Capture, focusing and energy selection of laser driven ion beams using conventional beam elements Morteza Aslaninejad Imperial College 13 December 2012

TNSA (Target Normal Sheath Acceleration) For relatively thick targets (μm thick AL foils) Acceleration of surface ions(60 MeV protons). For TNSA scaling considerations predict that laser intensities of a few W/cm 2 are required to reach the 200 MeV proton energies of interest for ion therapy. RPA (Radiation Pressure Acceleration ) Radiation Pressure Regime proposed by Esirkepov et al., 2004 Laser acts like a piston that accelerates foil as a whole. Promises several hundred MeV protons (or carbon) “mono-energetic”. Major challenges are to fabricate the required ultrathin target foils that have a typical thickness in the order of 10 nm only. Laser acceleration:  Beyond W/cm 2, laser pulses can produce particle beams.  There are different regimes of ion acceleration depending on target thickness and laser parameters:

Laser ion acceleration potential candidate for therapy applications :  available energies are getting interesting (65 MeV p)  high "quality" of beams at origin (small 6D phase space)  abundance of protons per shot (total ~ …10 13 )  high rep rate lasers emerging (10 Hz)  laser accelerator "compact" (acceleration length < 1mm) Focusing and the transport of laser driven protons a challenging problem.  Large energy spread  Large angular divergence,  Beam handling: though the emittance of particles bunches produced by laser acceleration are very favourable, the hardware necessary to match these beams to a therapy scanning system is unwieldy, expensive and impractical. BUT

Aperture collimation (no focusing) Development of a Laser –Driven Proton Accelerator for Cancer Therapy C.M. Ma et al., Laser Physics, 2006, Vol. 16, No. 4,pp Energy selection (bends + apertures) 250 MeV Passive elements to form dose Aperture collimation (no focusing) Development of a Laser –Driven Proton Accelerator for Cancer Therapy C.M. Ma et al., Laser Physics, 2006, Vol. 16, No. 4,pp Energy selection (bends + apertures) 250 MeV Passive elements to form dose Very compact particle selection not requiring much more than 1 m distance laser target to tumour. 1.Transverse collimation by a small aperture immediately behind the target limiting transmission to a production cone angle of Ω≈±10 mrad (0.5 degrees), 2.A dispersive energy separation device 3.A Second aperture close to the tumour Very compact particle selection not requiring much more than 1 m distance laser target to tumour. 1.Transverse collimation by a small aperture immediately behind the target limiting transmission to a production cone angle of Ω≈±10 mrad (0.5 degrees), 2.A dispersive energy separation device 3.A Second aperture close to the tumour Obviously, the available particle flux density form the source and the need for keeping the distance low are critical issues here.

PMQ Parameters Magnetic field strengths 55 and 60 T/m Repetition rate1 Hz Proton beam energy 2.4 MeV Focused to a spot with a size of 3 x 8 mm 2 at a distance of 650 mm from source Lens collection by quadrupole More flexibility can be expected if active lens focusing by a solenoid or quadrupole is applies which allows using a larger production cone angle Lens collection by quadrupole More flexibility can be expected if active lens focusing by a solenoid or quadrupole is applies which allows using a larger production cone angle Focusing and Spectral Enhancement of a Repetition- Rated, Laser-Driven, Divergent Multi-MeV Proton Beam Using Permanent Quadrupole Magnets M. Nishiuchi et al Appl. Phys.Lett.94,061107(2009) Focusing and Spectral Enhancement of a Repetition- Rated, Laser-Driven, Divergent Multi-MeV Proton Beam Using Permanent Quadrupole Magnets M. Nishiuchi et al Appl. Phys.Lett.94,061107(2009)

Solenoid Parameters Solenoid LengthL=360 mm Inner radiusR in =44 mm Outer radiusR out =76 mm Magnetic FieldB=13 T Focal distanceF=1 m Beam Parameters EnergyE=200 MeV Energy Spread∆E/E=±0.05 Special AngleΩ max =50 mrad RadiusR=10 μm Emittanceε=0.5 mm-mrad Pulse duration140 ps Lens Collection by Solenoid Collection and Focusing of Laser Accelerated Ion beam for Therapy Applications: Ingo Hofmann et al Physical Review Special Topics Accelerators and Beams 14, (2011). Lens Collection by Solenoid Collection and Focusing of Laser Accelerated Ion beam for Therapy Applications: Ingo Hofmann et al Physical Review Special Topics Accelerators and Beams 14, (2011). Sample trajectory through solenoid collector and ∆E/E=±0.05 Sample trajectory through solenoid collector and ∆E/E=±0.05 Distance target to solenoid= 240 mm

 Linear behaviour for energy spread  Quadratic dependence on the initial cone angle An emittance scaling law for the Chromatic emittance at the focusing : ε=α c Ω 2 (∆E/E) α c =0.3 m/rad  Linear behaviour for energy spread  Quadratic dependence on the initial cone angle An emittance scaling law for the Chromatic emittance at the focusing : ε=α c Ω 2 (∆E/E) α c =0.3 m/rad Phase space at the trajectory crossover

Usable intensities of 200 MeV protons as a function of selected production cone angle and energy window ∆E/E=±0.1 and Ω=40 mrad  I=3.5x times as high as the fluence requirement of 1.7x10 9 /cm 2 ∆E/E=±0.01 and Ω=40 mrad  I=3.5x times as high as the fluence requirement of 7x10 9 /cm 2 ∆E/E=±0.1 and Ω=40 mrad  I=3.5x times as high as the fluence requirement of 1.7x10 9 /cm 2 ∆E/E=±0.01 and Ω=40 mrad  I=3.5x times as high as the fluence requirement of 7x10 9 /cm 2

PHELIX laser system at GSI: The proton beams accelerated are contained within an angular distribution with up to a half-angle divergence of 7° for high energy protons and 20° for low energy protons. Exponentially decaying proton spectra had a maximum cut-off near 23 MeV. Intensities of 2.9*10 19 W/cm 2 With 72 J of normal incident linearly polarized μm laser light in 500 fs. The laser light focused to an 8.5μm by 17 um diameter spot size(FWHM) on a flat 25 μm thick Au foil. Laser Accelerated protons Captured and Transported by a Pulse power Solenoid T. Burrins-Mog, et al Physical Review Special Topics Accelerators and Beams 14, (2011) Laser Accelerated protons Captured and Transported by a Pulse power Solenoid T. Burrins-Mog, et al Physical Review Special Topics Accelerators and Beams 14, (2011) 8.5 Tesla solenoid for beam energy up to 23 MeV. Solenoid as the initial capture and collimating elements for a 250 MeV proton is scaled to 32 T.

Electrostatic lenses on the other hand have several advantages. Space charge plasma lenses of the Gabor type might be a cost effective alternative. See talk by Juergen Thank you Gabor Lenses for Capture and Energy Selection of Laser Driven Ion Beams In Cancer Treatment Juergen Pozimski Gabor Lenses for Capture and Energy Selection of Laser Driven Ion Beams In Cancer Treatment Juergen Pozimski Magnetic lenses are very sensitive to the energy of the particles and the extremely high –energy spread of the raw beam from the laser would render the design of such a transport system challenging. Conventional optical systems like solenoids or quadrupoles will be operating at the technical limits which would be contradictory to the cost and space argument.

Back up slides

Spectral Yield Pure aperture collimation: dN/dE=0.25x10 9 E=200 MeV ∆E/E=±0.01 →→∆E=±2 MeV →→∆E=4 MeV dN=(0.25x10 9 )=10 9 The standard proton fluence of 7x10 8 /cm 2 Required at the distal layer with a spot radius of 10 mm (resulting at 1 m distance according to the assumed divergence)requires, 2x10 9 which would be missed by a factor of 2 for a single laser shot.

Laser parameters considered by I. Hofmann: Spot radius = 10 μm Pulse duration = 66 fs Specific power = 3x10 21 W/cm 2 Peak power = 10 PW Pulse energy = 620 J Average power = 6 kW(10 Hz) Nishiuchi expriement Divergence half angle=10°±1° The beam diameter(FWHM)= 28±1 mm PMQ used. J-KAREN laser 100 TW