We report on measurement techniques of the charge, spectrum, divergence, transverse emittance and the first real-time observation of the accelerated electron pulse and the accelerating plasma wave. Our time-resolved study allows a singleshot measurement of the electron bunch duration providing a value of 5.8 +1.9 -2.1 fs full-width at half maximum (2.5+0.8 -0.9 fs root mean square) as well as the plasma wave with a density-dependent period of 12-22 fs. It reveals the evolution of the bunch, its position in the surrounding plasma wave and the wake dynamics. The results afford promise for brilliant, sub-angstrom-wavelength ultrafast electron and photon sources for diffraction imaging with atomic resolution in space and time.

The normalised transverse emittance is a measure of the quality of an electron beam from a particle accelerator. The brightness, parallelism and focusability are all functions of the emittance. Here we present a high-resolution single shot method of measuring the transverse emittance of a 125 ± 3 MeV electron beam generated from a laser wakefield accelerator (LWFA) using a pepper-pot mask. An average normalised emittance of εrms,x,y = 2.2 ± 0.7, 2.3 ± 0.6 π-mmmrad was measured, which is comparable to that of a conventional linear accelerator. The best measured emittance was εrms,x,=1.1 ± 0.1 π-mm-mrad, corresponding to the resolution limit of our system. The low emittance indicates that this accelerator is suitable for driving a compact free electron laser.

This paper presents the experimental investigation of laser-driven proton acceleration using a table top Ti:Sapphire laser system interacting with the thin-foil targets during the course of medical application of the laser-driven proton beam. The proton beam with maximum energy of upto 14~MeV is generated in 60 TW mode. The number of protons at ~10 MeV is estimated to be over 10⁵ proton/sr/MeV/shot with beam having half divergence angle of 5~degree. If 10 Hz operation is assumed 2 Gy dose is possible to irradiate during 10 min onto a ~1 mm tumor just under the skin. In contrast to the previous condition of our apparatus with which we demonstrated the DNA double-strand breaking by irradiating the laser-driven proton beam onto the human cancer cells in-vitro test, the result reported here has significant meaning in the sense that pre-clinical in-vivo test can be started by irradiating the laser-driven proton beam onto the skin of the mouse, which is unavoidable step before the real radiation therapy.

Recent theoretical work has provided new insight into the physics of Electro-Optic detection of ultrashort relativistic electron beams.¹ Typically, Electro-Optic detection has been restricted to bunches longer than ~ 100 fs. This limitation is due to the transverse optical (TO) phonon resonance that most Electro-Optic materials exhibit in the THz range. Once the electron bunch profile becomes short enough so that a significant portion of its frequency components reside above this resonance frequency, the temporal profile of the space charge field begins to distort as it propagates through the crystal. This distortion becomes more significant as the bunch becomes shorter and destroys the ability of current decoding techniques to resolve the original bunch profile. It is possible to circumvent this issue by realizing that for these higher frequency components it is no longer valid to rely on the formalism of Pockels effect. Instead, sum and difference frequency generation must be taken into account. Using nonlinear three-wave mixing to describe the process, a new technique that promises the order of magnitude increase in resolution necessary to measure the ultrashort bunches produced by laser wakefield accelerators has been developed. This technique provides both phase and amplitude information about the generated pulse from which, in principle, the temporal profile can be reconstructed.

In laser driven accelerators, the interaction of laser radiation with plasma leads to a variety of scattering mechansims. The scattered radiation can be used to understand the wake structure and its effect on electron acceleration. In the case of a resonantly driven quasi-linear wake, spectral broadening due to photon acceleration and deceleration is related to the coupling of energy into plasma waves. Simultaneous time and frequency analysis of the laser fields produces distinctive features in the photon phase space that give information on wake generation in long plasma channels. The ponderomotive guiding center algorithm is advantageous for modeling such interactions because it allows for averaging over optical cycles, and can be implemented in axisymmetric geometry. In the case of the nonlinear wakes that are driven in the self-guided regime, a region of electron cavitation is formed, which emits electro-optic shocks at the second harmonic of the drive laser. The form of this radiation can be correlated with electron trapping.

Laser powered accelerators have been under intensive study for the past decade due to their promise of high gradients and leveraging of rapid technological progress in photonics. Of the various acceleration schemes under examination, those based on dielectric structures may enable the production of relativistic electron beams in breadbox sized systems. When combined with undulators having optical-wavelength periods, these systems could produce high brilliance x-rays which find application in, for instance, medical and industrial imaging. These beams also may open the way for table-top atto-second sciences. Development and testing of these dielectric structures faces a number of challenges including complex beam dynamics, new demands on lasers and optical coupling, beam injection schemes, and fabrication. We describe one approach being pursued at UCLA-the Micro Accelerator Platform (MAP). A structure similar to the MAP has also been designed which produces periodic deflections and acts as an undulator for radiation production, and the prospects for this device will be considered. The lessons learned from the multi-year effort to realize these devices will be presented. Challenges remain with acceleration of sub-relativistic beams, focusing, beam phase stability and extension of these devices to higher beam energies. Our progress in addressing these hurdles will be summarized. Finally, the demands on laser technology and optical coupling will be detailed.

Modern accelerators and light sources subject bunches of charged particles to quasiperiodic motion in extremely high electric fields, under which they may emit a substantial fraction of their energy. To properly describe the motion of these particle bunches, we require a kinetic theory of radiation reaction. We develop such a theory based on the notorious Lorentz-Dirac equation, and explore how it reduces to the usual Vlasov theory in the appropriate limit. As a simple illustration of the theory, we explore the radiative damping of Langmuir waves.

The effects of coherently enhanced radiation reaction on the motion of subwavelength electron bunches in interaction with intense laser pulses are analyzed. The radiation reaction force behaves as a radiation pressure in the laser beam direction, combined with a viscous force in the perpendicular direction. Due to Coulomb expansion of the electron bunch, coherent radiation reaction takes effect only in the initial stage of the laser-bunch interaction while the bunch is still smaller than the wavelength. It is shown that this initial stage can have observable effects on the trajectory of the bunch. By scaling the system to larger bunch charges, the radiation reaction effects are strongly increased. On the basis of the usual equation of motion, this increase is shown to be such that radiation reaction may suppress the radial instability normally found in ponderomotive acceleration schemes, thereby enabling the full potential of laser-vacuum electron bunch acceleration to GeV energies. However, the applicability of the used equation of motion still needs to be validated experimentally, which becomes possible using the presented experimental scheme. For full details, see our paper [P. W. Smorenburg et al., Laser and Particle Beams 28, pp. 553-562, 2010].

One of problems of physics of laser particle acceleration is increase of transformation of laser pulse energy in particle kinetic energy. Changing parametres of a laser target, it is possible to operate such ion characteristics as the maximum and average ion energy, angular divergence and spatial distribution. Rather recently [1], it was revealed, that transformation of laser energy in ion energy increases at use a thin foils limited in size. Occupying smaller effective volume hot electrons have higher density and temperature, accelerating ions more effectively. There is an optimum range for target thickness since too thin targets lead to a warming up a thermal wave of borders of a target, and too thick to electron energy losses. Optimization of some targets under geometrical characteristics was made, for example in [2]. The absorption of laser radiation of such targets reaches considerable values, however are not 100 %. In paper [3] using a periodic micro-relief on a target surface has been shown, that, it is capable to increase absorption of laser radiation up to 90 %. In the present paper it is offered to increase in such way absorption of thin targets and to choose parameters of a relief and basic part of a target so that the additional absorbed energy is transferred mainly to the accelerated protons. The choice of optimum characteristics of a target is made by means of analytical and numerical PIC modeling of a target set with characteristics near to optimum values. The calculations have shown that there is no necessity for ideal periodicity and a regularity of target relief for essential growth of absorption and energy of a proton. Replacement of a regular relief on randomly rough with characteristic scale comparable with regular considerably does not reduce neither absorption, nor energy of the accelerated ions.

Radiation pressure acceleration (RPA) theoretically may have great potential to revolutionize the study of laserdriven ion accelerators due to its high conversion efficiency and ability to produce high-quality monoenergetic ion beams. However, the instability issue of ion acceleration has been appeared to be a fundamental limitation of the RPA scheme. To solve this issue is very important to the experimental realization and exploitation of this new scheme. In our recent work, we have identified the key condition for efficient and stable ion RPA from thin foils by CP laser pulses, in particular, at currently available moderate laser intensities. That is, the ion beam should remain accompanied with enough co-moving electrons to preserve a local "bunching" electrostatic field during the acceleration. In the realistic LS RPA, the decompression of the co-moving electron layer leads to a change of local electrostatic field from a "bunching" to a "debunching" profile, resulting in premature termination of acceleration. One possible scheme to achieve stable RPA is using a multi-species foil. Two-dimensional PIC simulations show that 100 MeV/u monoenergetic C⁶⁺ and/or proton beams are produced by irradiation of a contaminated copper foil with CP lasers at intensities 5 × 10²⁰W/cm², achievable by current day lasers.

It has been found that more intense proton beams are generated from plastic foils than metal foils irradiated by an ultraintense laser pulse. The acceleration model, ARIE (Acceleration by a Resistively Induced Electric field) accounts for the experimental observations from plastic foils compared with metal foils. Proton beams on foil thickness and laser prepulse have been observed, which is also well described by the ARIE model. An experiment with an aluminum-coated plastic target strongly suggests that front side acceleration is a dominant acceleration process in plastic targets. We also suggest that a vacuum-sandwiched double layer target could effectively enhance the laser contrast ratio, which was investigated in the combination of a two-dimensional hydro code and a two-dimensional PIC (Particle-In-Cell) code.

Interaction of an ultra-short intense laser pulses with thin foil targets is accompanied by acceleration of ions from the target surface. To make this ion source suitable for applications, it is of particular importance to increase the efficiency of laser energy transformation into accelerated ions and the maximum ion energy. This can be achieved by using thin foil target with a layer of microscopic spheres on the front, laser irradiated surface. The influence of microscopic structure on the target surface on the laser target interaction and subsequent ion acceleration is studied here using numerical simulations. The influence of the size of microspheres, the density profile and the laser pulse incidence angle are studied as well.

We have developed a 2.5 cell, 3 GHz RF accelerator specifically to inject electrons in a laser wakefield accelerator (LWA). The electron bunches are accelerated to around 3.5 MeV and focused at 1.14 m from the cathode of the accelerator using a pulsed solenoid. Bunches between 0 and 33 pC were focused onto a phosphor screen at the position of the entrance of a plasma channel. The (RMS) bunch size was 32 μm at 1 pC and increases to 61 μm at 33 pC. The energy of the bunches at the chosen settings was measured to be 3.71 MeV with 0.02 MeV energy spread (at 10 pC). Energy fluctuations were less than 2 keV. The pointing stability of the focused electron bunches was determined from 100 consecutive shots at 1 Hz to be 5 μm (RMS). GPT (General Particle Tracer) simulations have been performed using the measured bunches as input for LWA. The simulations show that up to 1 pC of charge can be accelerated to energies of around 1 GeV using realistic plasma and laser parameters. The measured bunch parameters in combination with the simulations show how external injection of pre-accelerated electrons can be a viable alternative to other injection mechanisms.

The ponderomotive force is derived for a relativistic charged particle entering an electromagnetic standing wave with a general three-dimensional field distribution and a nonrelativistic intensity, using a perturbation expansion method. It is shown that the well-known ponderomotive gradient force expression does not hold for this situation. The modified expression is still of simple gradient form, but contains additional polarization-dependent terms. These terms arise because the relativistic translational velocity induces a quiver motion in the direction of the magnetic force, which is the direction of large field gradients. Oscillation of the Lorentz factor effectively doubles this magnetic contribution. The derived ponderomotive force generalizes the polarization-dependent electron motion in a standing wave obtained earlier [A. E. Kaplan and A. L. Pokrovsky, Phys. Rev. Lett. 95, p. 053601, 2005]. Comparison with simulations in the case of a realistic, non-idealized, three-dimensional field configuration confirms the general validity of the analytical results.

The theoretical research works on proton and ion acceleration done by our group are reviewed. A complex target consisting of a front horizontal slice adjoining a conventional heavy ion and proton double-layer slab are used to produce more-fast moving hot electrons to enhance the target-normal sheath acceleration (TNSA) so that the protons in the proton layer can be accelerated to energies more than three times, and the energy spread halved, that from the simple double-layer slab. A sandwich target design with a thin compound ion layer between two light-ion layers and a micro-structured target design are proposed for obtaining efficiently monoenergetic heavy-ion beams. Radiation pressure acceleration from multi stage shock acceleration to continuous light sail acceleration for ultra-thin foils is used to generate GeV protons. The foil thickness for light pressure acceleration is studied. Quasi-single-cycle relativistic laser pulse is generated with an accelerated flying foil. In order to accelerate protons to higher energy up to TeV, wake acceleration is proposed. Mixed plasma is used to enhance the wake and radiation pressure acceleration is used to help protons to be easily trapped in the wake.

The interaction of femtosecond laser pulses with submicron water clusters is studied by two-dimensional particle-in-cell simulations. We search for optimum laser and cluster parameters to obtain quasimonoenergetic beam of protons accelerated from the cluster. For the laser amplitude a₀ = 3, the optimum cluster size is about 150 nm for the generation of pronounced peak in proton energy distribution function at maximum energy and the optimum laser pulse duration is about 40 - 80 fs. Various initial density profiles of cluster plasma, formed due to insufficient laser pulse contrast and prepulses, are involved in this study, including underdense clusters.

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 > E_p < 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.

Proton beams are more advantageous than high-energy photons and electrons for radiation therapy because of their finite penetrating range and the Bragg peak near the end of their range, which have been utilized to achieve better dose conformity to the treatment target allowing for dose escalation and/or hypofractionation to increase local tumor control, reduce normal tissue complications and/or treatment time/cost. Proton therapy employing conventional particle acceleration techniques is expensive because of the large accelerators and treatment gantries that require excessive space and shielding. Compact proton acceleration systems are being sought to improve the cost-effectiveness for proton therapy. This paper reviews the physics principles of laser-proton acceleration and the development of prototype laserproton therapy systems as a solution for widespread applications of advanced proton therapy. The system design, the major components and the special delivery techniques for energy and intensity modulation are discussed in detail for laser-accelerated proton therapy.

Human cancer cells are irradiated by laser-driven quasi-monoenergetic protons. Laser pulse intensities at the 5×10¹⁹-W/cm² level provide the source and acceleration field for protons that are subsequently transported by four energy-selective dipole magnets. The transport line delivers 2.25 MeV protons with an energy spread of 0.66 MeV and a bunch duration of 20 ns. The survival fraction of in-vitro cells from a human salivary gland tumor is measured with a colony formation assay following proton irradiation at dose levels up to 8 Gy, for which the single bunch does rate is 1 × 10⁷ Gy/s and the effective dose rate is 0.2 Gy/s for 1-Hz repetition of irradiation. Relative biological effectiveness at the 10% survival fraction is measured to be 1.20 ± 0.11 using protons with a linear energy transfer of 17.1 ± 2.8 keV/μm.

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).

It is widely accepted that proton or light ion beams may have a high potential for improving cancer cure by means of radiation therapy. However, at present the large dimensions of electromagnetic accelerators prevent particle therapy from being clinically introduced on a broad scale. Therefore, several technological approaches among them laser driven particle acceleration are under investigation. Parallel to the development of suitable high intensity lasers, research is necessary to transfer laser accelerated particle beams to radiotherapy, since the relevant parameters of laser driven particle beams dramatically differ from those of beams delivered by conventional accelerators: The duty cycle is low, whereas the number of particles and thus the dose rate per pulse are high. Laser accelerated particle beams show a broad energy spectrum and substantial intensity fluctuations from pulse to pulse. These properties may influence the biological efficiency and they require completely new techniques of beam delivery and quality assurance. For this translational research a new facility is currently constructed on the campus of the university hospital Dresden. It will be connected to the department of radiooncology and host a petawatt laser system delivering an experimental proton beam and a conventional therapeutic proton cyclotron. The cyclotron beam will be delivered on the one hand to an isocentric gantry for patient treatments and on the other hand to an experimental irradiation site. This way the conventional accelerator will deliver a reference beam for all steps of developing the laser based technology towards clinical applicability.

We study the production of radioisotopes for nuclear medicine in (γ, γ) photoexcitation reactions or (γ, xn+yp) photonuclear reactions for the examples of ^195mPt,^117mSn and ⁴⁴Ti with high flux [(10¹³ - 10¹⁵)γ/s], small beam diameter and small energy band width (ΔE/E ≈ 10^-3 - 10^-4) γ beams. In order to realize an optimum γ-focal spot, a refractive γ-lens consisting of a stack of many concave micro-lenses will be used. It allows for the production of a high specific activity and the use of enriched isotopes. For photonuclear reactions with a narrow γ beam, the energy deposition in the target can be reduced by using a stack of thin target wires, hence avoiding direct stopping of the Compton electrons and e⁺e^- pairs. The well-defined initial excitation energy of the compound nucleus leads to a small number of reaction channels and enables new combinations of target isotope and final radioisotope. The narrow-bandwidth γ excitation may make use of collective resonances in γ-width, leading to increased cross sections. (γ, γ) isomer production via specially selected γ cascades allows to produce high specific activity in multiple excitations, where no back-pumping of the isomer to the ground state occurs. The produced isotopes will open the way for completely new clinical applications of radioisotopes. For example ^195mPt could be used to verify the patient's response to chemotherapy with platinum compounds before a complete treatment is performed. In targeted radionuclide therapy the short-range Auger and conversion electrons of ^195mPt and ^117mSn enable a very local treatment. The generator ⁴⁴Ti allows for a PET with an additional γ-quantum (γ-PET), resulting in a reduced dose or better spatial resolution.

We are exploring the use of the ultra-high contrast 200 TW ALLS facility (5 J, 28 fs, 10 Hz repetition rate) as a basic tool to image in real time with X-rays (generated by the laser) tumors during their irradiation by protons (accelerated by the same laser). The feasibility of phase contrast imaging in in-line geometry and proton acceleration with 100 TW (3 J, 30 fs) on targets is studied and presented in the present paper. We demonstrate here that phase contrast x-ray imaging, of tests and complex objects located in air at 1m from the X-ray source, can be achieved in one shot using our betatron x-ray source generated in a supersonic gas jet. Using solid targets (thin and thick foils) our experiments indicate that protons are accelerated at a maximum energy of 12 MeV.

Recently, a high efficiency regime of acceleration in laser plasmas has been discovered, allowing table top equipment to deliver doses of interest for radiotherapy with electron bunches of suitable kinetic energy. In view of an R&D program aimed to the realization of an innovative class of accelerators for medical uses, a radiobiological validation is needed. At the present time, the biological effects of electron bunches from the laser-driven electron accelerator are largely unknown. In radiobiology and radiotherapy, it is known that the early spatial distribution of energy deposition following ionizing radiation interactions with DNA molecule is crucial for the prediction of damages at cellular or tissue levels and during the clinical responses to this irradiation. The purpose of the present study is to evaluate the radio-biological effects obtained with electron bunches from a laser-driven electron accelerator compared with bunches coming from a IORT-dedicated medical Radio-frequency based linac's on human cells by the cytokinesis block micronucleus assay (CBMN). To this purpose a multidisciplinary team including radiotherapists, biologists, medical physicists, laser and plasma physicists is working at CNR Campus and University of Pisa. Dose on samples is delivered alternatively by the "laser-linac" operating at ILIL lab of Istituto Nazionale di Ottica and an RF-linac operating for IORT at Pisa S. Chiara Hospital. Experimental data are analyzed on the basis of suitable radiobiological models as well as with numerical simulation based on Monte Carlo codes. Possible collective effects are also considered in the case of ultrashort, ultradense bunches of ionizing radiation.

Recent progress in using picosecond CO₂ lasers for Thomson scattering and ion-acceleration experiments underlines their potentials for enabling secondary radiation- and particle- sources. These experiments capitalize on certain advantages of long-wavelength CO₂ lasers, such as higher number of photons per energy unit, and favorable scaling of the electrons' ponderomotive energy and critical plasma density. The high-flux x-ray bursts produced by Thomson scattering of the CO₂ laser off a counter-propagating electron beam enabled high-contrast, time-resolved imaging of biological objects in the picosecond time frame. In different experiments, the laser, focused on a hydrogen jet, generated monoenergetic proton beams via the radiation-pressure mechanism. The strong power-scaling of this regime promises realization of proton beams suitable for laser-driven proton cancer therapy after upgrading the CO₂ laser to sub-PW peak power. This planned improvement includes optimizing the 10-μm ultra-short pulse generation, assuring higher amplification in the CO₂ gas under combined isotopic- and power-broadening effects, and shortening the postamplification pulse to a few laser cycles (150-200 fs) via chirping and compression. These developments will move us closer to practical applications of ultra-fast CO₂ lasers in medicine and other areas.

Compact sources of high energy protons (50-500MeV) are expected to be key technology in a wide range of scientific applications ^1-8. Particularly promising is the target normal sheah acceleration (TNSA) scheme ^9,10, holding record level of 67MeV protons generated by a peta-Watt laser ¹¹. In general, laser intensity exceeding 10¹⁸ W/cm² is required to produce MeV level protons. Enhancing the energy of generated protons using compact laser sources is very attractive task nowadays. Recently, nano-scale targets were used to accelerate ions ^12,13. Here we report on the first generation of 5.5-7.5MeV protons by modest laser intensities (4.5 × 10¹⁷ W/cm²) interacting with H₂O nano-wires (snow) deposited on a Sapphire substrate. In this setup, the plasma near the tip of the nano-wire is subject to locally enhanced laser intensity with high spatial gradients, and confined charge separation is obtained. Electrostatic fields of extremely high intensities are produced, and protons are accelerated to MeV-level energies. Nano-wire engineered targets will relax the demand of peak energy from laser based sources.

Charged Particle Therapy (the use of protons and other light ions such as carbon to treat certain forms of cancer) is experiencing a rapid expansion in many parts of the world, and there are now more than 30 such centres operating in hospitals. The current technologies available use cyclotrons and synchrotrons to deliver the dose to the cancer. While each of these technologies is mature, and capable of treating cancer successfully, there is always room for improvement in technique, to reduce costs, increase throughput and availability and improve outcomes. This talk will discuss some recent development, using both traditional and laser-based accelerator techniques.