The radially- or azimuthally-polarized beam is an example of vector beams on a higher-order Poincare sphere. Recently, much attention has been paid to the vector beams and their basic focusing properties have been studied elsewhere . Among the vector beams, because of the formation of a strong longitudinal electric field with a tight focusing geometry, the radially-polarized beam has been used for direct electron acceleration through the laser-matter interaction. Thus, it is interesting to investigate how strong longitudinal electric field can be formed by the femtosecond high-power laser pulse.
In this paper, the field strength of a longitudinal field of a tightly-focused, radially-polarized femtosecond PW laser pulse has been investigated through numerical calculations based on vector diffraction theory . Because a femtosecond laser pulse has a broad spectrum, the field strength for a given wavelength in the laser spectrum should be known for the accurate assessment of field strength of a tightly-focused femtosecond laser spot. In the research, the electric field strengths for all incident wavelength components are directly calculated from a laser spectrum and pulse energy, and used to assess the strength of a tightly-focused electromagnetic field in the focal region. The orbital angular momentum can be imposed to the light by introducing a spiral phase. A vector OAM beam is formed when the orbital angular momentum is imposed to a radially-polarized beam. Thus, the focusing property of the vector OAM beam is also discussed in the paper. The result will provide precise information on electric and magnetic fields which will be helpful in understanding electron motions under strong and unconventional electromagnetic field.
A novel regime of high frequency radiation generation via reflection at the electron density spikes in under- dense plasma is proposed. Intense driver laser pulse propagating in underdense plasma forms dense electron singularities near the front part of the bow waves, moving at relativistic velocity. By irradiating a source pulse counterpropagating to the electron density singularities, it is reflected and compressed, producing ultrashort coherent high order harmonics with frequency upshift.
Radiochromic film (RCF) based multichannel diagnostics utilizes the concept of a stack detector comprised of alternating layers of RCFs and shielding aluminium layers. An algorithm based on SRIM simulations is used to correct the accumulated dose. Among the standard information that can be obtained is the maximum ion energy and to some extend the beam energy spectrum. The main area where this detector shines though is the geometrical characterization of the beam. Whereas other detectors such as Thomson parabola spectrometer or Faraday cups detect only a fraction of the outburst cone, the RCF stack placed right behind the target absorbs the whole beam. A complete 2D and to some extend 3D imprint of the ion beam allows us to determine parameters such as divergence or beam center shift with respect to the target normal. The obvious drawback of such diagnostics is its invasive character. But considering that only a few successful shots (2-3) are needed per one kind of target to perform the analysis, the drawbacks are acceptable. In this work, we present results obtained with the RCF diagnostics using both conventional accelerators and laser-driven ion beams during 2 experimental campaigns.
The quadrupole lens free multiple profile emittance measurement method is an adaptation of the standard multiple profile monitor method for electron beam emittance measurement which was tested at PW laser system. This single shot technique allows to obtain the emittance from beam profile radii fit by means of Twiss (Courant-Snyder) parameters. Lanex scintillating screens were used as profile monitors due to their high yield of visible photons. However, on the other hand, the screen is a source of multiple Coulomb scattering which can influence the beam profile on the following screens at relatively low electron energies. Nevertheless, the contribution of the multiple scattering can be effectively subtracted from the signal by e.g. Bayes unfolding. For high energy beams (E > 0.5 GeV), the multiple scattering contribution is negligible. The presented diagnostics is easy to be implemented into standard experimental setups without any special requests for alignment procedure. Moreover, it can be useful in the optimization phase of the laser plasma accelerator where beam fundamental parameters (energy, energy spread, divergence, pointing) typically fluctuate shot-to- shot.
A tight focusing scheme using a low f-number focusing optic is frequently considered as an effort to efficiently increase a peak intensity of a high power laser. In this paper, we present a method for describing the focal spot of a femtosecond laser pulse which is formed in the spatio-temporal region under low f-number (f-number ≤ 1) focusing conditions. In the method, transverse and longitudinal electromagnetic (EM) fields for a monochromatic wave are calculated in the focal plane and its vicinity, and then, in order to precisely describe the femtosecond focal spot in the spatio-temporal domain, the calculated monochromatic EM fields are coherently superposed with a given amount of spectral bandwidth and phase. The accuracy and validity of the method are tested and compared to results obtained with Fourier transform method under high f-number conditions. The single electron trajectory under a strong longitudinal field formed by a low f-number optic is presented to emphasize the importance of the tight focusing scheme.
In the framework of the ELI-Beamlines project, the HELL (High energy ELectron by Laser) platform will host an electron beamline with a dual aim: to explore innovative concepts of laser driven electron acceleration and to deliver a stable and reliable electron beam to external users, according to their specific needs. Because of this, it is crucial to identify the possible applications and their respective range of parameters. In order to accomplish this goal, Monte Carlo simulations of electron radiography and radiotherapy are performed and discussed. Once identified those parameter spaces, a beam transport line is studied and presented for each energy range. Finally, beam diagnostics are discussed.
The performance of a 0.1-Hz-repetition-rate, 30-fs, 1.5-PW Ti:sapphire laser which is using for research on high field physics in APRI-GIST is presented. The charged particles (electrons and protons) are accelerated and an efficient x-ray generation is demonstrated using the PW laser. Protons are accelerated up to 80 MeV when an ultra-thin polymer target is irradiated by a circularly-polarized PW laser pulse. Electrons are accelerated to multi-GeV level with a help of injector and accelerator scheme. In the relativistic harmonic generation experiment, the harmonic order is dramatically extended, by optimizing the intensity of pre-pulse level, up to 164<sup>th</sup> that corresponds to 4.9 nm in wavelength and the experimental results can be explained by the oscillatory flying mirror model. The upgrade of the PW laser to the multi-PW level is under way.
A PW Ti:Sapphire laser with 30-J energy and 30-fs pulse duration has been developed at GIST and applied to generate
x-rays and energetic charged particles. We present the status and plan of developing ultrashort x-ray sources and their
applications. We successfully demonstrated x-ray lasers and their applications to high-resolution imaging. In addition,
we plan to generate high flux x-ray/gamma-ray sources using the PW laser.
New particle acceleration regimes driven by PW class lasers imply the development of new in-situ diagnostics. Before constructing new types of detectors one must test currently available diagnostics in these new regimes of high intensity laser-matter interaction. Experimental tests on two types of time of flight detectors are presented, demonstrating the possibility of their measuring capabilities in harsh conditions, namely the strong laser induced electromagnetic pulse. A recently developed silicon carbide detector was successfully tested and particle beams were characterized. Further tests were performed on a detector based on secondary emission of electrons during the transition of laser accelerated particle beams. The presented results show a clear consistency and sufficient capabilities for high intensity laser driven particle beam detection.
We have developed a 0.1-Hz-repetition-rate, 30-fs, 1.5-PW Ti:sapphire laser system for the research on high field physics. In this paper, we describe the design and output performance of the PW Ti:sapphire laser and its applications in the generation of relativistic high order harmonic generation and the acceleration of charged particles (protons and electrons). In the experiment on relativistic harmonic generation, the harmonic order dramatically extended up to 164<sup>th</sup> that corresponds to 4.9 nm in wavelength, and the dramatic extension was explained by the oscillatory flying mirror model. Recently, we could accelerate protons up to 45 MeV from a 10-nm polymer target and show the change in the acceleration mechanism from target normal sheath acceleration to radiation pressure acceleration. The femtosecond high power laser system is a good candidate for developing a compact electron accelerator as well. The generation of multi-GeV electron beam was observed from an injection scheme when a PW laser pulse was focused by a long focal length spherical mirror.
The enhancement of laser-driven proton acceleration mechanism in TNSA regime has been demonstrated through the use of advanced nanostructured thin foils. The presence of a monolayer of polystyrene nanospheres on the target frontside has drastically enhanced the absorption of the incident laser beam, leading to a consequent increase in the maximum proton beam energy and total laser conversion efficiency. The experimental measurements have been carried out at the 100 TW and 1 PW laser systems available at the APRI-GIST facility. Experimental results and comparison with particle-in-cell numerical simulations are presented and discussed.
The wavefront aberrations in the 100 TW laser pulses are measured and corrected to improve the intensity distribution of
the focal spot. Before correcting wavefront aberration of the laser pulses, the laser pulses have higher-order aberrations
such as coma, trefoil, and spherical aberration as well as defocus and astigmatism. The wavefront aberrations in the laser
pulses are corrected by the deformable mirror. The dynamic and static corrections are tested with the deformable mirror.
When correcting wavefront aberrations with the deformable mirror, the focal spot having a 1.2 times spot size of the
diffraction-limited focal spot is observed.