64 research outputs found
Laser-proton acceleration in the near-critical regime using density tailored cryogenic hydrogen jets
Modern particle accelerators are a key component of today’s research landscape and indispensable in industry and medicine. In special application areas, the portfolio of these facilities will be expanded by laser-driven compact plasma accelerators that generate short, high-intensity pulses of ions with unique beam properties. Though intensely explored by the community, scaling the maximum beam energies of laser-driven ion accelerators to the required level is one of the most significant challenges of this field. This endeavor is inherently linked to a fundamental understanding of the underlying acceleration processes.
The prospect to efficiently increase the beam energy relies on the ability to control the accelerating field structures beyond the well-established acceleration from the stationary target rear side. However, manipulating the interaction in such micrometer-sized accelerators proves to be challenging due to the transient nature of the plasma fields and requires precise tuning of the temporal laser pulse shape and the volumetric density distribution of the plasma target to a level that could so far not be achieved.
This thesis investigates laser-proton acceleration using a cryogenic hydrogen target that combines the capabilities of predictive three-dimensional simulation and the in-situ realtime monitoring of the density distribution in the experiment to explore the fundamental physical principles of plasma based acceleration mechanisms. The corresponding experiments were performed at the DRACO laser facility at the Helmholtz-Zentrum Dresden-Rossendorf. The key to the success of these studies was the advancement of the cryogenic target system that generates a self-replenishing pure hydrogen jet. Using a mechanical chopping device, which protects the target system from the disruptive influence originating from the high-intensity interaction, allowed, for the first time, systematic experiments with a large number of laser shots in the harsh environment of the ultra-short pulse DRACO petawatt laser. The performance of a cylindrical hydrogen jet can be substantially optimized by a flexible all-optical tailoring of the target profile. Guided by real-time multi-color probing, the target density, the decisive parameter of the interaction, was scanned over two orders of magnitude allowing the exploration of different advanced acceleration regimes in a controlled manner. This approach led to the experimental realization of proton beams with energies up to 80 MeV and application relevant high particle yield from advanced acceleration mechanisms occurring in near-critical density plasmas, a regime so far mostly investigated in numerical studies. Besides cylindrical jets, the formation of thin hydrogen sheets was studied to gain insight into the fluid and crystallization dynamics that can be used to tailor the target shape for laser-proton acceleration. Using these jets, the onset of target transparency was explored, a regime that promises increased proton energies when optimized. Furthermore, after irradiation of the hydrogen jet with a high-intensity laser pulse, an unexpected axial modulation in the plasma density distribution was observed that can play a role in structuring the proton beam profile. This modulation is caused by instabilities that originate from the laser-plasma interaction, for example due to laser-driven return currents or the plasma expansion dynamics
Laser-proton acceleration in the near-critical regime using density tailored cryogenic hydrogen jets
Modern particle accelerators are a key component of today’s research landscape and indispensable in industry and medicine. In special application areas, the portfolio of these facilities will be expanded by laser-driven compact plasma accelerators that generate short, high-intensity pulses of ions with unique beam properties. Though intensely explored by the community, scaling the maximum beam energies of laser-driven ion accelerators to the required level is one of the most significant challenges of this field. This endeavor is inherently linked to a fundamental understanding of the underlying acceleration processes. The prospect to effciently increase the beam energy relies on the ability to control the accelerating field structures beyond the well-established acceleration from the stationary target rear side. However, manipulating the interaction in such micrometer-sized accelerators proves to be challenging due to the transient nature of the plasma fields and requires precise tuning of the temporal laser pulse shape and the volumetric density distribution of the plasma target to a level that could so far not be achieved.
This thesis investigates laser-proton acceleration using a cryogenic hydrogen target that combines the capabilities of predictive three-dimensional simulation and the in-situ realtime monitoring of the density distribution in the experiment to explore the fundamental physical principles of plasma based acceleration mechanisms. The corresponding experiments were performed at the DRACO laser facility at the Helmholtz-Zentrum Dresden-Rossendorf. The key to the success of these studies was the advancement of the cryogenic target system that generates a self-replenishing pure hydrogen jet. Using a mechanical chopping device, which protects the target system from the disruptive influence originating from the high-intensity interaction, allowed, for the first time, systematic experiments with a large number of laser shots in the harsh environment of the ultra-short pulse DRACO petawatt laser. The performance of a cylindrical hydrogen jet can be substantially optimized by a flexible all-optical tailoring of the target profile. Guided by real-time multi-color probing, the target density, the decisive parameter of the interaction, was scanned over two orders of magnitude allowing the exploration of different advanced acceleration regimes in a controlled manner. This approach led to the experimental realization of proton beams with energies up to 80 MeV and application relevant high particle yield from advanced acceleration mechanisms occurring in near-critical density plasmas, a regime so far mostly investigated in numerical studies. Besides cylindrical jets, the formation of thin hydrogen sheets was studied to gain insight into the fluid and crystallization dynamics that can be used to tailor the target shape for laser-proton acceleration. Using these jets, the onset of target transparency was explored, a regime that promises increased proton energies when optimized. Furthermore, after irradiation of the hydrogen jet with a high-intensity laser pulse, an unexpected axial modulation in the plasma density distribution was observed that can play a role in structuring the proton beam profile. This modulation is caused by instabilities that originate from the laser-plasma interaction, for example due to laser-driven return currents or the plasma expansion dynamics
Spectral Control via Multi-Species Effects in PW-Class Laser-Ion Acceleration
Laser-ion acceleration with ultra-short pulse, PW-class lasers is dominated
by non-thermal, intra-pulse plasma dynamics. The presence of multiple ion
species or multiple charge states in targets leads to characteristic
modulations and even mono-energetic features, depending on the choice of target
material. As spectral signatures of generated ion beams are frequently used to
characterize underlying acceleration mechanisms, thermal, multi-fluid
descriptions require a revision for predictive capabilities and control in
next-generation particle beam sources. We present an analytical model with
explicit inter-species interactions, supported by extensive ab initio
simulations. This enables us to derive important ensemble properties from the
spectral distribution resulting from those multi-species effects for arbitrary
mixtures. We further propose a potential experimental implementation with a
novel cryogenic target, delivering jets with variable mixtures of hydrogen and
deuterium. Free from contaminants and without strong influence of hardly
controllable processes such as ionization dynamics, this would allow a
systematic realization of our predictions for the multi-species effect.Comment: 4 pages plus appendix, 11 figures, paper submitted to a journal of
the American Physical Societ
Dynamic convergent shock compression initiated by return current in high-intensity laser solid interactions
We investigate the dynamics of convergent shock compression in the solid wire
targets irradiated by an ultra-fast relativistic laser pulse. Our
Particle-in-Cell (PIC) simulations and coupled hydrodynamic simulations reveal
that the compression process is initiated by both magnetic pressure and surface
ablation associated with a strong transient surface return current with the
density in the order of 1e17 A/m^2 and a lifetime of 100 fs. The results show
that the dominant compression mechanism is governed by the plasma ,
i.e., the ratio of the thermal pressure to magnetic pressure. For small radii
and low atomic number Z wire targets, the magnetic pressure is the dominant
shock compression mechanism. As the target radius and atomic number Z increase,
the surface ablation pressure is the main mechanism to generate convergent
shocks based on the scaling law. Furthermore, the indirect experimental
indication of the shocked hydrogen compression is provided by measuring the
evolution of plasma expansion diameter via optical shadowgraphy. This work
could offer a novel platform to generate extremely high pressures exceeding
Gbar to study high-pressure physics using femtosecond J-level laser pulses,
offering an alternative to the nanosecond kJ laser pulse-initiated and pulse
power Z-pinch compression methods
Time-resolved optical shadowgraphy of solid hydrogen jets as a testbed to benchmark particle-in-cell simulations
Particle-in-cell (PIC) simulations are a superior tool to model
kinetics-dominated plasmas in relativistic and ultrarelativistic laser-solid
interactions (dimensionless vectorpotential ). The transition from
relativistic to subrelativistic laser intensities (), where
correlated and collisional plasma physics become relevant, is reaching the
limits of available modeling capabilities. This calls for theoretical and
experimental benchmarks and the establishment of standardized testbeds. In this
work, we develop such a suitable testbed to experimentally benchmark PIC
simulations using a laser-irradiated micron-sized cryogenic hydrogen-jet
target. Time-resolved optical shadowgraphy of the expanding plasma density,
complemented by hydrodynamics and ray-tracing simulations, is used to determine
the bulk-electron temperature evolution after laser irradiation. As a showcase,
a study of isochoric heating of solid hydrogen induced by laser pulses with a
dimensionless vectorpotential of is presented. The comparison
of the bulk-electron temperature of the experiment with systematic scans of PIC
simulations demostrates that, due to an interplay of vacuum heating and
resonance heating of electrons, the initial surface-density gradient of the
target is decisive to reach quantitative agreement at \SI{1}{\ps} after the
interaction. The showcase demostrates the readiness of the testbed for
controlled parameter scans at all laser intensities of
Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets.
We report on recent experimental results deploying a continuous cryogenic hydrogen jet as a debris-free, renewable laser-driven source of pure proton beams generated at the 150 TW ultrashort pulse laser Draco. Efficient proton acceleration reaching cut-off energies of up to 20 MeV with particle numbers exceeding 109 particles per MeV per steradian is demonstrated, showing for the first time that the acceleration performance is comparable to solid foil targets with thicknesses in the micrometer range. Two different target geometries are presented and their proton beam deliverance characterized: cylindrical (∅ 5 μm) and planar (20 μm × 2 μm). In both cases typical Target Normal Sheath Acceleration emission patterns with exponential proton energy spectra are detected. Significantly higher proton numbers in laser-forward direction are observed when deploying the planar jet as compared to the cylindrical jet case. This is confirmed by two-dimensional Particle-in-Cell (2D3V PIC) simulations, which demonstrate that the planar jet proves favorable as its geometry leads to more optimized acceleration conditions
Spectral and spatial shaping of laser-driven proton beams using a pulsed high-field magnet beamline
Spectral and spatial shaping of laser-driven proton beams using a pulsed high-field magnet beamline
Intense laser-driven proton pulses, inherently broadband and highly
divergent, pose a challenge to established beamline concepts on the path to
application-adapted irradiation field formation, particularly for 3D. Here we
experimentally show the successful implementation of a highly efficient (50%
transmission) and tuneable dual pulsed solenoid setup to generate a homogeneous
(8.5% uniformity laterally and in depth) volumetric dose distribution
(cylindrical volume of 5 mm diameter and depth) at a single pulse dose of 0.7
Gy via multi-energy slice selection from the broad input spectrum. The
experiments have been conducted at the Petawatt beam of the Dresden Laser
Acceleration Source Draco and were aided by a predictive simulation model
verified by proton transport studies. With the characterised beamline we
investigated manipulation and matching of lateral and depth dose profiles to
various desired applications and targets. Using a specifically adapted dose
profile, we successfully performed first proof-of-concept laser-driven proton
irradiation studies of volumetric in-vivo normal tissue (zebrafish embryos) and
in-vitro tumour tissue (SAS spheroids) samples.Comment: Submitted to Scientific Report
Observation of ultrafast solid-density plasma dynamics using femtosecond X-ray pulses from a free-electron laser
The complex physics of the interaction between short pulse high intensity
lasers and solids is so far hardly accessible by experiments. As a result of
missing experimental capabilities to probe the complex electron dynamics and
competing instabilities, this impedes the development of compact laser-based
next generation secondary radiation sources, e.g. for tumor therapy
[Bulanov2002,ledingham2007], laboratory-astrophysics
[Remington1999,Bulanov2015], and fusion [Tabak2014]. At present, the
fundamental plasma dynamics that occur at the nanometer and femtosecond scales
during the laser-solid interaction can only be elucidated by simulations. Here
we show experimentally that small angle X-ray scattering of femtosecond X-ray
free-electron laser pulses facilitates new capabilities for direct in-situ
characterization of intense short-pulse laser plasma interaction at solid
density that allows simultaneous nanometer spatial and femtosecond temporal
resolution, directly verifying numerical simulations of the electron density
dynamics during the short pulse high intensity laser irradiation of a solid
density target. For laser-driven grating targets, we measure the solid density
plasma expansion and observe the generation of a transient grating structure in
front of the pre-inscribed grating, due to plasma expansion, which is an
hitherto unknown effect. We expect that our results will pave the way for novel
time-resolved studies, guiding the development of future laser-driven particle
and photon sources from solid targets
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