Valery Yu. Bychenkov

Accelerating protons to therapeutic energies with ultraintense, ultraclean, and ultrashort laser pulses

Proton acceleration by high-intensity laser pulses from ultrathin foils for hadron therapy is discussed. With the improvement of the laser intensity contrast ratio to 10(-1) achieved on the Hercules laser at the University of Michigan, it became possible to attain laser-solid interactions at intensities up to 10(22) W/cm2 that allows an efficient regime of laser-driven ion acceleration from submicron foils. Particle-in-cell (PIC) computer simulations of proton acceleration in the directed Coulomb explosion regime from ultrathin double-layer (heavy ions/light ions) foils of different thicknesses were performed under the anticipated experimental conditions for the Hercules laser with pulse energies from 3 to 15 J, pulse duration of 30 fs at full width half maximum (FWHM), focused to a spot size of 0.8 microm (FWHM). In this regime heavy ions expand predominantly in the direction of laser pulse propagation enhancing the longitudinal charge separation electric field that accelerates light ions. The dependence of the maximum proton energy on the foil thickness has been found and the laser pulse characteristics have been matched with the thickness of the target to ensure the most efficient acceleration. Moreover, the proton spectrum demonstrates a peaked structure at high energies, which is required for radiation therapy. Two-dimensional PIC simulations show that a 150-500 TW laser pulse is able to accelerate protons up to 100-220 MeV energies.

Generation of GeV protons from 1 PW laser interaction with near critical density targets.

The propagation of ultraintense laser pulses through matter is connected with the generation of strong moving magnetic fields in the propagation channel as well as the formation of a thin ion filament along the axis of the channel. Upon exiting the plasma the magnetic field displaces the electrons at the back of the target, generating a quasistatic electric field that accelerates and collimates ions from the filament. Two dimensional particle-in-cell simulations show that a 1 PW laser pulse tightly focused on a near-critical density target is able to accelerate protons up to an energy of 1.3 GeV. Scaling laws and optimal conditions for proton acceleration are established considering the energy depletion of the laser pulse.

Nonthermal tails of the electron distribution functions with nonlocal transport

Quasi-self-similar solutions to the stationary electron Fokker-Planck equation in diffusive approximation have been found in inhomogeneous plasma. These solutions describe reduction in the number of bulk electrons and formation of the suprathermal tail. The characteristics of the stationary electron distributions have been treated in terms of the collisionality parameter, the ratio of the electron stoping rage to the plasma gradient scale length. The dependencies of the electron distribution functions on density profile has been studied. Fokker-Plank simulations performed demonstrate good agreement with a theory.

Optimization of a Laser-Based Proton Source and a New Mechanism of Ion Acceleration

The 3-D particle-in-cell simulations with the code MANDOR demonstrate effective ion acceleration from the interaction of intense ultrashort linearly polarized laser pulses with both ultrathin solid dense foils and low-density targets when the laser energy ranges from several millijoules to tens of joules. The optimum foil thickness and the corresponding maximum energy of the accelerated ions for a given energy of the laser pulse were found. Different mechanisms of ion acceleration, such as target normal sheath acceleration, directed Coulomb explosion, and ponderomotive acceleration, are involved. We discuss the transition from one acceleration regime to another when the target thickness and density and the laser pulse intensity change and show that reducing the target density significantly increases the ion energy. We present some examples of isotope production as a possible application of multimegaelectronvolt proton beams on the joule scale of a laser.

Ion acceleration from laser-irradiated thin targets

Summary form only given. The interaction of short intense laser pulses with solid targets allows record-breaking ion energies to be attained at the laboratory scale. It has already been shown, that significant increase of ion energy arises in laser pulse interaction with ultrathin foil. Recent improvements of the intensity contrast ratio of pulses and advances in the technology of producing ultrathin targets warrant laser pulse interaction with ultrathin foils to be of practical interest. The electrons of the foil are capable of being heated up to MeV energies and form extended halo near the ion core. This triggers plasma expansion into a vacuum that provides acceleration of ions. A strong laser field can even knock out all electrons from a thin target that causes Coulomb explosion. Quasineutral plasma outflow and the regime of plasma expansion with charge separation effects in collisionless isothermal expansion of a semi-bounded plasma have been theoretically studied in great detail1,2. However, at high electron energy (temperature) the model of semi-bounded plasma becomes inapplicable as far as the electron Debye length approaches the foil thickness. This is why analytical theory of plasma slab expansion is of high demand. From the other hand, analytically well studied regime of ion acceleration from plasma foil is a Coulomb explosion3. Going beyond previous studies we have developed a theory of plasma slab expansion into a vacuum with the Boltzmanns electrons for arbitrary electron temperature. The electron temperature is a controlling parameter of our theory and matches laser intensity. By increasing it our theory smoothly switches from the quasineutral expansion approach to the Coulomb explosion limit. We derived energetic characteristics of the accelerated ions for arbitrary value of electron temperature. In the limits of very small Debye radius and very large ones our theory agrees with known results for quasineutral outflow and Coulomb explosion2,3. The validity of the theory is proven by comparison with kinetic simulations4. We also compared theoretical results with 3D kinetic PIC simulations of ion acceleration triggered by relativistic laser interaction with thin foils. Qualitative agreement has been demonstrated.

SU‐GG‐T‐462: Observation of Quasi‐Monoenergetic Laser Accelerated Proton and Carbon Beams

Purpose: To perform preliminary experiments to achieve the Directed Coulomb Explosion (DCE) regime of proton acceleration to therapeutic energies in high‐intensity laser‐matter interactions. Method and Materials: Particle‐in‐Cell (PIC) simulations of the planned experiments at HERCULES laser at the University of Michigan have predicted a new regime of attainable laser‐target interactions for proton acceleration. The laser was recently upgraded to 300 TW with Amplified Spontaneous Emission(ASE) intensity contrast ratio of 10−11, allowing intensities of 2×1022 W/cm2 to be achieved in a near diffraction limited, 1.3 micron, focal spot. Dual plasma mirrors have been installed and characterized to reduce the prepulse at < 30 ps (from the uncompensated dispersion of optical elements during the pulse compression) before the main pulse providing 3 orders of magnitude contrast improvement. This allowed experiments on thin foil membranes (50 nm) with 50TW temporally clean pulses without compromising the target. Results: We found for the first time that for all target thicknesses proton spectra exhibit quasi‐monoenergetic features, which are more pronounced for ultra‐thin (50 nm Si3N4) targets resulted in AE/E∼30%. Moreover for these Si3N4 targets spectra for all the charge states of carbon ions C3+‐C6+ are also found to be quasi‐monoenergetic. Maximum proton energy drops from 6 MeV for 1 ⌈ m Mylar foil to 4 MeV for 50 nm Si3N4 membranes. Conclusion: Implementation of dual plasma mirrors substantially improved laser contrast and created more favorable proton and carbon flux‐energy distributions. Further improvements to the plasma mirrors are required, using better antireflection coatings on glass substrates, to achieve the DCE regime of proton acceleration.

MO‐EE‐A2‐05: Experimental Implementation of the Directed Coulomb Explosion Regime of Laser‐Proton Acceleration

Purpose: To quantify the laser pulse requirements and target parameters required to achieve the Directed Coulomb Explosion (DCE) regime of laser‐target interaction for the acceleration of protons to therapeutic energies. Method and Materials: Particle‐in‐Cell (PIC) simulations of the planned experiments along the funded upgrade path of the HERCULES laser at the University of Michigan have predicted a new regime of attainable laser‐target interactions for proton acceleration. The laser was recently upgraded to 300 TW and a temporal pulse contrast ratio of 10−11, allowing intensities of 2×1022 W/cm2 to be achieved in a near diffraction limited, 1.3 micron, focal spot. The 2 ns long amplified spontaneous emission(ASE) pre‐pulse was suppressed by a factor 10−3 through the implementation of a cross‐polarized wave (XPW) pulse cleaner to prevent pre‐plasma creation on the front surface and preserve the physical integrity of the thin‐film target. Dual plasma mirrors are being characterized to reduce the prepulse at < 30 ps before the main pulse (caused by variations in the index of refraction through the optic path) below a contrast ratio of 10−11. This will allow experiments on thin foil targets (< 100 nm) up to 300TW with no significant pre‐pulse to compromise the target. Results: Preliminary measurements show an additional reduction of the contrast ratio of the ASE pre‐pulse by a factor of 10−3 after the addition of dual plasma mirrors. Additional work is required to optimize the setup and parameters of the plasma mirrors to account for polarization effects and wave front distortions and variable intensity levels. Conclusion: Implementation of dual plasma mirrors is progressing with promising results and will soon allow experimental implementation of the laser pulse characteristics required to test the DCE regime of proton acceleration.

TH‐C‐230A‐06: High‐Energy Proton Acceleration Driven by Ultra‐Intense Ultra‐Clean Laser Pulses

Purpose: To improve the contrast ratio of the multi‐terawatt Chirped‐Pulse Amplification (CPA) Ti:Sapphire laser to 1011 to allow Coulomb explosion regime of ion acceleration in the interaction of ultra‐short high‐intensity laser pulses with ultra‐thin ( < 1 micron) foils. Method and Materials: The cross‐polarized wave generation (XPW) technique in BaF2 crystals was implemented. This technique improves contrast by rejecting the low‐intensity amplified spontaneous emission(ASE) preceding the main laser pulse. Particle‐in‐cell (PIC) simulations were conducted under the anticipated experimental conditions: 225 TW in a 6.75 J, 30 fs laser pulse with no prepulse, focused to a spot size of 2.4 microns (FWHM) on thin foils of varying thickness. Results: Implementation of the crosspolarized wave generation technique resulted in a contrast improvement of three orders of magnitude to approximately 1011. The performed PIC simulations show that for a 0.2 μm thick hydrogen foil, protons with energy of about 200 MeV can be generated. In the case of the two‐layer aluminum‐hydrogen foil the maximum energy of accelerated protons is about 150 MeV, but the proton spectrum has a flatter distribution, which may be more advantageous for therapy applications. Conclusion: We demonstrated that pulse cleaning based on cross‐polarized wave generation (XPW) using two BaF2 crystals yields a 1011 contrast ratio for a 50 TW laser system. Such contrast may be sufficient for a preplasma‐free interaction of sub‐Petawatt laser pulses with a sub‐micron thickness foils at intensity of ∼1022 W/cm2. Modeling of this interaction with PIC simulations demonstrated protonenergies that are of interest for the radiation therapy. This study was supported by the National Science Foundation through the Frontiers in Optical and Coherent Ultrafast Science Center at the University of Michigan and the National Institute of Health.