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Summary of the current status and carried out experiments of Laser-driven ion experiments at Draco

Laser-driven ion acceleration has been considered a potential alternative for conventional accelerators like cyclotrons or synchrotrons and thus could provide a more compact and cost-efficient particle therapy solution in the future. Instead of continuous ion beams, laser-driven ions exhibit fs to ps bunch length, carrying up to 1013 particles with broad energy spectrum and are highly divergent. Pulsed high-field magnets are a versatile and efficient way of shaping those bunches both spatially and spectrally for application, while preserving the short pulse lengths and high intensities leading to high dose rates when stopped in matter. We performed experiments with the PW beam of the Dresden laser acceleration source Draco to investigate the feasibility of worldwide first controlled volumetric tumour irradiations with laser-accelerated protons. Therefore, a setup of up to two solenoid magnets was used to efficiently capture and shape the proton beam, matching the radiobiological demands, which was then analysed by means of a Thomson parabola spectrometer, scintillator, ionization chamber and radiochromic film.

The talk summarizes activities of HZDR on the topic of laser-driven particle sources for medical applications. The focus lies on a the development and characterization of a pulsed high-field beamline that allows to transport and spacially as well as spectrally shape a laser-driven ion beam and thereby prepare it for irradiation studies, e.g. on radiobiological studies.

Summary of the current status and carried out experiments of Laser-driven radiobiology experiments at Draco Petawatt

In the last decade the investigation of laser-driven plasmas has gained great importance for the development of compact ion accelerator schemes with the efficient generation of multiple 10 MeV proton beams from TNSA experiments with PW laser systems like the Dresden laser acceleration source (DRACO) at the Helmholtz-Zentrum Dresden - Rossendorf. The exploration of the plasma dynamics and its microscopic parameters is crucial for the optimization of the acceleration process. Optical probing is one technique to investigate the temporal plasma evolution and complements numerical particle-in-cell simulations of the underlying physics. However, strong plasma self-emission at the driver lasers wavelength and its harmonics often masks the laser plasma interaction region and thus complicates the data analysis. Here, we present the implementation of a stand-alone probe laser system, which is temporally synchronized to the DRACO laser. The probe laser system consisting of a seed laser and one regenerative amplifier is based on Yb:YAG and thus provides a fundamental wavelength of 1030 nm, which is different from the wavelength of the DRACO driver laser (800 nm) and its harmonics. We present the advantages of this probing approach, which was tested during an experimental campaign with wire targets of different materials and diameters in the µm range, and give an inside on the current challenges and developments of the probing system.

Laser-driven ion acceleration promises to provide a compact solution for demanding applications like particle therapy, proton radiography or inertial confinement research. Controlling the beam parameters to achieve these goals is currently pushing the frontier of laser driven particle accelerators. We present an overview of recent achievements at the high power ultra-short pulse laser source DRACO at the HZDR in Dresden (Germany). The laser system was recently upgraded by new front end components and an additional Petawatt (PW) amplifier stage, finally providing high contrast pulses of 30J within 30fs at 1 Hz pulse repetition rate. The performance of the plasma acceleration is strongly dependent on the complex pre-plasma formation process at the target front surface which is determined by the temporal intensity contrast. Plasma mirror setups have proven to be a valuable tool to significantly improve the temporal contrast by reducing pre-pulse intensity and steepening the rising edge of the main laser pulse. Re-collimating single plasma mirror devices have therefore been implemented into the Draco laser beam lines, enabling investigation of laser proton acceleration and proton energy scaling within the TNSA regime using ultra-thin foil targets. The results of the simultaneously measured proton emission energies in laser forward direction, laser backward direction and the temporal contrast, measured on a single-shot base by means of self-referenced spectral interferometry with extended time excursion (SRSI-ETE) at unprecedented dynamic and temporal range, will be presented.

We present an experimental study investigating laser-driven proton acceleration via Target Normal Sheath Acceleration (TNSA) over a target thickness range spanning the typical TNSA-dominant region (~1 μm) down to below the relativistic laser-transparency regime (< 40 nm), enabled by freely adjustable target film thickness using liquid crystals along with high contrast (via plasma mirror) laser interaction (~ 2.65 J, 30 fs, I > 1 × 10^21 W/cm^2). Thickness dependent maximum proton energies scale well with TNSA models down to the thinnest targets, while those under ~ 40 nm indicate transparency-enhanced TNSA via differences in light transmission, maximum proton energy, and proton beam spatial profile. Oblique laser incidence (45°) allowed additional diagnostics to be fielded to diagnose the interaction quality: a suite of ion energy and spatial distribution diagnostics in the laser axis and both front and rear target normal directions as well as reflected and transmitted light measurements on-shot collectively verify the dominant acceleration mechanism as TNSA from high contrast interaction, even for ultra-thin targets. Additionally, 3D particle-in-cell simulations support the experimental observations of target-normal-directed proton acceleration from ultra-thin films.

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 development of high-intensity short-pulse lasers in the petawatt regime offers the possibility to design new compact accelerator schemes by utilizing high-density targets to generate proton beams with multiple 10 MeV energy per nucleon. The optimization of the acceleration process calls for a comprehensive exploration of the plasma dynamics involved, for example via spatially and temporally resolved optical probing. Experimental results can then be compared to numerical particle-in-cell simulations, which is particularly well suited in the case of cryogenic hydrogen jet targets [1]. However, strong plasma self-emission and conversion of the plasma’s drive laser wavelength into its harmonics often masks the interaction region and interferes with the data analysis. Recently, the development of a stand-alone and synchronizable probe laser system for off-harmonic probing at the DRACO laser at the Helmholtz-Zentrum Dresden – Rossendorf showed promising performance [2]. Here, we present an updated stand-alone probe laser system applying a compact CPA system based on a synchronized fs mode-locked oscillator (Light Conversion) operating at 1030 nm, far off the plasma’s drive laser wavelength of 800 nm. A chirped volume Bragg grating (Optigrate Corp) is used as a hybrid stretcher and compressor unit. The subsequent diode pumped regenerative Yb:CaF2 laser amplifier includes a spectral shaping element and chirped mirrors for GDD compensation. The system delivers 160 fs pulses with a maximum energy of 0.9 mJ and thus extends the recent developments [3] into the sub 200 fs region. Additionally, we present recent experimental results deploying the upgraded probe laser system and its harmonics in an experiment dedicated to laser-proton acceleration from a renewable cryogenic hydrogen jet at the DRACO laser.

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 simulation results on laser ion acceleration using hydrogen ribbon targets irradiated by ultra-intense, ultra-short laser pulses. These targets promise to produce pure proton beams applicable for cancer therapy at high repetition rates. We address critical issues concerning the acceleration process that potentially hinders the application of these beams in a clinical scenario. For achieving proton energies suitable for the treatment of deep seated tumors it is important to increase the laser intensity. At high laser intensities, plasma instabilities both at the target surfaces and target bulk can create electron filaments which result in non-uniform proton beams, detrimental for delivering uniform dose distributions. By varying the laser contrast it is possible to change the preplasma scale length to influence the formation of instabilities. Other means of controlling proton beams are either changing target geometry (e.g. going from planar ribbon targets to spherical droplet targets) or the polarization. We present results of 2D3V particle-in-cell simulations at realistic densities that show the influence on the plasma dynamics and final beam properties and discuss their relevance regarding applications of solid hydrogen targets for laser-driven proton tumor therapy.

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.

Demanding applications like radiation therapy of cancer are pushing the frontier of laser driven proton accelerators with controlled and well-defined proton beam properties. This talk will give an overview of recent achievements at the high-contrast high power laser source DRACO at the HZDR in Dresden (Germany). The laser system was recently upgraded by an additional Petawatt (PW) amplifier stage and new front end components finally providing high contrast pulses of >500 TW on target at 1 Hz pulse repetition rate. In first experiments the delivery of these pulses on target was demonstrated and the feasibility of worldwide first controlled volumetric irradiation of a specifically developed tumor model, grown on the ears of nude mice with laser-accelerated protons was investigated. In order to efficiently capture and shape the divergent TNSA proton beam, a setup of two pulsed high-field solenoid magnets will be presented to reliably generate homogeneous dose distributions in lateral direction and in depth. The performance of laser based proton and ion acceleration and the scaling of the laser energy to achieve increased ion energies strongly depend 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 significantly improve the temporal contrast by reducing pre-pulse intensity and steepening the rising edge of the main laser pulse. Re-collimating single plasma mirror devices have been implemented into the Draco laser beam lines and the talk will summarize on measurements of the resulting contrast enhancement comparing different techniques. With the achieved contrast enhancement, laser proton acceleration and proton energy scaling were investigated within the TNSA regime using ultra-thin foil targets and implications for the radiobiological experiments will be discussed.

We present various approaches to increase the generated proton energy in laser driven proton acceleration. Recent results gained at our in-house laser system deploying different laser and target parameters are shown.

We report on high pulse contrast operation modes of the Draco laser with focus on the on-shot characterization of the few-ps-contrast by novel techniques based on tilted beam self-referenced spectral interferometry with 108 dynamic range.

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.

Demanding applications like radiation therapy of cancer have pushed the development of laser plasma proton accelerators and defined levels of control and necessary proton beam stability in laser plasma experiments. The presentation will give an overview of the recent experiments for laser driven proton 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 pure condensed hydrogen jet as a renewable target. 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. A comparison of the data with results obtained using ultra-thin foils at high-contrast provided by a single plasma-mirror will be given. Additionally, the expanding jet could be monitored on-shot with a temporally synchronized probe beam perpendicular to the pump laser axis. Recorded probe images resemble those of z-pinch experiments with metal wires and indicate sausage-like instability along the jet axis.

Applications of laser-accelerated protons demand a stable source of energetic particles at high repetition rates. We present the results of our experimental campaign in cooperation with MEC/SLAC at the 10Hz Ti:Sa laser Draco of Helmholtz-Zentrum Dresden-Rossendorf (HZDR), employing a pure condensed hydrogen jet as a renewable target. Draco delivers pulses of 30fs and 5J at 800nm, focused to a 3µm spot by an F/2.5 off-axis parabolic mirror. The jet's nominal electron density is approximately 30 times the critical density and its thickness is 2µm, 5µm or 10µm, depending on the applied aperture on the source. Ion diagnostics reveal mono-species proton acceleration in a solid angle of at least +/-45 ° with respect to the incoming laser beam, with maximum energies of around 5 MeV. The expanding jet could be monitored on-shot with a temporally synchronized probe beam perpendicular to the pump laser axis. Recorded probe images resemble those of z-pinch experiments with metal wires and indicate an m=0 instability in the plasma.

Demanding applications like radiation therapy of cancer have pushed the development of laser plasma proton accelerators and defined levels of control and necessary proton beam stability in laser plasma experiments. The presentation will give an overview of the recent experiments for laser driven proton acceleration with high contrast using a plasma mirror at the high power laser Draco at HZDR, delivering pulses of 30fs and 3J on target. We present results of an experimental campaign employing a pure condensed hydrogen jet as a renewable target.

Concept of solid plasma diagnostics with Small Angle X-ray Scattering, experimental feasibility of SAXS at femtosecond and nanosecond laser driven solid material, and beyond electron-electron correlations: Resonant SAXS for ion distribution measurements

Applications of laser-accelerated protons demand a stable, high-energy and high-repetition rate particle source. We present the results of our experimental campaign in cooperation with the HED group at SLAC, performed at the 10 Hz Ti:Sa laser Draco of Helmholtz-Zentrum Dresden-Rossendorf (HZDR), employing a cryogenic Hydrogen jet as a renewable target. Draco delivers pulses of 30 fs and 5 J at 800 nm, focussed to a 3 µm spot by a F/2.5 off-axis parabolic mirror. The cylindrical jet has a diameter of 2 µm or 5 µm and a nominal electron density of 30 times the critical density. Preliminary results show a mono-species proton acceleration in a solid angle of at least +/-45° with respect to the incoming laser beam and proton energies exceeding 10 MeV. Radiochromic film stacks in forward direction show signatures of two acceleration mechanisms, one being the conventional TNSA and a second one leading to filament-like structures, possibly stemming from an instability within the plasma. Among other results, an on-shot monitoring of the stability of the jet by means of a temporally synchronized probe beam will be shown in the presentation.

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.

By focusing an ultra-short high-intensity laser pulse on a solid target, pulses of protons and other positively charged ions with energies of several 10 MeV per nucleon are generated. The properties of these particle beams such as their energy and absolute number are highly dependent on experimental conditions like laser and target parameters. In order to achieve principal comparability between different experimental campaigns at the Draco laser system at the Helmholtz-Zentrum Dresden-Rossendorf, a reference setup for the laser ion acceleration experiment was established. A configuration is sought in which proton beams of reproducible characteristics are generated. To ensure a high stability of the proton spectra, the application of longer focal length parabolas (f ~ 1000 mm) will be tested for this setup, according preparatory studies being presented in this talk.

The talk gives an introduction into the laser-ion acceleration program at HZDR.