Accessing novel ion acceleration mechanisms, such as Radiation Pressure Acceleration (RPA), is a promising route to generate high energy beams of both light and heavy ions [1]. In particular, the Light Sail (LS) regime predicts high efficiency, mono-energetic beams and can be accessed with currently available high power laser facilities with the use of ultra-thin foils and circular polarisation [2-4]. In recent experiments at the GEMINI laser facility (RAL, UK), target bulk (carbon) ions were favourably accelerated in the LS-RPA regime up to 33MeV/nucleon at an optimal carbon foil thickness of 15nm, whereas protons only reached energies of 18 MeV. This result, which differs from what is typically observed in laser-solid interactions, where protons are always accelerated more efficiently than heavier ions, is interpreted with the support of multi-dimensional Particle in Cell (PIC) simulations. While the 40fs pulse was temporally cleaned by a double plasma mirror arrangement to increase the laser contrast to 10-14 at the ns timescale, it is shown that the limited preceding laser fluence incident on the target on the ps scale causes target expansion, with protons, being lighter, escaping from the interaction region. This leaves a pre-dominantly carbon plasma which, for circular polarization, is accelerated by RPA, with proton energies determined instead by plasma expansion and sheath effects. It is shown through simulations that controlling the laser temporal profile and plasma mirror activation opens up a promising route for controlling which ion species is preferentially accelerated in the RPA regime. This has particular importance as <1PW systems are coming online currently where these accelerations will begin to inherently dominate, and the preceding laser intensity will need to be suitably controlled.
The spatio-temporal and polarisation properties of intense light is important in wide-ranging topics at the forefront of intense light-matter interactions, including laser-driven particle acceleration. In the context of experiments to optimize transparency-enhanced ion acceleration in expanding ultrathin foils, we investigate the polarisation and temporal properties of intense light measured at the rear of the target. An effective change in the angle of linear polarisation of the light results from a superposition of coherent radiation, generated by a directly accelerated bipolar electron distribution, and the light transmitted due to the onset of relativistic self-induced transparency. Simulations show that the generated light has a high-order transverse electromagnetic mode structure in both the first and second laser harmonics that can evolve on intra-pulse time-scales. The mode structure and polarisation state vary with the interaction parameters, opening up the possibility of developing this approach to achieve dynamic control of structured light fields at ultrahigh intensities [1].
We also report on frequency-resolved optical gating measurements of the light which demonstrate a novel and simple approach to diagnose the time during the interaction at which the foil becomes transparent to the laser light. This is a key parameter for optimising ion acceleration in expanding ultrathin foils. Coherent transition radiation produced at the foil rear interferes with laser light transmitted through the foil producing spectral fringes. The fringe spacing enables the relative timing of the onset of transmission with respect to the transition radiation generation to be determined. This self-referencing approach to spectral interferometry provides a route to optically controlling and optimising ion acceleration from ultrathin foils undergoing transparency [2].
[1] M.J. Duff et al., Scientific Reports 10, 105 (2020)
[2] S.D.R. Williamson et al., Phys. Rev. Applied 14, 034018 (2020)
The maximum energy to which ions are accelerated in the interaction of a high power laser pulse with a thin foil target scales with the laser intensity, with a power-law that varies with the acceleration mechanism and laser pulse parameters. For fixed laser energy and pulse duration, maximizing the intensity by focusing to a smaller focal spot does not, however, necessarily result in higher-energy ions. For the case of relatively thick foil targets, it has been shown that self-generated magnetic fields and unfavourable changes to the temperature and divergence of the fast electron population injected into the target can result in lower-energy sheath-accelerated ions compared to that expected from intensity scaling laws.
We report results from an investigation of the influence of laser focusing on ion acceleration in the ultrathin target regime, for which high energy protons have been achieved by our group [1]. We compare the interaction physics resulting from the use of f/3 and f/1 focusing geometries. Although f/1 focusing (achieved using a focusing plasma optic) produces a smaller nominal laser focal spot size and thus higher nominal peak intensity, more efficient ion acceleration to higher energies is achieved with the f/3 geometry for the case of expanding ultrathin foils undergoing relativistic self-induced transparency. Particle-in-cell simulations reveal that self-focusing in the expanding plasma produces a near-diffraction-limited focal spot, resulting in up to an order of magnitude higher focused intensity in the f/3 case. We also report on the extent to which this intensity enhancement is expected in the case of the short-pulse, ultrahigh-intensity regime that will soon be accessible using multi-petawatt lasers. The study is published in reference [2].
[1] A. Higginson et al., Nature Communications 9, 724 (2018)
[2] T. P. Frazer et al., Phys. Rev. Research 2, 042015(R) (2020)
The Scottish Centre for the Application of Plasma-based Accelerators (SCAPA) is a research centre dedicated to providing high energy particle beams and high peak brightness radiation pulses for users across all scientific and engineering disciplines. A pair of Ti:Sapphire femtosecond laser systems (40 TW peak power at 10 Hz pulse repetition rate and 350 TW at 5 Hz, respectively) are the drivers for a suite of laser-plasma accelerator beamlines housed across a series of radiation shielded areas. The petawatt-scale laser delivers 45 W of average power that establishes it as the world leader in its class. The University of Strathclyde has had an operational laser wakefield accelerator since 2007 as the centrepiece of the ongoing Advanced Laser Plasma High-energy Accelerators towards X-rays (ALPHA-X) project. SCAPA, which is a multipartner venture under the auspices of the Scottish Universities Physics Alliance, continues the dedicated beamline approach pioneered by ALPHA-X and represents a significant expansion in the UK’s experimental capability at the university level in laser-driven acceleration. The new centre supports seven radiation beamlines across three concrete shielded bunkers that each nominally specialise in a different aspect of fundamental laser-plasma interaction physics and radiation sources: GeVscale electron beams, MeV/c proton and ion beams, X-rays, gamma rays and so on. Development of application programmes based on these sources cover a wide range of fields including nuclear physics, radiotherapy, space radiation reproduction, warm dense matter, high field physics and radioisotope generation.
The radiation pressure of next generation high-intensity lasers could efficiently accelerate ions to GeV energies. However, nonlinear quantum-electrodynamic effects play an important role in the interaction of these lasers with matter. We show that these quantum-electrodynamic effects lead to the production of a critical density pair-plasma which completely absorbs the laser pulse and consequently reduces the accelerated ion energy and efficiency by 30-50%.
An investigation of the effects of the radiation reaction force on radiation pressure acceleration is presented. Through 1D(3V) PIC code simulations, it is found that radiation reaction causes a decrease in the target velocity during the interaction of an ultra-intense laser pulse with a solid density thin foil of varying thickness. This change in the target velocity can be related to the loss of backwards-directed electrons due to cooling and reflection in the laser field. The loss of this electron population changes the distribution of the emitted synchrotron radiation. We demonstrate that it is the emission of radiation which leads to the observed decrease in target velocity. Through a modification to the light sail equation of motion (which is used to describe radiation pressure acceleration in thin foils), which accounts for the conversion of laser energy to synchrotron radiation, we can describe this change in target velocity. This model can be tested in future experiments with ultra-high intensity lasers, and will lead to a better understanding of the process of relativistically induced transparency in the new intensity regime.
Point-like sources of X-rays that are pulsed (sub nanosecond), high energy (up to several MeV) and bright are very promising for industrial and security applications where imaging through large and dense objects is required. Highly penetrating X-rays can be produced by electrons that have been accelerated by a high intensity laser pulse incident onto a thin solid target. We have used a pulse length of ~10ps to accelerate electrons to create a bright x-ray source. The bremsstrahlung temperature was measured for a laser intensity from 8.5-12×1018 W/cm2. These x-rays have sequentially been used to image high density materials using image plate and a pixelated scintillator system.
We present experimental results, theory, and simulations demonstrating two novel sources of coherent X-ray radiation
generated in the relativistic laser (>1018W/cm2) interaction with easily accessible, repetitive, and debris-free gas jet
targets. The first source is based on a relativistic mirror reflecting a counter-propagating laser pulse. A strongly nonlinear
breaking wake wave driven by an intense laser pulse can act as a semi-transparent relativistic flying mirror. Such a
mirror directly converts counter-propagating laser light into a high-frequency (XUV or X-ray) ultrashort pulse due to the
double Doppler effect. In the experimental demonstration with the 9 TW J-KAREN laser, the flying mirror generated in
a He gas jet partially reflected a 1 TW pulse, providing up to ~1010 photons, 60 nJ (~1012 photons/sr) in the XUV range
(12.8-22 nm). The second source is demonstrated with the laser power ranging from 9 to 170 TW in experiments with
the J-KAREN and Astra Gemini lasers. The odd and even order harmonics generated by linearly as well as circularly
polarized pulses are emitted forward out of the gas jet. The 120 TW laser pulses produce harmonics with ~3×1013photons/sr (~600 μJ/sr) in the 120±5 eV spectral range. The observed harmonics cannot be explained by previously
known mechanisms (atomic harmonics, betatron radiation, nonlinear Thomson scattering, etc.). We introduce a novel
mechanism of harmonic generation based on the relativistic laser-plasma phenomena (self-focusing, cavity evacuation,
bow wave generation), mathematical catastrophe theory which explains the formation of structurally stable electron
density singularities, spikes, and collective radiation of a compact charge driven by a relativistic laser.
M. Borghesi, S. Kar, R. Prasad, F. K. Kakolee, K. Quinn, H. Ahmed, G. Sarri, B. Ramakrishna, B. Qiao, M. Geissler, S. Ter-Avetisyan, M. Zepf, G. Schettino, B. Stevens, M. Tolley, A. Ward, J. Green, P. Foster, C. Spindloe, P. Gallegos, A.. Robinson, D. Neely, D. Carroll, O. Tresca, X. Yuan, M. Quinn, P. McKenna, N. Dover, C. Palmer, J. Schreiber, Z. Najmudin, I. Sari, M. Kraft, M. Merchant, J. C. Jeynes, K. Kirkby, F. Fiorini, D. Kirby, S. Green
In view of their properties, laser-driven ion beams have the potential to be employed in innovative applications in the
scientific, technological and medical areas. Among these, a particularly high-profile application is particle therapy for
cancer treatment, which however requires significant improvements from current performances of laser-driven
accelerators. The focus of current research in this field is on developing suitable strategies enabling laser-accelerated
ions to match these requirements, while exploiting some of the unique features of a laser-driven process. LIBRA is a
UK-wide consortium, aiming to address these issues, and develop laser-driven ion sources suitable for applicative
purposes, with a particular focus on biomedical applications. We will report on the activities of the consortium aimed to
optimizing the properties of the beams, by developing and employing advanced targetry and by exploring novel
acceleration regimes enabling production of beams with reduced energy spread. Employing the TARANIS Terawatt
laser at Queen's University, we have initiated a campaign investigating the effects of proton irradiation of biological
samples at extreme dose rates (> 109 Gy/s).
J. Green, M. Borghesi, C. Brenner, D. Carroll, N. Dover, P. Foster, P. Gallegos, S. Green, D. Kirby, K. J. Kirkby, P. McKenna, M. Merchant, Z. Najmudin, C. A. Palmer, D. Parker, R. Prasad, K. Quinn, P. P. Rajeev, M. Read, L. Romagnani, J. Schreiber, M. Streeter, O. Tresca, C.-G. Wahlström, M. Zepf, David Neely
Next generation intense, short-pulse laser facilities require new high repetition rate diagnostics for the detection of
ionizing radiation. We have designed a new scintillator-based ion beam profiler capable of measuring the ion beam
transverse profile for a number of discrete energy ranges. The optical response and emission characteristics of four
common plastic scintillators has been investigated for a range of proton energies and fluxes. The scintillator light output
(for 1 MeV > Ep < 28 MeV) was found to have a non-linear scaling with proton energy but a linear response to incident
flux. Initial measurements with a prototype diagnostic have been successful, although further calibration work is required
to characterize the total system response and limitations under the high flux, short pulse duration conditions of a typical
high intensity laser-plasma interaction.
Applying Laser-driven Particle Acceleration III: Uses of Distinctive Energetic Particle and Photon Sources
24 April 2023 | Prague, Czech Republic
Applying Laser-driven Particle Acceleration II, Medical and Nonmedical Uses of Distinctive Energetic Particle and Photon Sources: SPIE Optics + Optoelectronics Industry Event
19 April 2021 | Online Only, Czech Republic
Applying Laser-driven Particle Acceleration Workshop: Using Distinctive Energetic Particle and Photon Sources
2 April 2019 | Prague, Czech Republic
Medical Applications of Laser-Generated Beams of Particles: Review of Progress and Strategies for the Future
24 April 2017 | Prague, Czech Republic
Medical Applications of Laser-Generated Beams of Particles: Review of Progress and Strategies for the Future
13 April 2015 | Prague, Czech Republic
Medical Applications of Laser-Generated Beams of Particles II: Review of Progress Made in Recent Years
18 April 2013 | Prague, Czech Republic
Medical Applications of Laser-Generated Secondary Sources of Radiation and Particles
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