Masahiro Tadokoro

Three dimensional optical emission tomography of an inductively coupled plasma

The three dimensional distribution of the net production rate of excited species in an inductively coupled plasma (ICP) reactor in Ar has been measured as a function of pressure and discharge power. The ICP reactor is driven by a single turn radio-frequency current coil at 13.56 MHz. Robot assisted optical emission tomography was used in order to obtain radial profiles of emission at different distances from the coil thus giving effectively a three dimensional distribution of the net excitation rate as well as the number density of excited states. Under some conditions azimuthal symmetry of the discharge is broken mostly at higher pressures and higher powers. We have covered a range of pressures between 15 and 500 mTorr and powers between 50 W and 400 W. The conditions for producing a uniform distribution of excited atoms as a representative of active species, are discussed.

Beam commissioning of the new proton therapy system for University of Tsukuba

Beam commissioning of the new proton therapy system for University of Tsukuba was started in September 2000. The present system employs a synchrotron with a maximum energy of 250 MeV and two rotating gantries. The beam was successfully accelerated up to 250 MeV and transported to the irradiation nozzles. The position of the beam extracted from the synchrotron was confirmed to be very stable and sufficient flatness for the irradiation area was realized by using the dual ring double scattering method developed at University of Tsukuba. Furthermore, synchrotron operation triggered by patient respiration signal was succeeded.

A combined function magnet for a compact synchrotron

At Kyoto University, a compact proton synchrotron with combined function lattice has been proposed for cancer therapy facility, KUMPF (Kyoto University Medical Proton Facility). KUMPF provides proton beams in the energy range of 70-250 MeV with average current of about 10 nA. The synchrotron consists of six identical combined function magnets, each of which deflects the proton beam 60 degrees. The magnet has an FDF structure where focusing (F) and defocusing (D) sectors deflect the beam 15 and 30 degrees, respectively. In order to realize the tune value of (1.75, 1.75), n-values of -5.856 and 6.164 are required for F and D sectors, respectively. In the present paper, the detailed design of the combined function magnet will be presented.

Novel Design for Electromagnet with Wide Excitation Range

A novel design for an electromagnet with a wide excitation range is described. The magnetic field distribution of an electromagnet usually deforms as the excitation approaches the saturation level. This effect can be minimized if the saturation of the ferromagnetic material is uniform; the integrated magnetic field along a flux line \intH ds in the pole is uniform for all flux lines crossing the gap area of interest. This condition can be realized by adding vacant portions into low flux density regions in iron poles, and thus the magnetic flux density in the iron region becomes uniform. This magnetic flux density equalizer can be applied to a variety of magnets: flat dipoles, n-indexed dipoles, quadrupoles and higher-order multipole magnets.

An untuned RF cavity using multifeed coupling

Abstract A ferrite-loaded untuned RF cavity was designed and fabricated as the accelerating structure for a compact proton synchrotron dedicated to cancer therapy. An invented power-feeding method, “multifeed coupling”, was used to achieve a gap voltage higher than 700 V with an applied RF power of 1.2 kW in the frequency range 1.5–8 MHz and a cavity length 0.4 m. The temperature in the ferrite cores had less than a 22°C rise from room temperature using only forced air cooling.

A compact proton synchrotron with combined-function lattice dedicated for cancer therapy

A compact proton synchrotron with combined function lattice has been designed as a dedicated machine for cancer therapy because of its merits of easy operation and low construction cost. The lattice has a six-fold symmetry and its radius of curvature and circumference are 1.9 m and 23.9 m, respectively. For the purpose of establishing a good reference design, we have constructed a model magnet based on the three-dimensional magnetic field calculation. A magnetic field measurement has been performed with use of a three-dimensional Hall-probe. In the present paper, the results of these developments is presented together with the outline of the reference design.

Performance of parallel plate ionization chamber for medical irradiation

We have developed parallel plate ionization chambers (PPIC) to measure not only the cumulative intensity but also the time structure of slow-extracted heavy-ion beams from a medical synchrotron. The characteristics of the PPIC with 3 mm and 1 mm gap distances for 760, 200, 111, and 55 Torr air were investigated with C/sup 6+/ beam (290 MeV/u) at HIMAC. The applied voltage to start the plateau region strongly depends on the beam intensity, and pressure of counter gas. The PPIC can be also used as a useful beam monitor for the time-structure measurement of heavy-ion beams.

Resonant beam extraction with constant separatrix

A new scheme for slow beam extraction using nonlinear resonance is presented to realize small emittance. In the scheme, the amplitude of the betatron oscillations is increased by perturbations, while keeping the separatrix constant. As a measure of perturbation, the transverse filtered noise is studied in computer simulations. It is shown that the emittance of the extracted beam is vanishingly small. It is also shown that the time structure of the extracted current is not affected by the ripple in the magnet current.<<ETX>>

Energy varying resonant beam extraction from the synchrotron

Two different operating schemes for the energy varying beam extraction from the synchrotron are presented based on the resonant extraction scheme in which the transverse RF perturbation is applied for increasing the amplitude of the betatron oscillations with keeping the separatrix of the resonance constant. The first operating scheme is that the primary acceleration is followed by the second acceleration, during which the beam is extracted by the above extraction scheme. In order to keep the separatrix constant during the extraction, the currents of the magnets are ramped with keeping the mutual ratio. In the other operating scheme, the currents of the magnets are ramped from the injection energy level to the maximum level and the beam is extracted with a constant energy at an intermediate stage. The ramping pattern can be commonly used for the beam extraction at the different energy level.

Dual ring multilayer ionization chamber and theory-based correction technique for scanning proton therapy

PURPOSE To develop a multilayer ionization chamber (MLIC) and a correction technique that suppresses differences between the MLIC and water phantom measurements in order to achieve fast and accurate depth dose measurements in pencil beam scanning proton therapy. METHODS The authors distinguish between a calibration procedure and an additional correction: 1-the calibration for variations in the air gap thickness and the electrometer gains is addressed without involving measurements in water; 2-the correction is addressed to suppress the difference between depth dose profiles in water and in the MLIC materials due to the nuclear interaction cross sections by a semiempirical model tuned by using measurements in water. In the correction technique, raw MLIC data are obtained for each energy layer and integrated after multiplying them by the correction factor because the correction factor depends on incident energy. The MLIC described here has been designed especially for pencil beam scanning proton therapy. This MLIC is called a dual ring multilayer ionization chamber (DRMLIC). The shape of the electrodes allows the DRMLIC to measure both the percentage depth dose (PDD) and integrated depth dose (IDD) because ionization electrons are collected from inner and outer air gaps independently. RESULTS IDDs for which the beam energies were 71.6, 120.6, 159, 180.6, and 221.4 MeV were measured and compared with water phantom results. Furthermore, the measured PDDs along the central axis of the proton field with a nominal field size of 10 × 10 cm(2) were compared. The spread out Bragg peak was 20 cm for fields with a range of 30.6 and 3 cm for fields with a range of 6.9 cm. The IDDs measured with the DRMLIC using the correction technique were consistent with those that of the water phantom; except for the beam energy of 71.6 MeV, all of the points satisfied the 1% dose/1 mm distance to agreement criterion of the gamma index. The 71.6 MeV depth dose profile showed slight differences in the shallow region, but 94.5% of the points satisfied the 1%/1 mm criterion. The 90% ranges, defined at the 90% dose position in distal fall off, were in good agreement with those in the water phantom, and the range differences from the water phantom were less than ±0.3 mm. The PDDs measured with the DRMLIC were also consistent with those that of the water phantom; 97% of the points passed the 1%/1 mm criterion. CONCLUSIONS It was demonstrated that the new correction technique suppresses the difference between the depth dose profiles obtained with the MLIC and those obtained from a water phantom, and a DRMLIC enabling fast measurements of both IDD and PDD was developed. The IDDs and PDDs measured with the DRMLIC and using the correction technique were in good agreement with those that of the water phantom, and it was concluded that the correction technique and DRMLIC are useful for depth dose profile measurements in pencil beam scanning proton therapy.