T. Nara

Performance and operation of a beam chopping system for a cyclotron with multiturn extraction

A beam chopping system for a cyclotron is in operation at the JAERI cyclotron facility. A combination of a pulse voltage chopper in the injection line and a sinusoidal voltage chopper after the exit of the cyclotron is adopted to produce beam pulses spaced at 1 μs–1 ms intervals from natural cyclotron beams. The chopping system was designed according to a simple formulation of the chopping process in which the multiturn extraction was taken into account. Performance of the chopping system was experimentally proved to satisfy requirements of the design. In actual operation to produce beam pulses at long intervals, however, the number of the multiturn extraction is usually larger than the assumed value in the design because of a large phase acceptance of the cyclotron. Careful tuning of the acceleration phase width or the base magnetic field of the cyclotron is necessary to reduce it. It is essential to strictly define the acceleration phase of injected beams in the central region of the cyclotron to improv...

Flat-top acceleration system for the variable-energy multiparticle AVF cyclotron

A flat-top acceleration system appropriate to minimization of energy spread in an ion beam was investigated for the JAERI AVF cyclotron. A combination of the fundamental- and the fifth-harmonic voltages to obtain a homogeneous energy gain distribution of accelerated particles is ideally suited to a variable-energy multiparticle cyclotron using acceleration harmonics of 1, 2, and 3. The flat topping of the energy gain distribution using the fifth harmonics has the advantages of minimizing amplifier power, reducing power dissipation in a resonator, and increasing the energy gain per turn. The flat-top acceleration system was designed to reduce the energy spread to 0.02%, which fulfills a beam focusing condition for production of a microbeam with a beam spot diameter of 1 μm. Tolerable fluctuations of acceleration voltages, required to achieve the energy spread of 0.02%, were 2.0×10−4 for the fundamental voltage and 1.0×10−3 for the fifth-harmonic voltage. Both fundamental- and fifth-harmonic phases were req...

Magnetic field stabilization by temperature control of an azimuthally varying field cyclotron magnet

A magnetic field drift, gradual decrease of the order of 10−4 in several tens of hours, was observed with the beam intensity decrease in an operation of an azimuthally varying field (AVF) cyclotron. From our experimental results, we show that the temperature increase of the magnet iron by the heat transfer from the excitation coils can induce such change of the magnetic field as to deteriorate the beam quality. The temperature control of the magnet iron was realized by thermal isolation between the main coil and the yoke and by precise control of the cooling water temperature of the trim coils attached to the pole surfaces in order to prevent temperature change of the magnet iron. The magnetic field stability of ±5×10−6 and the beam intensity stability of ±2% have been achieved by this temperature control.

An energy spread minimization system for microbeam generation in the JAERI AVF cyclotron

A heavy-ion microbeam with energy of hundreds of MeV is a significantly useful probe for research in biology and biotechnology. A single-ion hit technique using the heavy-ion microbeam is being developed at the JAERI AVF cyclotron facility for elucidation of biofunctions. For production of a microbeam with a spot size of one micro-meter in diameter, the energy spread in the beam is required to be reduced to 0.02% to minimize the effect of chromatic aberrations in the focusing lenses. The energy spread in the cyclotron beam depends on a waveform of the acceleration voltage and beam phase acceptance of the cyclotron. The typical energy spread of the cyclotron beam is around 0.1% in the ordinary acceleration mode using a sinusoidal voltage waveform. The energy spread can be reduced by superimposing a fifth-harmonic voltage waveform on the fundamental one to generate a flat-top waveform for uniform energy gain. The flat-top acceleration system has been designed for the variable-energy multi-particle AVF cyclotron with acceleration harmonic mode of 1, 2 and 3. An additional coaxial cavity has been installed to generate the fifth-harmonic voltage, coupled to the main resonator. The frequency range of the fifth harmonics, 55–110 MHz, was fully covered by the flat-top acceleration system.

TEMPERATURE CONTROL OF A CYCLOTRON MAGNET FOR STABILIZATION OF THE JAERI AVF CYCLOTRON BEAM

Frequent corrections of the magnetic field of the JAERI AVF cyclotron were required for keeping a beam current constant during long time operation. We observed correlation between the magnetic field and the temperature of the cyclotron magnet yoke by measuring the magnetic field with an NMR probe and the temperature with platinum resistance thermometers. As a result, this instability of a cyclotron beam was induced by temperature-change of the magnet yoke caused mainly by thermal conduction from the main coil. To restrain the thermal conduction to the yoke, we have inserted temperature-controlled copper plates between the yoke and the main coil. In addition, a temperature control system for the cooling water of the trim coils has been installed, which is independent of the total cooling system for controlling the pole tip temperature. An optimum condition of the temperature control systems for stabilizing the magnetic field has been investigated.

DESIGN OF THE FLAT-TOP ACCELERATION SYSTEM FOR THE JAERI AVF CYCLOTRON

A flat-top acceleration system for the JAERI AVF cyclotron has been designed. The fifth harmonic of the fundamental frequency is used to obtain uniform energy gain. To determine optimum parameters of the flat-top system, a cold model test was carried out and flat-top waveforms of the voltages were observed successfully in the whole range of the fundamental frequency. An rf power required for generating a flat-top dee voltage of 30 kV was estimated to be about 1 kW. The design of the flat-top cavity is being modified using the MAFIA code.

Metallic ion generation from ceramic rods by ECR ion source and thermodynamic feature of vaporization

Abstract Metallic ion generation from eight kinds of ceramic rods such as Al 2 O 3 , Mo 2 C, BN was examined by an ECR ion source with a direct insertion method, and stable ion beams were observed from six. Vapor pressure of possible products from a solid phase or a solid-liquid condensed phase of the examined materials was evaluated by thermodynamic calculation and ranged over more than nine orders of magnitude at high temperatures. A clear correlation was found between the beam stability of metallic ions and the surface condition of the rod ends exposed to the plasma, and it is completely explained by the magnitudes of the calculated vapor pressure. This indicates that the vaporization process can be understood as sublimation. Sputtering of the ceramic rods by ECR-plasma ions is considered not to be able to provide sufficient vapor pressure of metallic elements because of the low ion energy close to the sputtering threshold energy.

A liquid-helium-free superconducting coil system forming a flat minimum-magnetic-field distribution of an electron cyclotron resonance ion sourcea)

A flat distribution of the minimum magnetic field (flat-Bmin) of an electron cyclotron resonance ion source (ECRIS) is expected to perform better in highly charged ion production than classical Bmin. To form a flat-Bmin structure with a liquid helium-free superconducting device, a coil system of seven coils with four current leads has been designed. The lead number was reduced by connecting the plural coils in series to maintain the flat-Bmin structure even when the coil currents are changed for adjustment. This coil system can be operated with a helium-free cryostat, since the estimation of heat from the leads to the coils is nearly equivalent to the existing superconducting ECRIS of a similar type.

Magnetic field modification of 18 GHz electron cyclotron resonance ion source at the Japan Atomic Energy Research Institute

An 18 GHz electron cyclotron resonance ion source under development since. 1994 has a solenoid coil between a pair of mirror coils to vary the mirror ratio over a wide range. The solenoid coil of the original design formed a small bump in the bottom of the mirror field profile when the source parameters were optimized for high charge states. Source performance in generating highly charged ions improved with a number of minor changes on the vacuum, the gas feed, and the beam extraction system. The maximum beam current of Ar16+ was 2 enA, which was much lower than that expected from the microwave frequency and the magnetic field strength. Little effect of a biased disk on intensities of highly charged ions and dependence of the charge state distribution on the solenoid coil current suggested that the bump divided plasma into two regions and limited the source performance. The bump was removed by halving the solenoid coil length, and the beam current of Ar16+ increased to 1.3 eμA as a result.

Construction and development of the JAERI 18 GHz electron cyclotron resonance ion source

An 18 GHz electron cyclotron resonance ion source for multiply charged ions was constructed and is now in operation. A new distribution of the mirror field was adopted, and minimum strength is varied by a solenoid coil installed between the mirror coils. The measured mirror field distribution is close to the designed distribution and its maximum strength exceeds 1.4 T. The source is now being tuned by use of Ar ion generation. The relatively high base pressure in the plasma chamber has been improved by installing an additional vacuum pump. The source performance has been maturing gradually with the vacuum and Ar ions with charge states of up to 13+ have been observed so far.An 18 GHz electron cyclotron resonance ion source for multiply charged ions was constructed and is now in operation. A new distribution of the mirror field was adopted, and minimum strength is varied by a solenoid coil installed between the mirror coils. The measured mirror field distribution is close to the designed distribution and its maximum strength exceeds 1.4 T. The source is now being tuned by use of Ar ion generation. The relatively high base pressure in the plasma chamber has been improved by installing an additional vacuum pump. The source performance has been maturing gradually with the vacuum and Ar ions with charge states of up to 13+ have been observed so far.