving in the multi-beam acceleration system.

<figure 1> Four-beam type RFQ linac system

<figure 2> Single beam type RFQ linac system

　As a result of our previous study, we have reached a conclusion that a cavity structure called IH (Interdigital H) is most suitable for the multi-beam type RFQ acceleration cavity because of the power efficiency in low energy particle accelerations. GSI in Germany has proposed and studied the IH type cavity, however acceleration tests and detail studies for the multi-beam type cavity had not been conducted.
　We tried to develop a two-beam type as a prototype of the multi-beam type IH-RFQ linac. This accelerator has very complex internal structure, so the linac had been designed and built on close industry-university cooperation with a precision machining maker<figure 3>. A laser ion source with DPIS (Direct Plasma Injection Scheme) was adopted for an injection system of the two-beam type IH-RFQ linac. The ion source has a simple structure and a good performance to generate high intensity heavy ion. We had also newly developed a two-beam type laser ion source <figure 4>.

<figure 3> Photo of two-beam type IH-RFQ linac. Components of the electrodes have been made of copper having high electric conductivity and heat-transfer coefficient. The other components have been made of stainless steel with copper plating of about 50 micrometers thick. Two sets of the quadrupole electorode have been installed in the cavity.

<figure 4> Test system for laser ion source. A blue and long device on the back left of the photo is a Nd: YAG laser to generate carbon ions. Black devices in front of the laser are optical system consisted of mirrors and lenses, and the system focuses and guides the laser light to a ion source chamber on the top center of the photo. The ion source is applied dozens kilovolts to inject generated ions to the accelerator, so it is necessary to isolate between the ion source and the other devices using insulators. Devices on the right foreground of the photo are analyzers to measure generated ions.

Beam acceleration test

　The two-beam type IH-RFQ linac system consists of the laser ion source, a high voltage terminal for the ion source, the acceleration cavity, a radio frequency power source and an evacuation system mainly. The linac system accelerated carbon ion beams with 108 mA (54 mA per beam) in the beam current, and we succeeded in the beam acceleration by the two-beam type IH-RFQ linac for the first time in the world <figure 6>. This is also a world record as heavy ion intensity accelerated one RFQ linac.

<figure 5> Acceleration test system. The ion source on the right foreground of the photo is connected to the two-beam type IH-RFQ linac. A copper-colored pipe on the left side of the linac is a coaxial wave-guide to supply the electric power into the cavity.

<figure 6> Current waveform of accelerated carbon ion beam using the two-beam type IH-RFQ linac. Beam currents from an upper and an under beam channel in the cavity are plotted with red and blue color respectively. A signal in the zero point of the time axis has roots in an oscillation of the laser, and the carbon ions are generated in this time. A signal after about the 4.5 microseconds is accelerated beam current, and the RFQ linac accelerates carbon beams with the current of about 54 milliamperes per beam channel.

Toward higher intensity and performance

　There had been no quantitative evaluation of the multi-beam acceleration for heavy ions, because the engineering development is so difficult. This research is therefore a first case evaluating the superiority of the system through experiments. The result of this research is a technical breakthrough to generate higher heavy ion beams, and some applications to high-energy physics, heavy ion cancer therapy and heavy ion inertial confinement fusion are expected. This research is awarded a Seiichi Tejima prize in the 22th year of the Heisei era.

〜 Author : Accelerator Division III, Takuya Ishibashi 〜

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