Gabor lenses for capture and energy selection of laser driven ion beams in cancer treatment. J. Pozimski PASI meeting RAL 5 th April 2013 Imperial College

Introduction Wakefield acceleration allows for extremely compact accelerators integrated into the gantry with enormous effects on cost per unit. Energy spectrum achievable at present, combined with conventional beam formation technology contradict proposed savings. To capture the beam with large divergence angle SC solenoids (14T) are envisaged. To perform an energy selection lattice designs foresee a footprint of 5x5m. => An unconventional accelerator requires an unconventional optical system.

3 Space charge lenses of Gabor-Type In 1932 von Borries and Ruska showed that discharge plasmas can focus electron beams. B. Von Borries, E. Ruska; “Das kurze Raumladungsfeld einer Hilfsentladung als Sammellinse fuer Kathodenstrahlen”, Zeitschrift f. Phys. 76 (1932) Gabor published his idea of a space charge lens in Nature. D. Gabor, “Space charge lens for the focusing of ion beams”, Nature160, 89(1947)

Space charge lenses of Gabor-Type Between 1970 and 1990 various publications by : R. Booth, H.W. Levevre, “Space-Charge-Lens for high current Ion beams”, Nucl. Inst.&Meth. 151 (1978) M.D. Gabovich Mobley, “Gabor Lenses- Experimental results at Brookhaven”, BNL 1973 R. J. Noble, “Beam transport with Magnetic Solenoids and Plasma Lenses”, Proc. Linac 1988 J. A. Palkovic, “Measurements on a Gabor lens for neutralizing and focusing a 30 keV proton beam”, Ph.D. thesis, University of Wisconsin, Madison(1991). J. A. Palkovic et al., Rev. Sci. Instrum. 61(1) (1990) M. Reiser, Comparison of Gabor lens, gas focusing and electrostatic quadrupole focusing for low energy beams, Proceedings of the PAC Conference 1989, Chicago, USA demonstrated focussing of ion beams using Gabor type space charge Lenses. But in all cases the lens performance in terms of focussing strength and aberrations was poor compared to theoretical predictions. The reasons for this under performance where unknown at that time.

5 Radial trapping in a GPL = Brillouin flow Generated radial electric field (=focussing) is proportional to charge density of captured particles

6 Longitudinal trapping was considered later

7 Diffusion and thermal coupling can explain “underperformance” J. Pozimski and O. Meusel, “Space charge lenses for particle beams”, Rev. Sci. Instrum. 76 (2005)

8 Experimental results and comparison with simulations O. Meusel, A. Bechtold, J. Pozimski, U. Ratzinger, A. Schempp, H. Klein, “Low-energy beam transport using space-charge lenses”, Nuclear Instruments and Methods in Physics Research A 544 (2005) 447–453

Gabor lens geometries Space charge lenses of the Gabor type could be utilized for laser wake field accelerated ions instead of conventional magnets reducing the required magnetic field by: Parameter Lens Anode radius R A (mm) Cathode radius R C (mm) Total length L (mm) Electron Density  max (C/m 3 ) Anode voltage V A (kV) Magnetic field B Z (T) Model e Model e Model e PoP e-03<65< 0.09

GPL lattice for beam formation and energy selection – mono energetic beam (200 MeV) 3 lens setup with one aperture proposed :

GPL lattice for beam formation and energy selection – energy spread +-5% Energy spread of the beam can be significantly reduced (factor 2.6) with good transmission of “right energy” particles (80%).

GPL lattice – energy variation by lens settings Energy variation for spot scanning can be performed by variation of lens parameters only. Lens design was chosen to allow for one HV PS only. Energy (MeV) B Field (T) Anode Voltage (kV) (All three lenses) Charge Density (10 2 C/m 3 ) Lens 1Lens 2Lens 3Lens 1Lens 2Lens

Proof of principle experiment at available beam energies A PoP setup for energies in the range up to 30 MeV will be proposed in the near future. The experiments could be preformed in 3 phases : 1: one lens (“3”), no aperture, particle capture 2 : two lenses (shown), aperture, reduced energy spread, energy selection up to ~10 MeV 3 : three lenses & aperture, Energy selection up to 30 MeV