Takeshi Yoshiyuki

Focusing and spectral enhancement of a repetition-rated, laser-driven, divergent multi-MeV proton beam using permanent quadrupole magnets

A pair of conventional permanent magnet quadrupoles is used to focus a 2.4 MeV laser-driven proton beam at a 1 Hz repetition rate. The magnetic field strengths are 55 and 60 T/m for the first and second quadrupoles, respectively. The proton beam is focused to a spot with a size of less than ∼3×8 mm2 at a distance of 650 mm from the source. This result is in good agreement with the Monte Carlo particle trajectory simulation.

Development of Saddle-Shaped Coils Using Coated Conductors for Accelerator Magnets

A project to develop fundamental technologies for accelerator magnets using high-Tc superconductors is currently underway. The primary applications in this project are fixed field alternating gradient (FFAG) accelerators for carbon cancer therapy systems and accelerator-driven subcritical reactors. One of the challenging issues in this study is to accomplish the complicated coil shape required for the accelerator magnets using YBa2Cu3Ox (YBCO) coated conductors. An asymmetric, three-dimensional winding structure has been designed for the main ring of an FFAG accelerator, but the anisotropic bending flexibility of coated conductors is constrained because of their tape-like shape. To demonstrate a suitable winding technology for a three-dimensional coil using coated conductors, a prototype winding machine with a two-axis motion mechanism was constructed. Based on the present design of the FFAG magnet, a saddle-shaped coil wound on a curved surface with a radius of 700 mm was designed. A 100-turn coil was wound by the winding machine using an approximately 90 m length of 3 mm-wide YBCO coated conductor made by the IBAD-MOCVD method. The coil was impregnated with epoxy resin, and the voltage-current (V-I) characteristics were evaluated in a liquid nitrogen environment. From the V-I characteristics in an electric field region of 10-9V/cm, an index value of above 20 was obtained, which means that the superconducting properties were not degraded.

Magnetic Field Design of Dipole Magnet Wound With Coated Conductor Considering Its Current Transport Characteristics

When designing accelerator magnets using high Tc superconductor tapes, their current transport characteristics with anisotropic magnetic field dependence as well as their temperature dependence must be considered. The current transport characteristics of commercially-available GdBa2Cu3O7 coated conductor made by IBAD and PLD process were measured in various magnetic fields and temperatures, and they were formulated with the percolation depinning model. Using the formulated current transport characteristics, dipole magnets wound with coated conductors were designed to show the feasibility of accelerator magnets operated at a high temperature, where cryo-coolers can be applied. First, a two-dimensional magnet cross-section was designed, and, then, a three-dimensional magnet design was made, based on the constant perimeter restriction and considering the bending and torsion tolerances of coated conductors. Their enthalpy margins, temperature margins, and current margins were estimated and they were compared with that of a NbTi magnet.

Magnetic Field Design of Coil-Dominated Magnets Wound With Coated Conductors

Coil-dominated magnets wound with coated conductors were designed for an FFAG accelerator for carbon therapy, which was designed by applying linear approximation. When designing the coil-end of the magnets, we applied differential geometry. To apply the differential geometry to three-dimensional windings with coated conductors, we introduced the concept of generalized flat-wise bending. Thereby, the combination of flat-wise bending and torsion was considered as bending of developable surface. Generated magnetic field was calculated, and the positions of conductors were optimized to minimize the field error. Three dimensional shapes of magnets were successfully designed with constraints on generalized flat-wise bending and edge-wise bending of coated conductors.

Fabrication of YBCO Small Test Coils for Accelerator Magnet Development

A research program for studying coil winding technologies using YBa2Cu3Ox (YBCO) coated conductors is now in progress. In this program, we are attempting to develop high-Tc superconducting magnets for highly functional, efficient, and compact accelerator systems. In designing superconducting coils with thin tape conductors, we must be concerned about magnetization of the superconducting layer. In order to experimentally evaluate the influence of the additional field resulting from the magnetization current, prototype racetrack coils were constructed with YBCO coated conductors. A 62 m length of coated conductor was wound to form each of the coils, which were then impregnated with epoxy resin. For precise analysis of the additional magnetic field, it is essential to fabricate superconducting coils without degradation because the voltage generated at such a degraded part would affect the behavior of the additional magnetic field. In a liquid nitrogen environment, the racetrack coils were energized to evaluate the voltage-current (V-I) characteristics. It was found that there was no degradation in the superconductivity. The V-I characteristics showed sufficiently high index values of 28 and 34 throughout an electric field range of 10-9 V/cm. Magnet design derived from accelerator design, as well as construction of an experimental reduced-scale coil in a typical shape, are also discussed.

Prompt In-Line Diagnosis of Single Bunch Transverse Profiles and Energy Spectra for Laser-Accelerated Ions

Many applications of laser-accelerated ions will require beamlines with diagnostic capability for validating simulations and machine performance at the single bunch level as well as for the development of controls to optimize machine performance. We demonstrated prompt, in-line, single bunch transverse profile and energy spectrum detection using a thin luminescent diagnostic and scintillator-based time-of-flight spectrometer simultaneously. The Monte Carlo code, particle and heavy ion transport code systems (PHITS) simulation is shown to be reasonably predictive at low proton energy for the observed transverse profiles measured by the thin luminescent monitor and also for single bunch energy spectra measured by time-of-flight spectrometry.

Toward laser driven proton medical accelerator

Towards our final goal, such as to establish the laser-driven proton accelerator for the medical application, one of the most important things to establish is to develop the proton transport system. In this continuous work, we demonstrate the focusing system of the laser-driven proton beam with permanent magnet qadrupoles (PMQs), with which a 2.4 MeV laser-driven proton beam, having a divergence angle of ~10 degrees at birth, is focused to a spot whose size is 3×8mm2 at 640mm downstream from the target.

Laser driven proton beam for the applications: toward ultra‐high intensity & short pulse proton beam

As a benchmark experiment to realize a novel and compact laser driven proton accelerator whose significant features are high number density in a short pulse width (∼ns), 2.4 MeV laser driven proton beam is stably focused at 1 Hz repetition rate by using a pair of permanent quadrupole magnets (PMQs) with large apertures whose diameters and field strengths are 3.5 cm and 55 T/m for the first and 2.3 cm and 60 T/m for the second magnets, respectively. The proton beam has been focused to a focal spot of 3×8 mm2 in horizontal and vertical direction (full width at half maximum) at 650∼mm from the source, which is well reproduced by the simulation. The further optimization of the focusing system will sure to pave a way to the novel proton accelerator with which we can investigate the physics appeared in the short time scale as well as that in a high energy density matter, oncology, astrophysics, and so on.