Takuya Natsui

Laser-driven dielectric electron accelerator for radiobiology researches

In order to estimate the health risk associated with a low dose radiation, the fundamental process of the radiation effects in a living cell must be understood. It is desired that an electron bunch or photon pulse precisely knock a cell nucleus and DNA. The required electron energy and electronic charge of the bunch are several tens keV to 1 MeV and 0.1 fC to 1 fC, respectively. The smaller beam size than micron is better for the precise observation. Since the laser-driven dielectric electron accelerator seems to suite for the compact micro-beam source, a phase-modulation-masked-type laser-driven dielectric accelerator was studied. Although the preliminary analysis made a conclusion that a grating period and an electron speed must satisfy the matching condition of LG/λ = v/c, a deformation of a wavefront in a pillar of the grating relaxed the matching condition and enabled the slow electron to be accelerated. The simulation results by using the free FDTD code, Meep, showed that the low energy electron of 20 keV felt the acceleration field strength of 20 MV/m and gradually felt higher field as the speed was increased. Finally the ultra relativistic electron felt the field strength of 600 MV/m. The Meep code also showed that a length of the accelerator to get energy of 1 MeV was 3.8 mm, the required laser power and energy were 11 GW and 350 mJ, respectively. Restrictions on the laser was eased by adopting sequential laser pulses. If the accelerator is illuminated by sequential N pulses, the pulse power, pulse width and the pulse energy are reduced to 1/N, 1/N and 1/N2, respectively. The required laser power per pulse is estimated to be 2.2 GW when ten pairs of sequential laser pulse is irradiated.

Development of a Portable 950 keV X‐band Linac for NDT

We are developing a portable 950 keV X‐band (9.4 GHz) linac X‐ray source for on‐site nondestructive testing of erosion of metal pipes at a petrochemical complex. To develop it, we adopted a compact X‐band 9.4 GHz magnetron of 250 kW for RF generation device. The whole device, including power supply and cooling devices, were also downsized. The dose rate of X‐ray converted in a tungsten target is designed to be 0.2 Gy/min at 1‐m distance. We designed an accelerating tube that uses the π mode for the lower energy part and the π/2 mode cavity for the higher energy. We manufactured the accelerating tube and carried out beam acceleration tests, confirming that the electron beam was accelerated up to 950 keV.

Beam generation and acceleration experiments of X-band linac and monochromatic keV X-ray sorce of the University of Tokyo

In the Nuclear Professional School, the University of Tokyo (UTNS), we are constructing an X-band linear accelerator that consists of an X-band thermionic cathode RF gun and X-band accelerating structure. This system is considered for a compact inverse Compton scattering monochromatic X-ray source for the medical application. The injector of this system consists of the 3.5-cell coaxial RF feed coupler type X-band thermionic cathode RF gun and an alpha-magnet. The X-band accelerating structure is round detuned structure (RDS) type that developed for the future linear collider are fully adopted. So far, we have constructed the whole RF system and beam line for the X- band linac and achieved 2 MeV electron beam generation from the X-band thermionic cathode RF gun. In addition, we achieved 40 MW RF feeding to the accelerating structure. The laser system for the X-ray generation via Compton scattering was also constructed and evaluated its properties. In this paper, we will present the details of our system and progress of beam acceleration experiment and the performance of the laser system for the Compton scattering experiment.

Compact 950 keV X-band (9.4GHz) Linac X-ray Source for On-site Nondestructive Evaluation

We are developing a compact X-ray nondestructive evaluation (NDE) system using 9.4 GHz X-band linac with 250 kW magnetron. A conventional 1 MeV X-band machines use a large 1 MW magnetron system. We have chosen the 250 kW magnetron so that the RF heat loss is remarkably reduced. This design yields compactness and portable. This system consists of the X-band magnetron, modulator, thermionic 20 kV electron gun, X-band linac and metal target of X-ray generation. We aim that X-ray spot size is less than 1 mm. We designed the linac structure of the pi mode at low energy parts and the pi/2 mode at high energy parts by using updated commercial software. We finished to measure resonant frequency, and electromagnetic field on axis used by bead-pull method. These devices are to be applied to on-site NDE at petrochemical complex, nuclear- and thermal-power plants. We are also going to test the system at the Nuclear Professional School, the University of Tokyo this year. This paper presents the details of the system and experimental results.

Experiment of X-ray source by 9.4 GHz x-band linac for nondestructive testing system

We are developing a compact X-ray source for nondestructive testing (NDT) system. We aim to develop a portable X-ray NDT system by 950 keV X-band linac to realize on-site inspection. We use a low power (250 kW) magnetron as RF source for compactness of whole system. By using low power magnetron, we can use small magnetron power supply and cooling system. Additionally, the system has X-band linac and it has small spot size of electron beam. Our final goal of X-ray spot size is less than lmm. We have designed the linac structure of the pi mode at low energy parts and the pi/2 mode at high energy parts by using calculation codes. It was finished to measure resonant frequency and electromagnetic field of the linac. And the result of measurement consists with calculation data. The components of this system was completed and installed in the Nuclear Professional School, the University of Tokyo. We are carrying out electron beam accelerate testing.

Progress of 7-GeV SuperKEKB Injector Linac Upgrade and Commissioning

KEK injector linac is being upgraded for the SuperKEKB project, which aims at a 40-fold increase in luminosity over the previous project KEKB. SuperKEKB asymmetric electron and positron collider with its extremely high luminosity requires a high current, low emittance and low energy spread injection beam from the injector. Electron beams will be generated by a new type of RF gun, that will inject a much higher beam current to correspond to a large stored beam current and a short lifetime in the storage ring. The positron source is another major challenge that enhances the positron bunch intensity from 1 to 4 nC by increasing the positron capture efficiency, and the positron beam emittance is reduced by introducing a damping ring, followed by the bunch compressor and energy compressor. The recent status of the upgrade and beam commissioning is reported.

Commissioning of the Phase-I SuperKEKB B-Factory and Update on the Overall Status

The SuperKEKB B-Factory at KEK (Japan), after few years of shutdown for the construction and renovation, has finally come to the Phase-1 commissioning of the LER and HER rings, without the final focus system and the Belle II detector. Vacuum scrubbing, optics tuning and beam related background measurements were performed in this phase. Low emittance tuning techniques have also been applied in order to set up the rings for Phase-2 with colliding beams next year. An update of the final focus system construction, as well as the status of the injection system with the new positron damping ring and high current/low emittance electron gun is also presented.

950 keV X‐Band Linac For Material Recognition Using Two‐Fold Scintillator Detector As A Concept Of Dual‐Energy X‐Ray System

One of the advantages of applying X-band linear accelerator (Linac) is the compact size of the whole system. That shows us the possibility of on-site system such as the custom inspection system in an airport. As X-ray source, we have developed X-band Linac and achieved maximum X-ray energy 950 keV using the low power magnetron (250 kW) in 2 {mu}s pulse length. The whole size of the Linac system is 1x1x1 m{sup 3}. That is realized by introducing X-band system. In addition, we have designed two-fold scintillator detector in dual energy X-ray concept. Monte carlo N-particle transport (MCNP) code was used to make up sensor part of the design with two scintillators, CsI and CdWO4. The custom inspection system is composed of two equipments: 950 keV X-band Linac and two-fold scintillator and they are operated simulating real situation such as baggage check in an airport. We will show you the results of experiment which was performed with metal samples: iron and lead as targets in several conditions.

Beam Measurement of 11.424 GHz X‐Band Linac for Compton Scattering X‐ray Source

An inverse Compton scattering X‐ray source for medical applications, consisting of an X‐band (11.424 GHz) linac and Q‐switched Nd:YAG laser, is currently being developed at the University of Tokyo. This system uses an X‐band 3.5‐cell thermionic cathode RF gun for electron beam generation. We can obtain a multi‐bunch electron beam with this gun. The beam is accelerated to 30 MeV by a traveling‐wave accelerating tube. So far, we have verified stable beam generation (around 2.3 MeV) by using the newly designed RF gun and we have succeeded in beam transportation to a beam dump.

On‐site Real‐Time Inspection System for Pump‐impeller using X‐band Linac X‐ray Source

The methods of nondestructive testing (NDT) are generally ultrasonic, neutron, eddy‐current and X‐rays, NDT by using X‐rays, in particular, is the most useful inspection technique having high resolution. We can especially evaluate corroded pipes of petrochemical complex, nuclear and thermal‐power plants by the high energy X‐ray NDT system. We develop a portable X‐ray NDT system with X‐band linac and magnetron. This system can generate a 950 keV electron beam. We are able to get X‐ray images of samples with 1 mm spatial resolution. This system has application to real time impeller inspection because linac based X‐ray sources are able to generate pulsed X‐rays. So, we can inspect the rotating impeller if the X‐ray pulse rate is synchronized with the impeller rotation rate. This system has application in condition based maintenance (CBM) of nuclear plants, for example. However, 950 keV X‐ray source can only be used for thin tubes with 20 mm thickness. We have started design of a 3.95 MeV X‐band linac for bro...