This PDF file contains the front matter associated with SPIE Proceedings Volume 11778, including the Title Page, Copyright information, and Table of Contents.

We propose a table-top linearly polarized hard X-ray source by using a tilted shock-front injection in a laser wakefield accelerator (LWFA) to achieve comprehensive control of both polarization and energy of X-ray. By using shock-front injection, the electron bunches are injected during a sharp transition of plasma density. The length of density transition is significantly shorter than the plasma wavelength and offers a highly localized injection. In regular injection methods, such as self and ionization injection, the majority of electrons are injected radially symmetrically. Particle-in-cell (PIC) simulations show the tilted shock front breaks radial symmetry of injection and creates coherent in-plane oscillation of electrons. The coherence of electron bunches is maximized around 30 degrees which leads to a linearly polarized betatron radiation. The polarization of the resulting X-ray is analyzed by Bragg diffraction after collimation by a polycapillary lens.

The photon yield obtained during the betatron motion of electrons in the ion cavity is proportional to the number of such oscillations (or wiggling). In the case of high repetition rate laser systems the pulse energy is very limited, thus the laser pulse has to be tightly focused in order to achieve high peak intensity in the focus. In this way narrow gas jets are used, which results in short acceleration length of electrons, thus in low number of betatron oscillations. In order to increase the number of x-ray photons a mixture of clusters and gas target can be used, where the space-charge of nano-meter size droplets forces the electrons to oscillate at much higher frequency leading to the emission of more photons. In this work we present the interaction of such cluster targets with linearly polarized laser pulses and describe some possible positive effects when circular polarization is used.

Comparing a conventional undulator, the concept of plasma undulator is hard to realize because of a small-scaled cavity structure and a longitudinal acceleration field in the plasma cavity. So, the electron beam with betatron motion should stay in the longitudinal center of the plasma cavity. An other issue is the plasma cavity should propagate keeping a stable cavity formation and speed for entire plasma target. Moreover, high photon energy like UV and soft x-ray is much challenge since it requires longer plasma length and higher speed plasma cavity. To satisfy such conditions we investigate the electron beam-driven plasma cavity using PIC simulations. The life-time of the plasma cavity depends on how long the driving electron beam survives. We test several plasma profile, electron beam, and plasma lens parameters by testing matching conditions of the electron beam. In the presentation we also introduce new numerical technique in PIC to eliminate the numerical Cherenkov radiation causing unwanted increases of emittance. Finally we discuss a possibility of FEL from electron’s betraon motions.

The laser wake-field acceleration (LWFA) has been spotlighted as a compact electron accelerator, because of its accelerating gradient being several hundred times higher than conventional RF accelerators. In LWFA, a supersonic gas jet or discharged gas flow in a capillary is typically used as a plasma target, Recently, a plasma plume ablated from a solid target, such as, Teflon, Nylon or Aluminum, using a nano-second or pico-second laser pulse is proposed to maintain high vacuum and possibly operate at high repetition rate. In addition, it was demonstrated that metals, like aluminum, having higher charge states play an important role to increase the electron charge. Compared with the LWFA mechanism using helium or hydrogen gases, the LWFA using metallic targets involves the ionization effects. It boosts more electrons to be injected in a wake cavity, while reduces the acceleration length due to ionization diffraction. As increasing the injected electrons, more dynamic betatron oscillation is observed. For developing the new betatron emitter using LWFA, we suggest a dual-staged LWFA using metallic targets: the first is as a source, an energetic electron bunch, and the second is as a radiator. In this presentation, an overview and a study of the control of the betatron emission via 2D or 3D PIC (Paritlce-in-cell) code simulations are described.

Laser plasma wakefield accelerations (LWFA) are the most promising candidates for future compact accelerators and also can be used for next-generation free-electron lasers (FELs). However, due to the insufficient electron beam quality, such as a few percent of energy spread, stability, and reproducibility, the electron beam from the LWFA has difficulty to be directly used for FELs with a range from soft X-ray to hard X-ray. To overcome this limitation of the beam quality from the laser wakefield acceleration using various injection techniques, one of the most reliable way is to use the electron beam with short duration, lower energy spread, and emittance from the RF photocathode. This external injection technique is planning with conventional S-band RF photocathode gun and final energy of 70 MeV, few tens fs duration, and lower emittance at Pohang Accelerator Laboratory Injector Test Facility (PAL-ITF). In this presentation, we show a simulation result on the characterization of the electron beam from LWFA using external injection for soft X-ray free-electron lasers.

The accelerating structure of the laser wakefield accelerator (LWFA) is dynamic and highly sensitive to the local laser and plasma properties. It can expand and contract as it responds to the evolution of the laser and plasma fields. As a result, the position of, and environment within, the LWFA bubble are usually time dependent, which is not ideal for stable acceleration. Variations can have a negative impact on electron bunch properties, and are deleterious for ion channel lasers and plasma wigglers. We demonstrate how a laser pre-pulse improves the stability of the LWFA, and controls the evolution of the laser group and bubble velocity, which are important for determining LWFA dephasing and ultimately the electron bunch energy.

This talk is an introduction to ion-channel free-electron laser with longitudinal acceleration.

High-power terahertz (THz) sources are essential for many applications including THz-driven acceleration of electrons [1], molecular alignment [2], and material science [3]. Among many THz sources, a THz source with two-color laser mixing in gases has attracted much interest due to its capability of producing intense, broadband THz radiation [4]. In this scheme, a femtosecond laser pulse with its second harmonic pulse is focused to ionize a gaseous medium at the focus and create the plasma current, which can produce an intense, broadband THz pulse. With this scheme, we present an experimental study on efficient THz pulse generation from two-color laser filamentation of the mid-infrared laser pulse at 3.9 um in air. We find that mid-infrared yields higher THz energy with a laser-to-THz conversion efficiency of ~1%, which is about 10~100 times larger than conventional values with 800 nm laser pulse [4]. [1] E. A. Nanni et al., Nat. Commun. 6(1), 8486 (2015) [2] T. Kampfrath et al., Nat. Photonics 7(9), 680-690 (2013) [3] D. Nicoletti and A. Cavalleri, Adv. Opt. Photonics 8(3), 401-464 (2016). [4] D. Jang et al., Optica 6(10), 1338-1341 (2019)

We develop an analytical model for the generation of terahertz (THz) fields by the propagation of a higher-order Gaussian laser pulse in a magnetized plasma. A higher-mode Gaussian pulse is utilized for this purpose. The plasma nonlinearity is enhanced due to the modified gradient of the Gaussian spatial intensity profile. THz field generated by the transverse wakefields is significantly larger for this kind of laser pulses. The amplitude of THz fields can be enhanced by the flatness parameter of the higher-order Gaussian laser pulse. The fivefold enhancement of the THz field is reported in this study. Furthermore, the applied axial magnetic field also contributes to enhancing the THz field. For higher-order Gaussian laser pulse, the power conversion efficiency of THz field generation in the presence of a magnetized is four times higher than the ordinary Gaussian pulse case. The production of intense THz field with amplitudes belonging to the GV/m range is helpful in various applications such as THz extreme nonlinear optics and probing remote materials efficiently.

Radiation sources from Langmuir waves has been a topic of interest for their relevance to experimental approaches in plasma laboratories as well as for estimating physical models to explain cosmic radio bursts. Since the mechanism for converting energy from electrostatic Langmuir waves to electromagnetic waves is complex, diverse scenarios of such energy conversion have been studied, e.g. mode conversion, antenna radiation, nonlinear scattering, etc. Previously, we introduced a novel perspective of plasma dipole oscillation (PDO) which generates strong radiation bursts at the plasma frequency and high harmonics. In this paper, we report our discovery of radiation that result from electron-laser beam driven Langmuir waves and their interactions. In 2-D PIC simulations, we have observed that obliquely colliding Langmuir waves or even a single Langmuir wave generate localized radiation sources at the plasma frequency and high harmonics. These mechanisms differ from conventional two-plasmon mergers, where only the second harmonic of the plasma frequency is dominant: a strong radiation is observed even at the fundamental harmonic. In addition, from 3-D PIC simulations of electron laser beam driven plasma oscillators in magnetized plasma, the radiation from a local plasma oscillator, i.e. PDO, is found to be robust with diverse spectral peaks at the X-mode and the upper-hybrid mode. Nonlinear theory demonstrates that the relative strength of the harmonics of the plasma frequency depends on the shape of the PDO. The studies imply that the PDO has a more complicated internal structure than the simple model of a solid charge. We discuss the potential of the PDO generated from electron-beam driven plasmas or laser-driven plasmas as a radiation source and its relevance to cosmic radio bursts.

Generating high-field infrared and terahertz radiation during interaction of a super-intense laser pulse with a complex nanodimensional target consisting of nanowires or nanofoils is studied. During interaction, dense bunches of electrons are extracted out of the target and accelerated in the laser field, generating intense electromagnetic radiation. Depending on the duration and shape of the laser pulse, three interaction modes can be realized. In the first mode, the laser pulse is smooth, and the electrons are only partially displaced from the target. In this case, a unipolar radiation pulse is generated with duration of about that of the laser pulse. In the second mode, the laser pulse is nonadiabatic with the amplitude of the first half-cycle about the maximum pulse amplitude. Here, most of the electrons are extracted from the target at the beginning of interaction, and unipolar and bipolar pulses with duration of dozens of laser periods can be generated. Changing the target geometry allows one to control the period and number of oscillations in the generated radiation. Finally, in the intermediate mode of short laser pulses with an insufficiently steep front, oscillations of the formed electron bunches may occur in the Coulomb field of ions, leading to radiation with a frequency several times lower than that of the laser. Using numerical simulation, the characteristics of infrared and terahertz radiation in three interaction modes are found. It is shown that the amplitude of generated radiation can reach subrelativistic values, and the intensity conversion efficiency can be about one percent. The advantages of using complex nanowire targets are elucidated. Such targets allow to generate a train of terahertz and infrared pulses with controlled delay between them.

Classical electrodynamics basically assumes that a charged particle is a point charge, whose size is infinitesimal. This assumption caused a lot of problems with the radiation reaction (also called the self-force), by which a radiating particle feels friction. For example, Lorentz-Abraham-Dirac (LAD) model, which is the earliest theory about radiation reaction based on a point charge, leads to unphysical solutions such as runaway solutions and preacceleration. It has been tried to modify LAD model, and the most successful result was Landau-Lifshitz (LL) model, which also interpret well the recent experimental data of radiation reaction from laser-plasma interactions. However, even LL model has some problems, and particularly it cannot explain the radiation reaction of a uniformly accelerated charge. In this talk, we assume that the size of a particle is finite but the particle itself is completely indistinguishable from a point charge. By this assumption, we calculate the self-force of this particle undergoing uniform acceleration and uniform circular motion. The calculations show that the self-force of uniform acceleration can increase the effective mass of the particle, and it explain why a uniformly accelerated particle needs more energy to radiate. The calculations also indicate that the self-force of uniform circular motion only depends on the acceleration at the retarded time. In addition, we propose that there is a classical limit of the acceleration, and by combining it with the Schwinger limit we suggest that the classical radius of a particle is somehow related to the reduced Compton wavelength.

With the advance of high power laser technologies, we are approaching the possibility to study strong-field vacuum breakdown and an accompanying electron-positron pair plasma via a quantum electrodynamic (QED) cascade. To reach a high quantum nonlinear factor in the particle rest frame, two different configurations are envisioned either through collision of a laser pulse with an energetic electron beam or collision of two laser pulses. Producing QED pair plasmas all optically is generally believed to require next generation laser technology that can output 100 PW power. Based on the electron-beam-laser collision setup, our recent work, however, shows that signatures of collective pair plasma effects appear in exquisite detail through plasma-induced frequency upshifts in the laser spectrum. Remarkably, these signatures can be detected even in a pair plasma created by passing a dense multi-tens-GeV electron beam through a multi-PW laser pulse. This method substantially reduces the already low laser intensity requirement, and the use of lower laser intensities, compared to all-optical methods, significantly makes the QED collective effects easier to observe. Strong-field quantum and collective pair plasma effects can thus be explored with existing technology, providing vital information for QED cascades in general and QED plasma regimes in particular.

Recently, there have been some theoretical studies on the use of two-color intense laser pulses for the laser wakefield acceleration (LWFA) research. Here, we experimentally demonstrate a laser-plasma accelerator driven by relativistic copropagating two-color laser pulses (CTLP) in pure helium and in helium-nitrogen mixed gas targets where we observed significant enhancements in the energy spectra of the electron beams (Sci. Adv. 5. eaav7940, 2019). Such enhancement has been further confirmed in a real-application, where electrons from the CTLP-driven LWFA scheme are used in a bremsstrahlung-based positron beam generation configuration, which led to a significant boost in the positron beam energy as well. Numerical simulations suggested that the trailing second-harmonic relativistic laser pulse can sustain the acceleration structure (i.e., the plasma wave) for much longer distances after the preceding fundamental laser pulse is depleted in the plasma medium. Therefore, our experimental work confirms the advantage and robustness of the CTLP-driven LWFA scheme over the standard LWFA driven by a single-pulse of equivalent power. This paves the way towards a significant down-sizing of laser-plasma electron accelerators making their use in scientific and technological applications extremely attractive and affordable.

The theory of electrostatics is embedded within Maxwell’s electrodynamics, which is one of the most successful physical theories ever produced. However, a more direct generalisation of electrostatics to make it compatible with relativity would have been to replace it with the sourced massless Klein-Gordon equation. This paper gains insight into these two theories by comparing their structures and consequences in the static limit, finding that the two agree in terms of the field dynamics, but differ substantially in terms of how charged particles interact with the fields. These differences are most prominent for ultra-relativistic particles interacting with strong fields, conditions relevant to the new generation of high power laser facilities. Further connections between electrodynamics and scalar dynamics are explored by unifying the theories in higher dimensional spacetime, analogously to the Kaluza-Klein unification of electrodynamics and gravity.

For current state-of-the-art terawatt lasers, the primary laser scattering mechanisms in plasma include Forward Raman Scattering (FRS), excitation of plasma waves, and the self-modulational instability (SMI). Using 2D PIC simulations, we demonstrate the dominance of the FRS in the regime with medium-to-low density plasma and non-relativistic laser fields. However, the use of multi-colored lasers with frequency detuning exceeding the plasma frequency suppresses the FRS. The laser power can then be transmitted efficiently.

The increasing demand for high laser powers is placing huge demands on current laser technology. This is now reaching a limit, and to realise new areas of research promised at high intensities, new cost-effective and technically feasible ways of scaling up the laser power and controlling its optical characteristics are required. It is likely that ultra-compact, time-dependent, plasma-based amplifiers and optical elements (such as mirrors, polarisers, waveplates, etc.) will be required to reach powers exceeding tens of petawatts and possibly exawatts, and to manipulate the laser beams. Plasma is a robust optical medium as it is broken-down and can sustain extremely high electric fields. Plasma-based optical elements will most likely require the production of transient plasma gratings (TPGs), which take advantage of the ponderomotive force of the beat of at least two laser pulses. To create an amplifier, the TPG is formed through the action of the beat wave of a pump and seed pulse, which has a phase velocity satisfying the conditions for energy and momentum conservation of the three waves, where the third wave is a Langmuir or ion acoustic wave. Other optical elements will normally require static TPGs produced by degenerate driver pulses. Here we present the results of an experimental campaign conducted at the Central Laser Facility, where we have studied chirped pulse Raman amplification at high intensities. We have used a relatively long duration, frequency chirped, pump pulse to limit the growth of noise amplification, while ensuring amplification of the short seed pulse. We show that by changing the sign of the frequency chirp of the pump, the measured back-scattered and amplified seed energies change significantly. A negative chirp leads to a strong reduction in scattering from thermal density fluctuations, but seed amplification saturates. In contrast, for a positive frequency chirp, scattered energy continues to increase with increasing plasma density, without showing any sign of saturation, for the range of densities studied. From simulations we attribute this observation to the production of a local, long-lived, static TPG that continues to scatter the pump pulse long after it has passed. We will discuss the specific conditions that should be satisfied to produce such a grating. The ability to produce and maintain robust TPGs may provide a breakthrough in technologies for manipulating, reflecting and compressing ultra-intense laser pulses.

Volume density gratings produced by degenerate, counterpropagating laser pulses in plasma have several useful optical properties. Here we report on one of these in an investigation into creation of a transient plasma density grating that functions as a waveplate.

Accelerators driven by 10s TW-class lasers can produce electron bunches with femtosecond-scale duration and energy of 100s of MeV. A potential application of such short bunches is high-dose rate radiotherapy, which could transition to FLASH radiotherapy if a sufficiently large dose is delivered in a single shot. Here we present Monte Carlo simulations to study the bunch length evolution of an electron beam propagating in a water phantom. We show that for electron energies above 100 MeV the bunch lengthens to 1–10 ps duration after interaction with a 30 cm long water phantom, both for a collimated and weakly focused geometry. The corresponding dose rates are on the order of 200 Gy/s per primary electron, much higher than in conventional radiotherapy.

Femtosecond MeV electron beam generated by laser-plasma accelerators (LPA) is a promising source for ultrafast electron diffraction (UED) application. Compared to conventional UED instruments which limit temporal resolution to a few tens of fs, plasma electron accelerator-based UED is possible to make sub-10 fs temporal resolution because of no intrinsic time jitter between pump-probe pulses and ultrashort electron bunch length. Some groups have shown that a few MeV electron beam can be produced by using a few mJ laser pulse as it has shorter pulse duration (single- or few-cycle). In this regime, the laser pulse is tightly focused onto gas target, and thus electrons in relatively high density plasma (1020 cm-3) are self-injected and accelerated. However, the electron beam quality like energy spread and emittance should be still improved for applications. Here, we introduce plan of two laser pulses-based plasma electron acceleration research for UED application at Pohang Accelerator Laboratory (PAL). A laser pulse is separated to two pulses that one is used to drive plasma wakefield and the other one is delivered to induce electron injection in a plasma bubble. Since the driving pulse intensity is retained under threshold of self-injection to suppress electron injection, the electron injection occurs in a localized region the injection pulse is focused, resulting in the high quality electron generation. In addition, researches conducting for better electron beam quality are presented in this presentation.