Optimizing laser-driven proton acceleration from overdense targets

The discovery that ultra-intense laser pulses (I > 1018 W/cm2) can produce short pulse, high energy proton beams has renewed interest in the fundamental mechanisms that govern particle acceleration from laser-solid interactions. Experiments have shown that protons present as hydrocarbon contaminants on laser targets can be accelerated up to energies > 50 MeV. Different theoretical models that explain the observed results have been proposed. One model describes a front-surface acceleration mechanism based on the ponderomotive potential of the laser pulse. At high intensities (I > 1018 W/cm2), the quiver energy of an electron oscillating in the electric field of the laser pulse exceeds the electron rest mass, requiring the consideration of relativistic effects. The relativistically correct ponderomotive potential is given by Up = ([1 + Iλ2/1.3 x 1018]1/2 - 1) m{sub o}c2, where Iλ2 is the irradiance in W μm2/cm2 and moc2 is the electron rest mass. At laser irradiance of Iλ2 ~ 1020 W μm2/cm2, the ponderomotive potential can be of order several MeV. A few recent experiments--discussed in Chapter 3 of this thesis--consider this ponderomotive potential sufficiently strong to accelerate protons from the front surface of the target to energies up to tens of MeV. Another model, known as Target Normal Sheath Acceleration (TNSA), describes the mechanism as an electrostatic sheath on the back surface of the laser target. According to the TNSA model, relativistic hot electrons created at the laser-solid interaction penetrate the foil where a few escape to infinity. The remaining hot electrons are retained by the target potential and establish an electrostatic sheath on the back surface of the target. In this thesis we present several experiments that study the accelerated ions by affecting the contamination layer from which they originate. Radiative heating was employed as a method of removing contamination from palladium targets doped with deuterium. We present evidence that ions heavier than protons can be accelerated if hydrogenous contaminants that cover the laser target can be removed. We show that deuterons can be accelerated from the deuterated-palladium target, which has been radiatively heated to remove contaminants. Impinging a deuteron beam onto a tritiated-titanium catcher could lead to the development of a table-top source of short-pulse, 14-MeV fusion neutrons. We also show that by using an argon-ion sputter gun, contaminants from one side of the laser target can be selectively removed without affecting the other side. We show that irradiating a thin metallic foil with an ultra-intense laser pulse produces a proton beam with a yield of 1.5-2.5 1011 and temperature, kT = 1.5 MeV with a maximum proton energy > 9 MeV. Removing contaminants from the front surface of the laser target with an argon-ion sputter gun, had no observable effect on the proton beam. However, removing contaminants from the back surface of the laser target reduced the proton beam by two orders of magnitude to, at most, a yield of ~ 109 and a maximum proton energy < 4 MeV. Based on these observations, we conclude that the majority (> 99%) of high energy protons (E > 5 MeV) from the interaction of an ultra-intense laser pulse with a thin foil originate on the back surface of the foil--as predicted by the TNSA model. Our experimental results are in agreement with PIC simulations showing back surface protons reach energies up to 13 MeV, while front surface protons reach a maximum energy of 4 MeV. Well diagnosed and controllable proton beams will have many applications: neutron radiography, material damage studies, production of medical isotopes, and as a high-resolution radiography tool for diagnosing opaque materials and plasmas. Well collimated and focusable ion beams may also prove beneficial for alternative inertial-fusion concepts such as proton fast ignition, a potentially viable method for achieving a controlled fusion reaction in the laboratory earlier than expected.

The interaction of short and intense laser pulses with plasmas or solids is a very efficient source of high-energy ions. This paper reports the detailed study, with particle-in-cell simulations, of the interaction of such a laser pulse with thin, dense targets, and the resulting proton acceleration. Depending on the laser intensity and pulse duration, the most energetic protons are found to come from the front, the core, or the back of the target. The main accelerating mechanisms discussed in this paper are plasma expansion acceleration, where proton acceleration is driven by the hot electron population, and shock acceleration, originating from the laser ponderomotive potential imposed at the front target surface. Three main regimes of proton acceleration are defined and the parameters for which each regime is dominant are obtained. For irradiances close to 1020W∕cm2, the highest proton energies are obtained from thin foils efficiently heated by relativistic transparency. At larger intensities, a complex interplay between collisionless shock acceleration and plasma expansion acceleration is evidenced.

Proton emission from solid foil targets irradiated by relativistically intense femtosecond laser pulses is studied experimentally. Broad plateaus in energy spectra are measured from micron-thick targets when the incident laser pulses have relatively low intensity contrasts. It is proposed that such proton spectra can be attributed to the combined processes of laser-driven collisionless shock acceleration and target normal sheath acceleration. Simple analytic estimation and two-dimensional particle-in-cell simulations are performed, which support our interpretation. The obtained plateau-shape spectrum may also serve as an effective tool to diagnose the plasma state and verify the ion acceleration mechanisms in laser-solid interactions.

Laser-driven particle acceleration has been under intense investigation for the last decade and is of particular interest for key applications such as medical physics, fast ignition and inertial confinement fusion. The dominant ion acceleration mechanism is known as target normal sheath acceleration (TNSA); although easy accessible it suffers from a low energy conversion efficiency. Recently reported maximum ion energies1 are in the range of a few tens of MeV, which is well below the energies needed for most applications. Recently proposed acceleration mechanisms utilize ultra-high contrast laser pulses and ultra-thin solid targets allowing for generation of ions in the GeV range from intense laser-plasma interaction. A high laser contrast suppresses pre-ionization of the solid target by any pre-pulses or laser pedestals and in theory enables acceleration beyond TNSA in regimes such as the break-out afterburner (BOA)2 or radiation pressure acceleration (RPA).

Target normal sheath acceleration (TNSA) by self-guided laser pulses is demonstrated at a range of Al target thickness (2-10 microns), and proton spectra are compared with those accelerated by un-guided laser pulses. For un-guided pulses, the maximum proton energy decreases with the decrease of target thickness, exhibiting the detrimental effect of laser pre-pulse on TNSA. On the other hand, the dependence of proton energy on target thickness was not observed when using self-guided laser pulses, which is possibly due to the suppressed pre-pulse intensity. However the laser pulse appears to lose a significant amount of energy (>~50%) during ~4mm self-guiding in the plasma, and thus the accelerated proton energy is much lower than the case of unguided pulses.

Energetic protons and ions can be produced by the irradiation of a μm-thick solid target with an intense laser pulse via Target Normal Sheath Acceleration (TNSA). Typically the maximum proton energy increases with decreasing target thickness. However, amplified spontaneous emission (ASE) from the drive laser system may alter thinner targets before arrival of the main pulse and limit the attainable proton energy. Here we experimentally test a method of reducing ASE intensity and increasing TNSA proton energy by using relativistic self-guiding in an underdense plasma.

Ion acceleration resulting from the interaction of 11 fs laser pulses of 35 mJ energy with ultrahigh contrast (<10−10) and 1019 W cm−2 peak intensity with foil targets made of various materials and thicknesses at normal (0°) and 45° laser incidence is investigated. The maximum energy of the protons reached ∼1.4 MeV accelerated in the laser propagation direction and ∼1.2 MeV in the opposite direction from a formvar target. The energy conversion efficiency from the laser to the proton beam is estimated to be as high as ∼1.4% at 45° laser incidence using a 51 nm thick target. The high laser contrast indicates the predominance of vacuum heating via Brunel’s effect as an absorption mechanism involving a tiny pre-plasma at the target front. The experimental results are in reasonable agreement with theoretical estimates, where proton acceleration from the target front side in the backward direction is well explained by the Coulomb explosion of a charged cavity formed in a tiny pre-plasma, while forward proton acceleration is likely to be a two-step process: protons are first accelerated in the target front-side cavity and then further boosted in energy through the target back side via the target normal sheath acceleration (TNSA) mechanism.

Thin foils of graphene oxide (GO) are irradiated by a fs titanium sapphire laser at an intensity of about 1019 W/cm2 in high vacuum. The produced plasma in the forward direction accelerates ions in a regime of target-normal-sheath-acceleration, thanks to the relativistic electron emission from the target surface and to their emission from the rear foil surface, generating a high electric field pulse with the positive target. The ion acceleration is measured mainly using SiC detectors in the time-of-flight configuration. Adding gold as nanoparticles or as a thin coverage film, the ion acceleration is enhanced as a result of a higher plasma electron density. The optimal acceleration is reached by varying the GO thickness, the Au nanoparticle concentration, the thin Au film thickness, and the irradiation conditions. Particularly important is the laser focal position with respect to the target surface, which is responsible for different acceleration values. In the used experimental conditions, a maximum proton energy of 2.6 MeV was obtained and the best modality to add Au atoms to the target is discussed.

Compared to former laser ion acceleration schemes like target normal sheath acceleration (TNSA) [1], the laser acceleration from ultra-thin diamond-like carbon (DLC) foils is more efficient and for the high-power short-pulse laser ATLAS proton energies up to 100 MeV are expected [2]. Also for the generation of very dense relativistic electron bunches the use of ultra-thin diamond foils leads to much better results [3] than for laser bubble acceleration [4]. By reflection of coherent electromagnetic fields from these relativistic electron bunches it seems possible to generate brilliant, intense X-ray beams [5]. In the longer term we plan to use the laser-driven ion beams for cancer therapy and the X-ray beams in medical diagnostics.We describe the present status and the expected beam properties for the upgraded ATLAS laser at MPQ (Garching) and the setup of our medical beam line.

The impact of contaminants on laser-driven ion acceleration is investigated using particle-in-cell simulations. The conventional ion acceleration mechanism, target normal sheath acceleration, has been revisited for targets with proton-rich contaminants in the form of water vapor. The targets considered have a deuterated plastic layer on the rear surface of an aluminum target, and the influence of the contaminant layer on the deuteron acceleration is investigated. In the early stage of ion acceleration, the space-charge electrostatic field on the rear target surface accelerates only the outermost, proton-rich layer of ions, which inhibits the deuteron acceleration by shielding it from the field. When the proton layer is depleted, the deuterons become exposed to the space-charge field and are promptly accelerated. This scenario was tested with a two-dimensional particle-in-cell simulation model by varying the contaminant layer thickness and laser fluence (laser energy per unit area). For laser fluences Flaser&amp;lt;1 J/μm2, the contamination layer over the surface inhibits the deuteron acceleration from the rear surface, while in the opposite case of laser fluences Flaser&amp;gt;1 J/μm2 deuterons and heavier ions can be successfully accelerated with conversion efficiency of laser energy into ions of more than 1%. Experimental data from a 6 μm thick aluminum foil coated with a 1 μm deuterated plastic layer on the back surface are suggestive of the detrimental role of contaminants on deuteron acceleration.

Many parameters of non-equilibrium plasma generated by high intensity and fast lasers depend on the pulse intensity and the laser wavelength. In conditions favourable for the target normal sheath acceleration (TNSA) regime the ion acceleration from the rear side of the target can be enhanced by increasing the thin foil absorbance through the use of nanoparticles and nanostructures promoting the surface plasmon resonance effect. In conditions favourable for the backward plasma acceleration (BPA) regime, when thick targets are used, a special role is played by the laser focal position with respect to the target surface, a proper choice of which may result in induced self-focusing effects and non-linear acceleration enhancement. SiC detectors employed in the time-of-flight (TOF) configuration and a Thomson parabola spectrometer permit on-line diagnostics of the ion streams emitted at high kinetic energies. The target composition and geometry, apart from the laser parameters and to the irradiation conditions, allow further control of the plasma characteristics and can be varied by using advanced targets to reach the maximum ion acceleration. Measurements using advanced targets with enhanced the laser absorption effect in thin films are presented. Applications of accelerated ions in the field of ion source, hadrontherapy and nuclear physics are discussed.

The first laser ion acceleration experiments have begun with a method based on the TNSA (target normal sheath acceleration) that did not have an adiabatic process of the accelerating structure created by laser. A great deal of efforts have been dedicated to ever increase the adiabatic process of the accelerating structure with the ions since then. This acquiring of adiabaticity may be done by reducing the group velocity of the laser significantly to match that of ions, or to increase the ion velocity as fast as possible in order to match the fast accelerating structure created by laser. The methods of RPA (radiation pressure acceleration) and CAIL (coherent acceleration of ions by laser) belong to the latter. Finally, the recent entry of the single-cycle laser pulse acceleration (SCPA) marks another notch in the progress to make ion acceleration condition even more accessible and the acceleration results far more attractive by eradicating the oscillatory interference effects.

Some applications of fast ions driven by a short (⩽1 ps) laser pulse (e.g. fast ignition of ICF targets, x-ray laser pumping, laboratory astrophysics research or some nuclear physics experiments) require ion beams of picosecond (or shorter) time durations and of very high ion current densities (∼1010 A cm−2 or higher). A possible way of producing ion beams with such extreme parameters is ballistic focusing of fast ions generated by a target normal sheath acceleration (TNSA) mechanism at relativistic laser intensities. In this paper we discuss another method, where the production of short-pulse ion beams of ultrahigh current densities is possible in a planar geometry at subrelativistic laser intensities and at a low energy (⩽1 J) of the laser pulse. This method—referred to as skin-layer ponderomotive acceleration (S-LPA)—uses strong ponderomotive forces induced at the skin-layer interaction of a short laser pulse with a proper preplasma layer in front of a solid target. The basic features of the high-current ion generation by S-LPA were investigated using a simplified theory, numerical hydrodynamic simulations and measurements. The experiments were performed with subjoule 1 ps laser pulses interacting with massive or thin foil targets at intensities of up to 2 × 1017 W cm−2. It was found that both in the backward and forward directions highly collimated high-density ion beams (plasma blocks) with current densities at the ion source (close to the target) approaching 1010 A cm−2 are produced, in accordance with the theory and numerical calculations. These ion current densities were found to be comparable to (or even higher than) those estimated from recent short-pulse TNSA experiments with relativistic laser intensities. Apart from the simpler physics of the laser–plasma interaction, the advantage of the considered method is the low energy of the driving laser pulses allowing the production of ultrahigh-current-density ion beams with a high repetition rate. It opens a prospect for unique tabletop experiments in various fields of physical and technological research.

Target normal sheath acceleration (TNSA) is a method employed in laser–matter interaction experiments to accelerate light ions (usually protons). Laser setups with durations of a few 10 fs and relatively low intensity contrasts observe plateau regions in their ion energy spectra when shooting on thin foil targets with thicknesses of the order of 10 μm. In this paper, we identify a mechanism which explains this phenomenon using one-dimensional particle-in-cell simulations. Fast electrons generated from the laser interaction recirculate back and forth through the target, giving rise to time-oscillating charge and current densities at the target backside. Periodic decreases in the electron density lead to transient disruptions of the TNSA sheath field: peaks in the ion spectra form as a result, which are then spread in energy from a modified potential driven by further electron recirculation. The ratio between the laser pulse duration and the recirculation period (dependent on the target thickness, including the portion of the pre-plasma which is denser than the critical density) determines if a plateau forms in the energy spectra.

SiC detectors based on a Schottky junction represent useful devices to characterize fast laser-generated plasmas. High-intensity fs lasers have been used to irradiate thin foils and to characterize the produced accelerated electrons and ions in the target normal sheath acceleration (TNSA) regime, detecting their emission in the forward direction and at different angles with respect to the normal to the target surface. The electrons' energies have been measured using relativistic relationships applied to their velocity measured by SiC detectors in the time-of-flight (TOF) approach. In view of their high energy resolution, high energy gap, low leakage current, and high response velocity, SiC detectors reveal UV and X-rays, electrons, and ions emitted from the generated laser plasma. The electron and ion emissions can be characterized by energy through the measure of the particle velocities with a limitation at electron relativistic energies since they proceed at a velocity near that of the speed of light and overlap the plasma photon detection. The crucial discrimination between electrons and protons, which are the fastest ions emitted from the plasma, can be well resolved using SiC diodes. Such detectors enable the monitoring of the high ion acceleration obtained using high laser contrast and the absence of ion acceleration using low laser contrast, as presented and discussed.