THz (terahertz) waves have a wide range of important applications in science and technology, but solid-state-based generation of intense THz waves is quite limited. One of methods for generation of intense THz pulses is using a fs laser and plasma. In fact, a strong THz pulse can be generated by focusing an intense fs laser pulse in gas. In our laboratory, we produced intense THz pulses using several methods with a single laser pulse or two laser pulses of different wavelengths. The generated THz pulse forms were diagnosed by THz-TDS (THz time-domain-spectroscopy) and/or a spectral-encoding-based single-shot method. Furthermore, we used the broadband THz pulses for plasma diagnostics. In this experiment, the THz pulses were sent through an inductively-coupled plasma and the transmitted THz wave pulses were analyzed for phase shift. In this way, we obtained the plasma density information successfully, which can be applied for a wide range of plasma density diagnostics. In this talk, those details for laser-plasma-based THz wave generation and applications will be presented.

This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.

A laser electron accelerator using a laser-ablated metallic target has been developed for the applications of compact light sources at high-vacuum and/or high repetition rate. Unlike any plasma targets using gases, the total amount of molecules generated by ablating a ns or ps laser pulse on metal target is too small to change the vacuum level while the electron plasma density is reached the proper level required for laser acceleration through the optical ionization process by high-intensity Ti:Sapphire laser (here, called the main laser). The density enhancement as well as the electron injection in laser acceleration will be different for target elements (aluminum, copper, titanium, and so on). The ring-type permanent dipole magnet suggested has a periodic modulation of magnetic-field strength along the trajectory due to H-type iron yoke, indicating the possibility of improvement of beam stability and the angular acceptance of injection. We added the hill-valley ratio of ring-type iron pole gap to adjust the modulation strength of the azimuthally-varying magnetic field. The vertical focusing may increase as increasing the hill-valley ratio, at the expense of average field strength. In this paper, the beam quality and stability depending on the metallic target structure and the magnetic field modulation due to ring-type iron pole structure are presented.

Laser induced plasma acceleration of ion beams has been attracted long time due to its ultrahigh acceleration field, which leads to a compact accelerator in addition to the characteristics of the accelerated ion beams such as high intensity and short pulse duration. There have been enormous efforts to develop the technology along with understanding the underlying mechanisms. However, the ion beam has a characteristic broad or thermal-like energy distributions, which puts a limit in the application of the ion beams. A novel acceleration mechanism, RPA (Radiation Pressure Acceleration) has been considered to overcome such a limit but yet to be demonstrated. The authors considers a layered target, which composed of a metal layer and a plastic layer, utilizing a bulk electrostatic field diffused into the target from the rear surface of target. Using a 150 TW Ti:Sapphire laser at CoReLS, interesting carbon ions and protons showing non-Maxwellian energy spectra were generated from a copper foil coated with a polymer at the rear side and maximum energy was enhanced by a factor 1.7 compared with a single copper foil. A bulk electrostatic field formed at the metal-polymer interface and multi-species interactions are consider to cause such an energy distribution and enhancement.

High power lasers have become large because of they are based on low damage-threshold optical media. A more robust and compact medium for amplifying and manipulating intense laser pulses is plasma. Here we demonstrate that a few-millijoule, ultrashort seed pulses interacting with 3.5-J counter-propagating pump pulse in plasma can back-scattering nearly 100 mJ pump energy with high intrinsic efficiency, even when detuned away from the Raman resonance frequency. Scattering occurs off a plasma Bragg grating that is formed by ballistically evolving ions. Electrons bunched by the ponderomotive force of the beat-wave produce space-charge fields that impart phase correlated momenta to ions. Ions and electrons evolve into a volume Bragg grating that backscatters the pump pulse in the direction of the seed pulse. This, ultra-compact, two-step, inertial bunching mechanism can be used to manipulate and compress intense laser pulses. We also observe evidence of stimulated Compton (kinetic) and Raman backscattering. Experiments have been undertaken at the Central Laser Facility, Rutherford Appleton Laboratory using the Gemini laser.

Density gratings in plasma created using intersecting intense laser pulses can serve as transient photonic structures with interesting optical properties. We present analytical modelling of the formation of such structures and their dispersive properties, in particular birefringence.

Laser pulses with a few petawatt peak power is available from chirped-pulse-amplification, where the compression gratings of meter scale is vulnerable to the damage. For the exawatt or zettawatt lasers, as the diameter of gratings increased to hundreds of meters is challenging, a fundamentally different approach is required. One promising method can be found in plasma physics; as the plasma is damage-free and at the same time, strongly dispersive to electromagnetic waves. By properly designing the plasma structure, the powerful laser pulses can be compressed in a similar way to solid-based grating mirrors. Various methods exploiting the plasma have been investigated: Raman backward amplification, Brillouin amplification, and transient plasma gratings. Here I introduce a totally different idea to obtain high compression ratio with small energy loss. In the new method, a smooth gradient, high density (near-critical) plasma is used. Photons in a long negatively chirped incident pulse are reflected at different position inside the plasma, depending on their frequencies. The path difference of photons leads to concentration of the photons in a narrow region. From one-dimensional simulations, hundreds of times compression was obtained for the intensity orders of magnitude larger than the intensity sustainable by solid state of materials.

Coherent light sources, such as free electron lasers, provide bright beams for biology, chemistry, physics and advanced technological applications. As their brightness increases, these sources are also becoming progressively larger, with the longest being several km long (e.g. LCLS). Can we invert this tendency, and bring these sources back to thousands of university, hospital, and industry labs? Plasmas accelerator sources are certainly an attractive solution to this question, but only if their brightness increases several orders of magnitude. Drivers with a higher energy and power will increase the photon yield, but such a dramatic improvement depends on the onset of temporal coherence and superradiance. We intuitively expect superradiance emission when many light emitting particles fit within a radiation wavelength. When this condition is met, all particles radiate coherently as a single giant particle. Instead, here we show that superradiance can occur even when particles are arbitrarily far from each other [J. Vieira et al. Nat. Physics 17, 99 (2021)]. Moreover, by re-analysing the basics of superradiant emission, we explore previously unrecognised configurations leading to superradiance in plasma accelerator based light sources. We explore these concepts in theory and through particle-in-cell simulations complemented by the Radiation Diagnostic for Osiris (RaDiO) [M. Pardal et al, Comp. Phys. Communs. 285, 108634 (2023)].

Since they have been proposed, laser-plasma accelerators have interested the scientific community for their ability to generate electric fields exceeding the ones of Linacs and RF cavities. Several efforts have been made in order to produce monochromatic electron beams and to increase their maximum energy, often at the expense of the charge. However, some applications like femtosecond chemistry, radio-biology and industrial radiography do not need monochromatic beams, but rather highly charged ones (i.e., > 1 nC). For some of these applications it is also necessary to reduce the amount of high energy electrons (i.e., > 10 MeV), in order to avoid the activation of materials. Such beams can be produced using high Z gases like Nitrogen and Argon, exploiting the ionization injection of several plasma period. Here we numerically and experimentally investigate this little-known regime, employing different laser energies, f-numbers and plasma densities. This allowed us to find the conditions to produce electron beams with charges up to tens of nC and exceeding 100 mrad in divergence. We will also show and explain the dependencies of these beams (e.g., their charges and energy spectra) as functions of the aforementioned laser and plasma parameters.

Electrons can be accelerated to multi-GeV energies by the mechanism called Direct laser acceleration. The acceleration is secured by the resonance between betatron oscillations in the plasma channel and doppler-shifted oscillations in the laser field. We propose the scaling of electron energy that can be achieved by the mechanism as a function of laser intensity and plasma density. The scaling is in good agreement with quasi-3D particle-in-cell simulations. Also, the role of a laser spotsize in the acceleration is demonstrated which allows us to estimate the optimal laser focusing which maximizes electron energy.

Laser wakefield acceleration (LWFA) using laser-ablated metallic plasma targets has been developed for high-vacuum and high-repetition rate operations. Unlike the supersonic gas jet targets or capillary gas discharge target, the ionization effect of metallic ions due to high intensity fs laser increases the ionization diffraction and ionization injection, resulting in larger energy spread with higher charge. We proposed a structured metal target using two different metals to improve the beam quality. By adding a thin Ti or Cu wire in aluminum target and changing the focal position of fs laser pulse with respect to the position of the thin-layered zone, the injection timing of electrons depleted from ions of thin wire can be adjusted to improve the beam quality. We present and discuss the simulation results depending on the thickness and the position of the thin layer. *This work was supported by the National Research Foundation of Korea(NRF) grant founded by the Korea government(MSIT). (NRF Grant No. : NRF-2021R1A2C2094300, and RS-2022-0014317)

This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.

Ion acceleration by compact laser-plasma sources promises a variety of applications, but reaching the required beam quality parameters demands a high level of understanding and control over the laser-plasma interaction process. Several advanced acceleration schemes, including the Relativistically Induced Transparency (RIT) regime, have been proposed and investigated in search of a stable acceleration regime for proton energies beyond 100 MeV. Central component in the RIT scheme is the absorption of the electromagnetic laser field by the target and the generated plasma respectively. In joint studies, we experimentally explore these interactions at the onset of transparency at the two laser systems DRACO PW (HZDR) and J-KAREN (KPSI). With our transmission diagnostics, we characterize this onset with respect to the laser temporal profile in order to learn about the sensitivity of laser input parameters to increase the process’s robustness. Using ultra-short, high and low-contrast laser pulses on thin solid density foil targets, we observe high performance proton beams in an expanded foil case. The results of spectral, spatial, and energy analysis of the effects on the transmission and its correlation with the acceleration performance indicate changes in the plasma interaction itself.

The extremely high electric fields sustainable by a plasma make the Laser Wakefield Acceleration (LWFA) the most compact technique to generate very highly relativistic electron beams in the GeV regime. The limited repetition rate and low efficiency of this technology has, to date, prevented to unleash its full potential as a unique source for basic research, biomedical applications and high flux sources of secondary radiations as hard X-rays and gamma-rays. In very recent years different works show a new research direction on electron acceleration at 1 kHz repetition rate. In this talk I will show the laser-driven acceleration of unprecedented, collimated (2 mrad) and quasi-monoenergetic (ΔE/E = 25%) electron beams with energy up to 50 MeV at 1 kHz repetition rate. The laser driver is the in-house developed L1-Allegra multi-cycle (15 fs) 1 kHz OPCPA system, operating at 26 mJ (1.7 TW). Said innovative results have been achieved in the new Laser Wakefield ALFA platform for user experiments developed at ELI-Beamlines. The scalability of the driver laser technology and the electron beams reported in this work pave the way towards developing high brilliance X-ray sources for medical imaging, innovative devices for brain cancer treatment and represent a step forward to the realization of a kHz GeV electron beamline.

This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.

The development of laser plasma accelerators has been centred around the optimisation of particle yield resulting from the interaction of high energy, single-shot lasers with very low average power. Here we aim at the already existing, high average power lasers with kHz repetition rate but low pulse energy. In our experiment, the SEA laser of ELI-ALPS provided ultrahigh contrast laser pulses of 12fs pulse duration and 20mJ energy on the 200 nm thick dPE foils. A rotating wheel target system allowed the laser interaction run at 1 Hz repetition rate in bursts of 75 shots. The accelerated deuterons, measured by Thomson spectrometers for each shot, hit a 0.1mm thick, 20 mm diameter deuterated-polyethylene disk. Neutrons were then generated via d+d fusion reaction with a mean energy of 2.45MeV. The neutron events were detected by four plastic scintillators at various angles around the chamber. From the ToF distributions we have concluded that an average of >3000 neutrons were generated in a shot, with a slight anisotropy to the forward direction. The upcoming development of kHz repetition-rate primary-target systems would boost the yield of neutrons in a second, that may exceed which can be achieved with PW class lasers..

The ultrahigh laser intensities enabled by high power lasers facilitate the generation of high energy ions using accelerating gradients many million times that of conventional accelerators. The maturation of these sources relies on breakthroughs in the generated beam parameters and improved reproducibility and repetition rate. We used two independent state-of-the-art femtosecond laser systems capable of repetitive operation to accelerate protons and carbons to high energies (>50 MeV and 30 MeV/nucleon respectively) in the relativistically induced transparency (RIT) regime. We demonstrate that acceleration is optimised for different laser prepulse levels by varying the initial target thickness, relaxing laser requirements for energetic ion generation. We elucidated the acceleration dynamics with cutting-edge 3D simulation, showing a) the role of the laser prepulse in pre-expanding the target, and b) radiation pressure assisted electron expulsion from the target during relativistically induced transparency, generating a strong space charge field which rapidly accelerates ions. Our demonstration of a robust acceleration mechanism that does not require complicated targetry nor a single-shot prepulse suppressing plasma mirror is an important step forward for developing high repetition rate applications of laser driven ion sources.

A heavy water target system for high repetition rate multi-TW lasers as deuteron source will be presented. This self-regenerated ultrathin liquid leaf surface composed from two liquid jets collided from nozzles of 11 um. At the current state of the development, a stable leaf with a length of 1.5 mm is maintained in 10E-4 mbar vacuum. The thickness of the liquid sheet is <200 nm, measured from the modulation of the spectrally resolved interference arising from white light reflection on the front and the back surfaces of the liquid sheet.

We report on the commissioning of the ELIMAIA beamline laser-plasma Ion Accelerator carried out at relativistic intensities (~1021 W/cm2) with the high repetition-rate, high peak-power L3-HAPLS (>10J in 30 fs) laser available at the ELI Beamlines user facility. Targets of different composition and thickness were used to optimize the performance of the Ion Accelerator. In the best conditions, we were able to reach proton cutoff energies around 30 MeV and fluxes above 1011/sr. Moreover, we have demonstrated an excellent reliability and shot-to-shot stability (1-2% in energy) of the Ion Accelerator up to a repetition rate of 0.5 Hz for several hundreds of consecutive shots, along with on-shot target positioning and data acquisition and analysis systems. These results demonstrate the robustness of the developed technology available for users at the ELIMAIA beamline, thus paving the way towards its future use for fundamental and applied research, including biomedical ones.

The advance in laser-driven accelerators is progressively allowing to consider these sources for many different applications. Indeed, relatively compact laser systems can deliver few joules on target reaching intensities of 1019 Wcm-2. Such lasers are present in many facilities and are nowadays available as standard products for purchase. The capability of exploiting these sources would thus be beneficial also in terms of larger availability for users willing to leverage proton irradiation. In this regime, the Target Normal Sheath acceleration (TNSA) is routinely triggered and provides few MeV of protons in short bunches, delivering high dose per shot. Nevertheless, the accelerated proton beam is typically characterized by a divergence of 15° half-angle. Thus, for its effective employment, it is necessary to implement a magnetic transport line to transfer the protons from the TNSA source to the irradiation site. This issue has been faced by many groups and a cost-effective, compact magnetic beamline (hereinafter MBL) has been proposed in a previous work to sensibly enhance the proton flux on secondary targets for protons energy up to 10 MeV. Geant4 simulations were carried out to assess the feasibility of using an upgraded version of the mentioned MBL to employ laser-driven accelerator as proton source for in-air irradiation of secondary targets. The great versatility of this approach allows to explore multiple irradiation schemes that can be involved in different applications such as Ion Beam Analysis (in particular here we refer to PIXE and XRF) and radioisotope production for PET scan. The outcomes of the simulations were used for the preparation of two dedicated experimental campaigns on the two thematics. Here we present the obtained results and compare them with simulation. The main achievements are then discussed taking as reference the performance of the currently adopted methodologies, reporting on the main advantages and limits of laser-driven sources with respect to conventional one.

We report on the time-resolved observation of transient laser-induced breakdown (LIB) during the leading edge of high-intensity petawatt-class laser pulses with peak intensities up to 6x10^21 W/cm^2 in interaction with dielectric cryogenic hydrogen jet targets. The results show that LIB occurs much earlier than what is typically expected following the concept of barrier suppression ionization and that the laser pulse duration dependence of LIB and laser-induced damage threshold (LIDT) is very relevant to high-intensity laser-solid interactions. We demonstrate an effective approach to determine the onset of LIB, i.e. the starting point of target pre-expansion, by comparing a laser contrast measurement with a characterization study of the target specific LIB thresholds.

We report on experimental observation on periodic modulation in the energy spectrum of laser accelerated proton beams. Interestingly, theoretical model and two dimensional particle-in-cell simulations, in good agreement with the experimental finding, indicated that such modulation is associated with periodic modulated electron density induced by transverse instability. These results, may have implications for further understanding for the accelerating mechanisms as well as optimization strategies for laser driven ion acceleration.

This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.