Fixed-field, alternating-gradient particle accelerators.

Laser-driven ion sources are a rapidly developing technology producing high energy, high peak current beams. Their suitability for applications, such as compact medical accelerators, motivates development of robust acceleration schemes using widely available repetitive ultraintense femtosecond lasers. These applications not only require high beam energy, but also place demanding requirements on the source stability and controllability. This can be seriously affected by the laser temporal contrast, precluding the replication of ion acceleration performance on independent laser systems with otherwise similar parameters. Here, we present the experimental generation of >60 MeV protons and >30 MeV u-1 carbon ions from sub-micrometre thickness Formvar foils irradiated with laser intensities >1021 Wcm2. Ions are accelerated by an extreme localised space charge field ≳30 TVm-1, over a million times higher than used in conventional accelerators. The field is formed by a rapid expulsion of electrons from the target bulk due to relativistically induced transparency, in which relativistic corrections to the refractive index enables laser transmission through normally opaque plasma. We replicate the mechanism on two different laser facilities and show that the optimum target thickness decreases with improved laser contrast due to reduced pre-expansion. Our demonstration that energetic ions can be accelerated by this mechanism at different contrast levels relaxes laser requirements and indicates interaction parameters for realising application-specific beam delivery. 

https://www.nature.com/articles/s41377-023-01083-9.pdf

Enhanced ion acceleration from transparency-driven foils demonstrated at two ultraintense laser facilities

Laser Acceleration of Electrons, Protons, and Ions II; and Medical Applications of Laser-Generated Beams of Particles II; and Harnessing Relativistic Plasma Waves III

Spatial characterization of 0.5 MeV proton beam, driven by 12 fs, 35 mJ, 1019 W/cm2 intense laser-foil interaction is presented. The accelerated proton beam has been applied to obtain a high-resolution, point-projection static radiograph of a fine mesh using a CR-39 plate. The reconstruction of mesh edge blurring and particle ray tracing suggests that these protons have an effective source size (FWHM) of just 3.3 ± 0.3 µm. Furthermore, the spatial distribution of the proton beam recorded on the CR-39 showed that the divergence of these particles is less than 5-degree (FWHM). The low divergence and small source size of the proton beam resulted in an ultralow transverse emittance of 0.00032 π-mm-mrad, which is several orders of magnitude smaller than that of a conventional accelerator beam. 

/pdf/low-divergent-mev-class-proton-beam-with-micrometer-source-vo0ld43h.pdf

Low divergent MeV-class proton beam with micrometer source size driven by a few-cycle laser pulse

A new accelerator facility of Center for Accelerator and Beam Applied Science is under construction on Ito Campus to promote research and education activities at Kyushu University. The facility consists mainly of a 10 MeV proton cyclotron as an injector and a 150 MeV Fixed Field Alternating Gradient (FFAG) accelerator, which was developed at KEK as a prototype of proton FFAG for various applications. In this paper, the status of the development of devices and the facility is described. INTRODUCTION Center for Accelerator and Beam Applied Science has been established in April 2007 to promote activities in all the related scientific, medical, engineering and educational fields at Kyushu University. To realize the purpose of the center, Kyushu University has decided to construct a new facility merging three institutes that are Cockcroft-Walton Accelerator Laboratory of Faculty of Engineering, Institute for Irradiation and Analysis of Quantum Radiations and Kyushu University Tandem Accelerator Laboratory of Faculty of Sciences, on its new campus (Ito Campus). The facility mainly consists of a 10 MeV injector cyclotron and a 150 MeV Fixed Field Alternating Gradient (FFAG) accelerator [1-7] as a replacement of the Cockcroft-Walton accelerator. The 150 MeV FFAG, a prototype machine to prove its usefulness for various applications such as proton beam therapy, was developed at KEK. The construction of the 150 MeV FFAG was started in September 2002 at the east counter hall in KEK, and the beam extraction with 100 Hz operation was successfully demonstrated in November 2005. The 150 MeV FFAG, disassembled in June 2006, was transported by land from KEK to Ito Campus in March 2008. The construction of the first stage building of the facility has already been completed. The development of the 150 MeV FFAG, the beam commissioning, the pilot researches on nuclear, medical and life science will be carried out from 2009 to 2013. In this stage, we are also planning to introduce a high intensity proton injector for the FFAG. In the second stage, from 2014 to 2016, experimental halls will be extended, and a new tandem accelerator for AMS and heavy-ion beam injection to the FFAG will be installed into the facility. OVER VIEW OF 150 MEV FFAG Details of devices of the 150 MeV FFAG are described in this section. The main parameters of the 150 MeV FFAG are summarized in Table 1. Figure 1 shows the schematic layout of the accelerator. Table 1: Designed parameters of 150 MeV FFAG Energy 10 125 MeV(proton) Type of magnet Triplet radial ( DFD)

Status of center for accelerator and beam applied science of Kyushu University

The `Laser-hybrid Accelerator for Radiobiological Applications', LhARA, is conceived as a novel, uniquely-flexible facility dedicated to the study of radiobiology. The technologies demonstrated in LhARA, which have wide application, will be developed to allow particle-beam therapy to be delivered in a completely new regime, combining a variety of ion species in a single treatment fraction and exploiting ultra-high dose rates. LhARA will be a hybrid accelerator system in which laser interactions drive the creation of a large flux of protons or light ions that are captured using a plasma (Gabor) lens and formed into a beam. The laser-driven source allows protons and ions to be captured at energies significantly above those that pertain in conventional facilities, thus evading the current space-charge limit on the instantaneous dose rate that can be delivered. The laser-hybrid approach, therefore, will allow the vast ``terra incognita'' of the radiobiology that determines the response of tissue to ionising radiation to be studied with protons and light ions using a wide variety of time structures, spectral distributions, and spatial configurations at instantaneous dose rates up to and significantly beyond the ultra-high dose-rate `FLASH' regime. 
It is proposed that LhARA be developed in two stages. In the first stage, a programme of in vitro radiobiology will be served with proton beams with energies between 10MeV and 15MeV. In stage two, the beam will be accelerated using a fixed-field accelerator (FFA). This will allow experiments to be carried out in vitro and in vivo with proton beam energies of up to 127MeV. In addition, ion beams with energies up to 33.4MeV per nucleon will be available for in vitro and in vivo experiments. This paper presents the conceptual design for LhARA and the R&D programme by which the LhARA consortium seeks to establish the facility.

The Laser-hybrid Accelerator for Radiobiological Applications.

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The Laser-hybrid Accelerator for Radiobiological Applications.