Experimental characterisation of a novel, compact high-field beamline for application in laser-driven ion beam therapy

Introduction:

Compact laser-driven ion accelerators are a potential alternative to complex, large and expensive conventional accelerators. High-power short-pulse lasers, impinging on e.g. thin metal foils enable multi-MeV ion acceleration on µm length and ps time scale. The generated ion bunches (typically protons) show unique beam properties, like ultra-high pulse dose. Nevertheless, laser accelerators still require substantial development in reliable beam generation and transport. We present first experimental results on a beamline prototype based on high-field magnet technology specifically designed for capture and transport of laser-accelerated particles.

Material and methods:

Since the mid-1950s, pulsed high-field magnets serve as versatile research tools for solid state physics and material research. Recently developed pulsed magnet technology, specifically designed to meet the demands of laser acceleration [1], open up new research opportunities: We present a pulsed solenoid for effective collection and focusing of laser-accelerated ions which could function as a first component in a laser-driven gantry system [2]; furthermore, we present a dipole magnet for beam deflection and energy dispersion. The magnets were combined to form a first, pulsed high-field beamline and are powered by portable pulse. Characterization of both magnets has been carried out using a 10 MeV proton beam from a conventional Tandetron accelerator at Helmholtz-Zentrum Dresden – Rossendorf (HZDR). The transported beam was detected by means of radiochromic film and scintillator.

Result:

The transport experiments clearly show the functionality of both magnets: The solenoid focuses the large, divergent ion beam to a millimeter-sized spot showing no aberration. The dipole deflects the protons by a 45° angle and can be used as energy selection device via energy dispersion. For the several 10 µs long proton bunches no effect of the time structure of the magnetic pulse (~ several 100 µs) was observed thus making the magnets well suited for even shorter, laser-driven ion bunches. The beam position accuracy, after passing the beamline, was measured to be of the same order as the beam position fluctuations itself, showing the precision of the installed beamline. The maximum magnetic field strength achieved was 20 T for the solenoid and 5 T for the dipole. For operation at higher proton energies, i.e. above 200 MeV, an increase by a factor of only 1.5 – 2 is requested.

Summary:

Our experimental results show that compact high-field magnets can be used to precisely guide charged particle bunches. The pulsed nature of laser-accelerated particles is matched by the magnet technology. The maximum field strengths reached are sufficient for experiments with protons of several 10 MeV kinetic energy. For clinically relevant energies above 200 MeV, however, a slight increase is required. Experimental studies at a laser-accelerator are scheduled.
References
[1] T. Burris-Mog, et al., Laser accelerated protons captured and transported by a pulse power solenoid, PRSTAB 14, 121301 (2011)
[2] U. Masood, et al., A compact solution for ion beam therapy with laser accelerated protons, Appl. Phys. B 117, 41 (2014)