Hirotsugu Kashiwagi

Nd-YAG laser ion source for direct injection scheme

The feasibility of a “direct injection scheme” using Nd–yttritium–aluminum–garnet laser has been studied experimentally. The relationship between charge distribution and laser power density was measured. The obtained yield rate of C6+ was 46.8% at the peak current pulse with 1.6×1011 W/cm2 laser intensity. The current dependence of the distance from the target at this intensity was measured. The peak current of C6+ will be 60 mA with φ4.8 mm extraction aperture. It was found from the input beam condition simulation by the code PTEQ–HI that high current (from 12 to 28.6 mA) can be accelerated by the TIT–RFQ.

Scheme for direct plasma injection into an RFQ linac

A new efficient injection method from a laser ion source to a Radio Frequency Quadropole (RFQ) was proposed and is being tested in RIKEN, Japan. A laser plasma is induced just before the entrance of the RFQ and is injected directly into the RFQ channel. Using an existing RFQ, first verification tests have been completed successfully. Finally, the preliminary specifications for the first RFQ dedicated to the new injection scheme are presented.

Acceleration of high current fully stripped carbon ion beam by direct injection scheme

Acceleration of a 17mA, 100keV∕u C6+ ion beam has been successfully achieved with an radio frequency quadrupole (RFQ) linac by means of “direct injection scheme.” The C6+ beam produced by a laser ion source with a Nd:YAG laser was injected to the high current RFQ linac. It has been experimentally proved that the fully stripped carbon ion beam with a current more than 10mA was accelerated by the RFQ linac.

High current carbon beam production with direct plasma injection scheme

We have been studying a new heavy-ion production technique called “direct plasma injection scheme,” DPIS, since 2000. A new radio frequency quadrupole (RFQ) designed especially for the DPIS was commissioned in 2004 and very intense carbon beam was successfully obtained, reaching more than 60mA accelerated current from the RFQ. Most of the contents of the accelerated beam was carbon 4+ as verified by beam analysis.

Useful technique for analysis and control of the acceleration beam phase in the azimuthally varying field cyclotron

We have developed a new technique for analysis and control of the acceleration beam phase in the cyclotron. In this technique, the beam current pattern at a fixed radius r is measured by slightly scanning the acceleration frequency in the cyclotron. The acceleration beam phase is obtained by analyzing symmetry of the current pattern. Simple procedure to control the acceleration beam phase by changing coil currents of a few trim coils was established. The beam phase width is also obtained by analyzing gradient of the decreasing part of the current pattern. We verified reliability of this technique with 260 MeV (20)Ne(7+) beams which were accelerated on different tuning condition of the cyclotron. When the acceleration beam phase was around 0 degrees, top of the energy gain of cosine wave, and the beam phase width was about 6 degrees in full width at half maximum, a clear turn pattern of the beam was observed with a differential beam probe in the extraction region. Beam phase widths of ion beams at acceleration harmonics of h=1 and h=2 were estimated without beam cutting by phase-defining slits. We also calculated the beam phase widths roughly from the beam current ratio between the injected beam and the accelerated beam in the cyclotron without operating the beam buncher. Both beam phase widths were almost the same for h=1, while phase compressions by a factor of about 3 were confirmed for h=2.

Enhancement of beam pulse controllability for a single-pulse formation system of a cyclotron

The single-pulse formation technique using a beam chopping system consisting of two types of high-voltage beam kickers was improved to enhance the quality and intensity of the single-pulse beam with a pulse interval over 1 μs at the Japan Atomic Energy Agency cyclotron facility. A contamination rate of neighboring beam bunches in the single-pulse beam was reduced to less than 0.1%. Long-term purification of the single pulse beam was guaranteed by the well-controlled magnetic field stabilization system for the cyclotron magnet. Reduction of the multi-turn extraction number for suppressing the neighboring beam bunch contamination was achieved by restriction of a beam phase width and precise optimization of a particle acceleration phase. In addition, the single-pulse beam intensity was increased by a factor of two or more by a combination of two types of beam bunchers using sinusoidal and saw-tooth voltage waveforms. Provision of the high quality intense single-pulse beam contributed to improve the accuracy of experiments for investigation of scintillation light time-profile and for neutron energy measurement by a time-of-flight method.

Drift distance survey in direct plasma injection scheme for high current beam production

In a laser ion source, plasma drift distance is one of the most important design parameters. Ion current density and beam pulse width are defined by plasma drift distance between a laser target and beam extraction position. In direct plasma injection scheme, which uses a laser ion source and a radio frequency quadrupole linac, we can apply relatively higher electric field at beam extraction due to the unique shape of a positively biased electrode. However, when we aim at very high current acceleration such as several tens of milliamperes, we observed mismatched beam extraction conditions. We tested three different ion current at ion extraction region by changing plasma drift distance to study better extraction condition. In this experiment, C(6+) beam was accelerated. We confirmed that matching condition can be improved by controlling plasma drift distance.

Time structure of an accelerated beam using a radio-frequency quadrupole linac with direct plasma injection scheme

In direct plasma injection scheme, the relation between the ion current from a laser ion source and the accelerated beam current by a radio-frequency quadrupole (RFQ) linac was studied by a series of the following analyses. First, ion current at beam extraction was calculated from measured plasma parameters using a scaling law. Second, the beam emittance at the entrance of acceleration electrode region was evaluated by the simulation of the ion extraction out of the ion source plasma using the result of the first calculation. Last, the accelerated beam current could be reproduced by the simulation of beam acceleration using the results of the second. The second calculation revealed the time variation in the injected beam parameter to the RFQ linac. Then it was proved that the time structure of the accelerated beam was different from that of the injection beam because the degree of the matching between the injected beam emittance and the RFQ acceptance varied with time.

Study of a four hole ECR ion source for a HIF injector

Abstract We have designed an ECR ion source which extracts four equal beams from one chamber as a study of an ion source for an accelerator that has several RFQ channels in one accelerating cavity for a heavy ion, HIF accelerator. Solenoid coils for the mirror magnetic field were designed using the magnetic field calculation program OPERA. Extraction electrodes were designed by using particle orbital calculation program FUGUN. Based on the above, we designed and manufactured four hole ECR ion source.

A transverse emittance and acceptance measurement system in a low-energy beam transport linea)

A transverse beam emittance and acceptance measurement system has been developed to visualize the relationship between the injected beam emittance and the acceptance of a cyclotron. The system is composed of a steering magnet, two pairs of slits to limit the horizontal and vertical phase-space, a beam intensity detector just behind the slits for the emittance measurement, and a beam intensity detector in the cyclotron for the acceptance measurement. The emittance is obtained by scanning the slits and measuring the beam intensity distribution. The acceptance is obtained by measuring the distribution of relative beam transmission by injecting small emittance beams at various positions in a transverse phase-space using the slits. In the acceptance measurement, the beam from an ion source is deflected to the defined region by the slits using the steering magnet so that measurable acceptance area covers a region outside the injection beam emittance. Measurement tests were carried out under the condition of accelerating a beam of (16)O(6+) from 50.2 keV to 160 MeV. The emittance of the injected beam and the acceptance for accelerating and transporting the beam to the entrance of the extraction deflector were successfully measured. The relationship between the emittance and acceptance is visualized by displaying the results in the same phase-plane.