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Presentation of past and ongoing campaigns aimed at the efficient generation of high energy proton beams at the Draco PW laser facility of Helmholtz-Zentrum Dresden - Rossendorf (HZDR).

The spatial distribution of protons accelerated from submicron-thick plastic foil targets using multi-terawatt, frequency-doubled laser pulses with ultra-high temporal contrast has been investigated experimentally. A very stable, ring-like beam profile of the accelerated protons, oriented around the target's normal direction has been observed. The ring's opening angle has been found to decrease with increasing foil thicknesses. Two-dimensional particle-in-cell simulations reproduce our results indicating that the ring is formed during the expansion of the proton density distribution into the vacuum as described by the mechanism of target-normal sheath acceleration. Here - in addition to the longitudinal electric fields responsible for the forward acceleration of the protons - a lateral charge separation leads to transverse field components accelerating the protons in the lateral direction.

The presentation will give an overview of the recent experiments for laser driven proton acceleration with high contrast at the high power laser system Draco at HZDR. We present results of an experimental campaign employing a pure condensed hydrogen jet as a renewable and debris free target. Different ion diagnostics reveal mono-species proton acceleration in the laser incidence plane around the wire-like target, reaching cut-off energies of up to 20 MeV and exceeding 109 protons per MeV per steradian. Evaluation of two different target geometries (cylindrical with a diameter of 2µm, 5µm or 10µm and planar with 2x20 µm²) demonstrating more optimized acceleration conditions from the planar hydrogen jet. We report on modulations of laser accelerated protons by strong filamentary electromagnetic fields. Such modulations are related to the appearance of electron Weibel instability in the preplasma at the rear side of the target and impose important constraints on the preplasma level for high-quality proton acceleration [1]. Furthermore we present the usage of optical probing to study the laser plasma interaction providing plasma density measurements at the time of the interaction and to precisely determine the jet position. Recorded probe images taken up to 100 ps after the laser pulse arrived at the target, indicating plasma density modulations from pinching effects along the jet axis.

Developments in the field of laser plasma accelerators have been pushed by applications like radiation therapy in the past years. The acceleration of particle to higher energies is one of the key questions for such applications. Probing methods offer the capability for investigating the laser target interaction and thus allow for studying the acceleration process. In this talk results obtained by optical probing in recent experiments for laser driven proton acceleration with high contrast at the high power laser Draco at HZDR are presented. Draco delivers pulses of 30 fs and 5 J at 800 nm, focused to a 3 µm spot by a F/2.5 off-axis parabolic mirror. We present results of an experimental campaign using a cryogenic hydrogen jet as a renewable debris free target. The hydrogen jet’s nominal electron density is approximately 30 times the critical density for 800 nm and its shape and size can be varied. For instance cylindrical geometries with diameters of 2µm, 5µm or 10µm were used. The laser plasma interaction could be monitored on-shot with two temporally synchronized optical probe beams perpendicular and longitudinal to the Draco laser axis. Recorded probe images taken up to 100 ps after the laser pulse arrived at the target indicate plasma density modulations from pinching effects along the jet axis. A possible driver for those density modulations is a strong surface current which has been studied with 2D-PIC simulations. Such density modulations may modify the shape of the electron sheath and thereby alter the beam profile of the laser accelerated protons.

We present a pure condensed hydrogen jet as a renewable and debris-free target for high power laser experiments. The jet's nominal electron density is approximately 30 times the critical density and its diameter can be varied to be 2µm, 5µm or 10µm and thus allowing to study the regime of relativistic transparency. Different ion diagnostics reveal mono-species proton acceleration in the laser incidence plane around the wire-like target. Radiochromic film stacks in forward direction display signatures of filament-like structures, stemming from a Weibel-like instability generated at the rear side of the target in the underdense plasma region.

Applications like radiation therapy of cancer have pushed the development of laser plasma accelerators and defined levels of control and necessary particle beam stability in laser plasma experiments. The poster will give an overview of a recent experiment for laser driven particle acceleration with high contrast at the high power laser Draco at HZDR, delivering pulses of 30fs and 5J. We present results of an experimental campaign employing a cryogenic hydrogen jet as a renewable target. The jet's nominal plasma density is approximately 30 times the critical density and its diameter can be varied to be 2µm, 5µm or 10µm and thus allowing to study the regime of relativistic transparency. In addition a planar aperture was commissioned, providing a different geometry of the hydrogen jet. Different ion diagnostics reveal mono-species proton acceleration in the laser incidence plane around the wire-like target. Radiochromic film stacks in laser forward direction display filament-like structures, stemming from a Weibel-like instability generated at the rear side of the target. Furthermore the micro-jet target could be monitored on-shot with a temporally synchronized optical probe beam perpendicular and almost parallel to the pump laser axis. Recorded probe images taken on a timescale of several 10’s of picoseconds indicating plasma density modulations from pinching effects along the jet axis.

To advance the development of laser proton accelerators for highly demanding applications like cancer treatment a stable source of energetic particles at high repetition rates is required. During our last experimental campaign at the Helmholtz-Zentrum Dresden-Rossendorf we therefore employed a pure condensed hydrogen jet as a renewable target for the 100TW Draco laser. Draco is a ultra-high power Ti:Sa laser system which delivers pulses of 30fs and 3J on target at 800nm with a repetition rate of 10Hz. A recollimating single plasma mirror results in an improved temporal contrast represented by an ASE level of 10-13. The expanding jet was monitored on-shot with a separate phase locked diode-pumped ps-laser at a wavelength of 515 nm. By that over-exposure of the CCD resulting from strong plasma self-emission which had been observed in earlier experiments to be at the harmonics of the pump laser, could be avoided. The probe beam was split in two parts oriented perpendicular and parallel with respect to the pump laser axis in order to precisely determine the jet position and its density profile.

We present results of our experimental campaign on laser proton acceleration, in which liquid crystal film targets of tunable thickness were irradiated with plasma mirror cleaned laser pulses. The data show a significant increase in proton cut-off energy up to 25 MeV for a target thickness of 10 nm as compared to the few- micron scale reference for this target configuration yielding roughly 12 MeV. The performance of laser based ion acceleration strongly depends on the laser temporal contrast and its effect on the target plasma scale length. Plasma mirror setups have proven to be a valuable tool to improve the temporal contrast by several orders of magnitude, reducing the intensity of pre-pulses that emanate from the laser chain and steepening the rising edge of the main laser pulse. We present recent results obtained at the Titanium Sapphire laser system Draco, delivering 30 fs long laser pulses at an intensity exceeding 10^20 W/cm^2. Our recently commissioned single plasma mirror improves the contrast by four orders of magnitude while reflecting 80% of the initial pulse energy. Its influence on the laser proton acceleration process was studied in a campaign in collaboration with the High Energy Density Physics Group of Ohio State University using their tunable liquid crystal film target source. This device allows an on-demand variation of the target thickness from tens of micrometers down to 10 nm while keeping the target composition constant. The target was positioned under 45 degrees with respect to the incoming laser beam while accelerated protons and ions were monitored in both laser- and target normal direction by means of Thomson Parabolas and Radiochromic Film stacks. Hence, being sensitive to the identification of acceleration regimes beyond the well-known Target Normal Sheath Acceleration, preliminary results display a significant increase in proton cut-off energy when reaching thin targets. Up to 25 MeV could be observed for an optimum target thickness of 10 nm as compared to the few- micron scale reference for this target configuration yielding roughly 12 MeV.

In this paper, a reflective optical pump-probe technique for laser-driven plasmas at solid density target surfaces is presented. The technique is termed high depth-of-field time-resolved microscopy and it exploits the angular redistribution of the probe beam intensity after the probe’s reflection from an expanded and hence non-planar iso-density surface in the plasma. The main application of the robust technique, which uses simple imaging of the probe beam, is the spatio-temporal resolution of the plasma formation and expansion at the target rear surface. Analytic and numerical modeling of the experimental setup are applied for the analysis of the experimental results. The relevance and potential of the optical plasma probing method is highlighted by the application to targets of different geometries, helping to understand the target shape-related differences in the ion acceleration performance.

Reproducible series of mass-limited targets, manufactured with lithographic techniques, and varying in size, thickness, and mounting geometry were irradiated at the 150 TW Draco Laser facility of the Helmhotz-Zentrum Dresden-Rossendorf with ultrashort (30 fs) laser pulses of intensities of about 8 · 10^20 W/cm^2. A robust maximum energy enhancement of almost a factor of two was found when compared to reference irradiations of plain foils of the same thickness and material. The performance gain was, however, restricted to lateral target sizes of about 50 μm which was attributed to edge and mounting structure influences. In a recent experiments these mass-limited targets of similar size and thickness but different materials, such as Al, Ti and Au were irradiated at the diode-pumped high-power laser system POLARIS at the Helmholtz-Zentrum Jena, in order to study the influence of the material on to the laser proton acceleration performance. The effect of target size dependent bulk heating was studied with characteristic X-ray imaging and spectroscopy.

Large effort is currently put into translating laser-driven particle sources from the status of experimental machines to accelerators ready for applications. The program at HZDR thereby focuses on laser-ion accelerators for medical applications [1]. Besides qualifying laser-accelerators for stable and reliable acceleration, the main challenge is the achievement of sufficient (~ 200 MeV for protons) particle energies. Fs-PW laser systems, going into operation in different laboratories worldwide now, might provide the necessary laser powers [2]. Hence, careful testing of the laser power scaling of the established acceleration mechanisms (e.g. TNSA) is needed, considering not only the achievable particle energies but properties as the spatial/angular distribution of particles (beam profile) as well. In that context, we report on the experimental observation of spatially modulated proton beams emitted from micrometer thick targets which were irradiated with ultrashort (30 fs) laser pulses of a peak intensity of 5•1020W/cm2. The net-like proton beam modulations were recorded using radiochromic film and the investigation of different target systems for a laser energy range of 0.9 to 2.9 J revealed a clear dependence on laser energy and target thickness for the onset and strength of the modulations. Numerical simulations performed suggest filamentary instabilities, such as the parametric two plasmon decay and a Weibel-like instability, which occur in the laser-produced target front side plasma, as the source of the observed proton beam modulations [3]. The study is supported by pump-probe experiments of the plasma dynamics at the target front and rear surface. The method allows to connect the typical third-order autocorrelation traces [4] of the temporal pulse intensity contrast with the on-targets plasma conditions at pulse peak arrival. In that way, numerical simulations, necessary to investigate phenomena as plasma instabilities, can be matched to the real target plasma conditions more precisely. We propose that the presented results on laser energy dependent plasma instabilities may have implications for the scaling of present acceleration mechanisms, such as target normal sheath acceleration, to higher proton energies and hence higher laser powers.

Durch die Fokussierung eines ultrakurzen und hochintensiven Laserpulses auf ein Festkörpertarget können Pulse von Protonen und anderen positiv geladenen Ionen erzeugt werden. Auf Basis des etablierten TNSA (target-normal sheath acceleration) Prozesses konnten am Helmholtz-Zentrum Dresden-Rossendorf mit dem 150 TW Ultrakurzpulslaser Draco Protonenpulse mit Energien bis zu 20 MeV erzeugt und charakterisiert werden. Die Charakterisierung dieser Teilchenstrahlung erfordert die Identifizierung der Ionenspezies und die Bestimmung ihrer spektralen Verteilung möglichst nach jedem Puls, wofür standardmäßig Thomsonspektrometer verwendet werden. In den letzten Jahren wurde das DRACO-Lasersystem bis zu einer Pulsleistung über 500 TW erweitert. Aufbauend auf dem bisherigen Spektrometerdesign wurde in dieser Arbeit ein kompaktes Spektrometer für einen höheren Energiebereich bis über 80 MeV entworfen. Besonders wichtig dabei ist die Identifizierung möglicher das Messergebnis verfälschender Sekundärstrahlungsquellen, die mit Hilfe von Monte-Carlo Simulation analysiert werden müssen.

This paper reports on a systematic investigation of the ultrashort pulse laser driven acceleration of protons from thin targets of finite size, so-called reduced mass targets (RMTs). Reproducible series of targets, manufactured with lithographic techniques, and varying in size, thickness, and mounting geometry, were irradiated with ultrashort (30 fs) laser pulses of intensities of about 8 × 1020 Wcm−2. A robust maximum energy enhancement of almost a factor of two was found when comparing gold RMTs to reference irradiations of plain gold foils of the same thickness. Furthermore, a change of the thickness of these targets has less influence on the measured maximum proton energy when compared to standard foils, which, based on detailed particle-in-cell simulations, can be explained by the influence of the RMT geometry on the electron sheath. The performance gain was, however, restricted to lateral target sizes of greater than 50μm, which can be attributed to edge and mounting structure influences.

We report on the experimental observation of transverse modulations in proton beams accelerated from micrometer thick targets which were irradiated with ultra-short (30 fs) laser pulses of a peak intensity of 5·10^20 W/cm^2. The net-like proton beam modulations were recorded using radiochromic film and the data suggest a dependence on laser energy and target thickness for their onset and strength. Numerical simulations suggest that intensity-dependent instabilities in the laser-produced plasma at the target front side lead to electron beam break-up or filamentation, then serving as the source of the observed proton beam modulations.

Proton beams by their well-confined energy-loss in matter are a promising tool for the improvement of radiotherapy of cancer and are currently under intense medical investigation. Wider clinical use, however, is limited by the complexity and expense of current proton and ion accelerators. Compact laser driven proton therapy accelerators are discussed as a promising alternative, yet require substantial development in reliable beam generation and transport, but also in dosimetric protocols as well as validation in radiobiological studies. In our talk, we will present the first direct and dose controlled comparison of the radiobiological effectiveness (RBE) of intense proton pulses from a laser driven accelerator with conventionally generated continuous proton beams, showing no dependence of the RBE on the different beam properties [1]. Controlled dose delivery, precisely online and offline monitored for each of the ~ 4000 proton pulses, resulted in an unprecedented relative dose uncertainty of below 10%, using approaches scalable to radiotherapy applications. In parallel to the development of laser driven proton therapy accelerators, an advancement in instrumentation for laser driven protons is essential. Most importantly, these new diagnostic tools need to speedwise match the repetition rates of state-of-the-art high power laser systems and need to be adapted to the harsh plasma environment of laser based accelerators, not neglecting their fitment to the properties of laser accelerated proton pulses such as the high flux and the broad energy spectrum. We will present three types of scintillator-based detectors, all being optimized for specific stages of the experimental chain: a one-dimensional space- and energy-resolved detector for online spectral stability control of the acceleration performance [2], a two-dimensional space- and energy-resolved detector for source characterization measurements, and a three-dimensional detector for precise dose verification in a water-equivalent medium with regards to medical quality assurance [3]. [1] K. Zeil, et al.: Dose-controlled irradiation of cancer cells with laser-accelerated proton pulses, Appl. Phys. B (2012) [2] J. Metzkes, et al.: A scintillator-based online detector for the angularly resolved measurement of laser-accelerated proton spectra, Review of Scientific Instruments, Rev. Sci. Instrum. 83, 123301 (2012) [3] F. Kroll, et al.: Preliminary investigations on the determination of three-dimensional dose distributions using scintillator blocks and optical tomography, Med. Phys. 40, 082104 (2013)

Since the mid-1950s, pulsed (iron-free) high-field magnets have become a common, versatile research tool. Applications in solid state physics have promoted the development of sophisticated magnets that nowadays can achieve fields above 90 T repeatedly. We report here another area of application for pulsed power magnet technology in combination with high-intensity lasers; namely the use of pulsed magnets as research tools in laser-based laboratory astrophysics and as effective optical devices for laser-accelerated particle beams. Pulsed power solenoids for focusing of laser-accelerated particle beams might allow for the use of these new radiation qualities in medical radiation therapy or could function as a crucial part of a compact, laser-based ion source. The talk gives a survey of developments at Helmholtz-Zentrum Dresden-Rossendorf in portable pulse generator technology and pulsed high-field magnets, especially designed to match the requirements of different laser-plasma physics environments. The beam optical properties of a capacitor-driven air core solenoid are derived from experimental data and most recent experimental results collected within the LIGHT collaboration at the PHELIX laser system at GSI, Darmstadt are presented.

Thomson backscattering of intense laser pulses from relativistic electrons not only allows for the generation of bright X-ray pulses but also for the investigation of the complex particle dynamics at the interaction point. For this purpose a complete spectral characterization of a Thomson source powered by a compact linear electron accelerator was performed with unprecedented angular and energy resolution. A rigorous statistical analysis comparing experimental data to 3D simulations enabled, e.g., the extraction of the angular distribution of electrons with 1.5% accuracy and, in total, provides predictive capability for future high brightness hard X-ray and potential gamma-ray sources. We further present a novel Thomson scattering geometry in order to avoid the restrictions on the X-ray photon yield imposed by the Rayleigh limit. The suggested traveling-wave setup (TWTS) allows an overlap of electron and laser beams, even after propagating over distances in the centimeter to meter range. Experimental designs are discussed and optimized for different scattering angles.

We present a systematic study of the ultrashort pulse laser driven acceleration of protons from thin targets of finite lateral size, so-called reduced mass targets (RMTs). Reproducible series of targets, manufactured with lithographic techniques, and varying in size, thickness, and mounting geometry were irradiated at the 150 TW Draco Laser facility of the Helmhotz-Zentrum Dresden-Rossendorf with ultrashort (30 fs) laser pulses of intensities of about 8 • 10^20 W/cm^2. A robust maximum energy enhancement of almost a factor of two was found when compared to reference irradiations of plain foils of the same thickness and material. Furthermore, these targets exhibit a reduced performance dependence on target thickness compared to standard foils, which, based on detailed PIC simulations can be explained by the influence of the RMT geometry on the electron sheath. The performance gain was, however, restricted to lateral target sizes of about 50 μm which was attributed to edge and mounting structure influences. The contribution of the large electric fields at the target edges to the proton acceleration performance was investigated with measurements of the proton beam profile as well as optical pump and probe experiments.

Proton beams by means of the well-confined dose deposition in matter are a promising tool for improving radiotherapy of cancer. Wider clinical use, however, is limited by the complexity and expense of current proton accelerators. Compact laser-driven proton therapy accelerators are a potential alternative, yet require substantial development in reliable beam generation, transport and dosimetric monitoring as well as validation in radiobiological studies. In vitro cell irradiations with laser-accelerated protons show radiobiological effectiveness similar to conventionally accelerated protons. In vivo animal irradiations – the next step in the translational research chain towards laser-driven proton therapy of cancer – are currently restricted by the low proton yield and energy at the irradiation site. We report on systematic investigations of the ultrashort pulse laser-driven acceleration of protons from thin targets of narrow lateral dimension, so-called reduced mass targets (RMTs). A robust maximum energy enhancement (almost doubled) was found when compared to reference irradiations of plain foils of same thickness and material. Combining RMTs with a pulsed high-field solenoid for particle capturing, as developed at Helmholtz-Zentrum Dresden-Rossendorf, gives the potential to enhance and localise the dose per pulse applied to the tumour, thus making further radiobiological studies on volumetric tumours feasible in the future.

Es werden zwei am HZDR entwickelte Online-Detektoren für laserbeschleunigte Protonen basiert auf Plastikszintillatoren vorgestellt.

Laser-plasma based accelerators, where a high intensity laser interacts with a solid target, represent a promising concept for compact and cost-efficient ion sources. As the acceleration takes place on ultra-fast timescales and very small spatial scales of only a few µm, information about the acceleration processes in the plasma are generally derived indirectly, e.g. through particle or radiation diagnostics. Optical probing techniques however can give direct access to the plasma dynamics on the fs to ps timescale. In this poster we will present pump-probe experiments performed at the high-intensity laser DRACO at the HZDR, Germany. Seeking information about the conditions of the critical density surface in the plasma, we simply image the probe reflected from the target surface without doing tomography. The curvature of the reflecting critical surface translates into characteristic angular patterns in the reflected probe beam which can be traced back to the original surface shape using a ray-tracing model of the experiment. In our experiments, the pump beam was the 150 TW DRACO beam with a pulse length of 30 fs and a central wavelength of 800 nm. The probe beam was derived from the mean beam, frequency doubled, and had a pulse duration of ~100fs. Both beams were synchronized and the probe could be delayed with respect to the main beam with a resolution of ~200 fs. The imaging of the probe was performed with a large-aperture objective yielding a high spatial resolution of ~2µm. As a result, we compared the critical surface deformation and dynamics for different target materials (metal and silicon) and target shapes, observing a strong influence of the lateral target size.

In 2009, at the LANL Trident laser facility a new world record in laser accelerated proton energy has been set, exceeding 65 MeV, using hollow conical targets. We performed 2D collissional PIC simulations and identify two novel electron acceleration mechanisms that have not been considered before to enhance ion acceleration: the direct acceleration of electrons comoving with the driving laser along the the cone-wall inner surface (DASE) and the acceleration of electrons in surface plasma waves (PWA). We nd that they are responsible for a signicant increase in both electron number and energy in the case of a grazing laser incidence onto the inner cone wall surface compared to regular flat foils. We study the scaling of the electron and ion energies for various target and laser parameters.

High-Energy, short-pulse lasers can be used to accelerate ion beams. Laser-driven acceleration promises to provide very compact ion sources suitable for a variety of applications such as ion beam tumor therapy. Up until recently, however, the ion energies observed in experiments were too low to be useful for treating tumors. Using a novel target design, we have broken a long-lasting record for maximum proton energy measured in a laser acceleration experiment, reaching for the first time proton energies suitable for radiation therapy of intraocular tumors. We present realistic particle-in-cell simulations of the laser interacting with the target that can explain the increase in energy seen in our experiment. The results from these simulations will help to improve the target design, paving the way towards even higher proton energies.

In this work we report on a recent experiment where an energetic, well-collimated electron beam has been observed in the laser direction following the short pulse (600 fs) high-intensity laser interaction with ultra-thin solid foils. These results are in contrast to the typical low-energy divergent electrons accompanying ions in the target normal direction usually seen in solid targets.We observe the foils being preheated and expanded by ASE prior to the main pulse which makes them transparent for the laser. The experimental evidence as well as 2D particle-in-cell simulations suggest the excitation of a wakefield that can accelerate electrons to energies of tens of MeV.

We present a new theoretical understanding of and selected results from experiments performed in 2009 at the TRIDENT laser at Los Alamos National lab. Those experiments yielded the wolrd's highest proton energies from laser-solid interaction and were based on novel flat-top copper cone targets. Simulations show that a new mechanism of laser-plasma interaction was responsible for the observed high proton energies and suggest that an optimization of the cone geometry may further enhance the achievable proton energies.