Session TO8: High-Energy Density Physics, High-Field Physics, and Intense Beams

Multipactor Breakdown Thresholds at 110 GHz

A high power, 110 GHz source has been used to experimentally measure the maximum sustainable electric fields on dielectric materials in vacuum. The purpose of this work is to evaluate the suitability of these materials for potential future applications in high frequency linear accelerators or passive high power microwave components. There was previously a lack of data at frequencies above 11 GHz. Materials tested include alumina, sapphire, fused quartz, crystal quartz, and high resistivity silicon. Dielectric samples are tested both as windows, with electric fields parallel to the surface, and sub-wavelength dielectric rod waveguides, with electric fields perpendicular to the surface. The experiment was designed to test surface electric fields in excess of 100 MV/m. Samples were tested with 3 microsecond pulses of microwave power. Visible light emission, absorbed/scattered microwave power, and emitted electrons are used to detect and monitor discharges on dielectric surfaces. The results of these experiments have been compared to theoretical calculations of multipactor breakdown thresholds, testing these theories at significantly higher frequencies than has been done before.   

Oversized 250~GHz Traveling Wave Tube with a Photonic Band-Gap Structure

The challenge in manufacturing traveling wave tubes (TWTs) at high frequencies is that the sizes of the structures scale with, and are much smaller than, the wavelength. We have designed and are building a 250~GHz TWT that uses an oversized structure to overcome fabrication and power handling issues that result from the small dimensions. Using a photonic band-gap (PBG) structure, we succeeded to design the TWT with a beam tunnel diameter of 0.72~mm. The circuit consists of metal plates with the beam tunnel drilled down their center. Twelve posts are protruding on one side of each plate in a triangular array and corresponding sockets are drilled on the other side. The posts of each plate are inserted into the sockets of an adjacent plate, forming a PBG lattice. The vacuum spacing between adjacent plates forms the ‘PBG cavity’. The full structure is a series of PBG coupled cavities, with microwave power coupling through the beam tunnel. The PBG lattice provides confinement of microwave power in each of the cavities and can be tuned to give the right amount of diffraction per cavity so that no sever is needed to suppress oscillations in the operating mode. CST PIC simulations predict over 38~dB gain with 67~W peak power, using a 30~kV, 310~mA electron beam, 0.6~mm in diameter.   

Harmonic Frequency-Locking In The Multi-Frequency Recirculating Planar Magnetron

Sources that generate high power electromagnetic radiation at multiple microwave frequencies are relevant for technologies such as counter electronics. At The University of Michigan, the Multi-Frequency Recirculating Planar Magnetron (MFRPM) [1,2] has been experimentally demonstrated to produce simultaneous oscillations at 1 GHz and 2 GHz, generating up to 32 MW and 13 MW, respectively. The MFRPM is driven by MELBA-C at -300 kV, 1-5 kA for 0.3-1.0 $\mu $s with a 0.11-0.3T magnetic field. A novel harmonic frequency-locked state was observed between the 1 GHz and 2 GHz oscillators. Simulations are being conducted to design new experiments to investigate the physics of this locking behavior. [1] G.B. Greening, Ph.D Dissertation, ``Multi-Frequency Recirculating Planar Magnetrons,'' University of Michigan, 2017, [2] G.B. Greening, N. M. Jordan, S. C. Exelby, Y. Y. Lau, and R. M. Gilgenbach, ``Multi-frequency recirculating planar magnetrons,'' Applied Physics Letters, 109, 074101 (2016).   

Design, Simulation and Experiments on the Recirculating Crossed-Field Planar Amplifier

The Recirculating Planar Crossed-Field Amplifier (RPCFA) is the focus of simulation and experimental work. This amplifier, inspired by the Recirculating Planar Magnetron [1], is driven by the Michigan Electron Long Beam Accelerator (MELBA), configured to deliver a -300 kV, 1-10 kA, 0.3-1.0 \textmu s pulse. For these parameters, a slow wave structure (SWS), cathode, and housing were designed using the finite element frequency domain code Ansys HFSS, and verified using the PIC code, MAGIC [2]. Simulations of this device demonstrated amplification of 1.3 MW, 3 GHz signal to approximately 29 MW (13.5 dB) with nearly 53{\%} electronic efficiency. Simulations have also shown the device is zero-drive stable, operates under a range of voltages, with bandwidth of \textasciitilde 10{\%}, on par with existing CFAs. The RPCFA SWS has been fabricated using 3D printing, while the rest of the device has been developed using traditional machining. Experimental RPCFA cold tube characteristics matched those from simulation. Experiments on MELBA have demonstrated zero-drive stability and amplifier experiments are underway. [1] R.M. Gilgenbach, Y.Y. Lau, D.M. French, B.W. Hoff, J. Luginsland, and M. Franzi, ``Crossed field device,'' U.S. Patent US 8 841 867B2, Sep. 23, 2014. [2] Developed by Alliant Techsystems   

High dose-rate irradiation of materials with pulsed ion beams at NDCX-II

Charged particle radiation effects in materials is important for the design of fusion plasma facing components. Also, radiation effects in semiconductor devices are of interest for many applications such as detectors and space electronics. We present results from radiation effects studies with intense pulses of helium ions that impinged on thin samples at the induction linac at Berkeley Lab (Neutralized Drift Compression Experiment-II). Intense bunches of 1.2 MeV He$^{\mathrm{+}}$ ions with peak currents of 2 A, 1-mm beam spot radius and 2-30 ns FWHM duration create controlled high instantaneous dose rates enabling the exploration of collective damage effects. We use in-situ diagnostics to monitor transient effects due to rapid heating and the ionization and damage cascade dynamics. For tin, single pulses deposit sufficient energy in the foil to drive phase transitions. A new Thomson parabola to measures ion energy loss and charge state distributions following transmission of a few micron thick samples. In silicon, ion pulses induce free electron densities of order 10$^{\mathrm{21}}$ cm$^{\mathrm{-3}}$.   

Probing of high density plasmas using the multi-beam, high power TiSa laser system ARCTURUS

The understanding of relativistic laser plasma interaction at ultra-high intensities has advanced considerably during the last decade with the availability of multi-beam, high power TiSa laser systems. These laser systems allow pump-probe experiments to be carried out. The ARCTURUS laser at the University of Duesseldorf is ideally suited for various kinds of pump-probe experiments as it consists of two identical, high power beams with energies of 5J in 30 fs and a third beam for optical probing with energy of 30mJ in a 30fs pulse. All three beams are synchronised and have flexible time delays with respect to each other. Several different processes were studied where one of the beams was used as an interaction beam and the second one was incident on a thin solid gold foil to generate a proton beam. For example, thin foil targets were irradiated either with a linear or circular polarized pulse and probed with protons at different times. The expansion of foils for the two cases was clearly different consistent with numerical simulations. In addition, the interaction of gas targets was probed with protons and separately with an optical probe. With both diagnostics the formation of a channel was observed. In the presentation various two beam measurements will be discussed.   

Picosecond Thermal Dynamics in an Underdense Plasma Measured with Thomson Scattering

Field-ionized underdense plasmas have many promising applications within the laser--plasma interaction field: nuclear fusion, particle accelerators, x-ray sources, and laser--plasma amplification. Having complete knowledge of the plasma dynamics is essential to establishing optimal parameters for a given application. Here picosecond-resolved Thomson scattering measurements have been used to determine the electron thermal dynamics of an underdense ($\sim $10$^{\mathrm{19}}$/cm) H$_{\mathrm{2}}$ plasma irradiated by a 60-ps, 1053-nm laser pulse with an intensity of 2 $\times $ 10$^{\mathrm{14}}$ W/cm$^{\mathrm{2}}$. The picosecond-resolved spectra were obtained with a novel pulse-front tilt compensated streaked optical spectrometer. The electron temperature was observed to rise from an initial 5 eV to a density-dependent plateau in 23 ps. Simulation results indicate that inverse bremsstrahlung heating, radiative cooling, and radial conduction cooling all play an important role in modeling the thermal dynamics. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Nanostructure array plasmas generated by femtosecond pulses at highly relativistic intensities

The irradiation of high aspect ratio ordered nanostructure arrays with ultra-high contrast femtosecond laser pulses of relativistic intensity provides a unique combination of nearly complete optical absorption and drastically enhanced light penetration into near-solid density targets. This allows the material to be volumetrically heated deep into the ultra-high energy density regime$^{\mathrm{1}}$. In previous experiments we have shown that irradiation of Ni and Au nanostructures with femtosecond pulses focused to an intensity of 5x10$^{\mathrm{18}}$ Wcm$^{\mathrm{-2}}$ generate multi-KeV near solid density plasmas in which atoms are ionized to the Ni$^{\mathrm{+26}}$ and Au$^{\mathrm{+52}}$ charge states$^{\mathrm{2}}$. Here we present the first results of the irradiation of nanostructure arrays with highly relativistic pulses of intensities up to 5x10$^{\mathrm{21}}$Wcm$^{\mathrm{-2}}$. Silver and Rhodium nanowire arrays were irradiated with frequency-doubled pulses of 30 fs duration from a petawatt-class Ti:Sa laser. Time integrated x-ray spectra show the presence of He-like and Li-like emission. Results of experiments conducted with a variety of different nanowires diameters with a range of interwire spacings will be presented and compared to the result of 3D particle-in-cell-simulations. $^{\mathrm{1}}$Bargsten et al Sci. Advances Vol.3 No.1 (2017) $^{\mathrm{2}}$Purvis et al Nature Photonics 7, 769 (2013).   

Back-reflection mitigation solutions for 10 PW high-power laser experiments

Recent measurements with PW class lasers demonstrated that energies of up to 3{\%} of the incident laser energy can be back-reflected in the laser system [1] and that modulations of the target surface can occur due to the radiation pressure [1, 2]. Given the foreseen intensities in the ELI-NP experiments in the range of 1022-1023 W/cm2, back-reflections of the main laser pulse can occur from the distorted plasma, leading to irreversible damages of the beam transport system optics or even to the laser amplification chain. Moreover, the debris generated from the laser -- target interaction can damage the focusing optics and decrease their performance from only a few shots. We are presenting simulated results of different configurations for suppressing the back-reflection based on sacrificial mirrors and a single plasma mirror, and their limitations. [1] S. Ter-Avetisyan, et al., Optics Express 24 (24), 28104 (2016) [2] H. Vincenti, et al., Nature Commun. 5, 3403 (2014)   

Flying Focus: Spatiotemporal Control of the Laser Beam Intensity

A ``flying focus'' is presented: this advanced focusing scheme provides unprecedented spatiotemporal control over the laser focal volume. A chromatic focusing system combined with chirped laser pulses enabled the speed of a small-diameter laser focus to propagate over nearly 100$\times $ its Rayleigh length. Furthermore, the flying focus decouples the speed at which the peak intensity propagates from the group velocity of the laser pulse, allowing the laser focus to co- or counter-propagate along its axis at any velocity. Experiments have demonstrated a nearly constant intensity over 4.5 mm while the velocity of the focus ranged from subluminal (0.01$c)$ to superluminal (15$c)$. These properties could provide the opportunity to overcome current fundamental limitations in laser-plasma amplifiers, laser-wakefield accelerators, photon accelerators, ion accelerators, and high-order frequency conversion. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.   

Plasma-based amplification of laser beams with higher-order polarization modes

Parametric amplification of laser pulses in plasma has been used to amplify both simple Gaussian pulses as well as higher-order Laguerre-Gaussian modes. However, the amplified beams always had a simple mode of polarisation, only linear or circular. Here we present a novel scheme to amplify seed laser pulses with complex modes of polarisation, so-called Poincar\'e beams, using default Gaussian pump laser beams with simple linear or circular polarisation. As particular examples, we will discuss the amplification of seed laser pulses with radial or azimuthal polarisation, as well as a pulse with polarisation shaped like a M\"obius strip.   

PW-class laser-driven super acceleration systems in underdense plasmas

Probing laser driven super-acceleration systems can be important tool to understand physics related to vacuum, space time, and particle acceleration. We show two proposals to probe the systems through Hawking-like effect using PW class lasers and x-ray free electron lasers. For that we study the interaction of ultrahigh intense laser pulses with intensity $10^{22}-10^{24}$ W/cm$^2$ and underdense plasmas including ion motion and plasma radiation for the first time. While the acceleration $w\sim a_0\omega_{p}/\omega_L$ in a wake is not maximal, the pulse propagation is much stable. The effect is that a constantly accelerated detector with acceleration w sees a boson's thermal bath at temperature $ \hbar w/2\pi k_B c$. We present two designs for x-ray scattering from highly accelerated electrons produced in the plasma irradiated by intense laser pulses for such detection. Properly chosen observation angles enable us to distinguish spectral broadening from Doppler shift with a reasonable photon number. Also, ion motion and radiation damping on the interaction are investigated via fully relativistic 3D particle-in-cell simulation. We observe high quality electron bunches under super-acceleration when transverse plasma waves are excited by ponderomotive force producing plasma channel.   

High-energy vacuum birefringence and dichroism in an ultrastrong laser field

The interaction between real photons in vacuum is a long-standing prediction of quantum electrodynamics, which has never been observed experimentally. Upcoming 10 PW laser systems like the Extreme Light Infrastructure (ELI) will provide laser pulses with unprecedented intensities [1]. If combined with highly energetic gamma photons -- obtainable via Compton backscattering from laser-wakefield accelerated electron beams -- the QED critical field becomes accessible. In [2] we have derived how a generally polarized probe photon beam is influenced by both vacuum birefringence and dichroism in a strong linearly polarized plane-wave laser field. We put forward an experimental scheme to measure these effects in the nontrivial high-energy regime, where the QED critical field is reached and the Euler-Heisenberg approximation, valid for low-frequency electromagnetic fields, breaks down. Our results suggest the feasibility of verifying/rejecting the QED prediction for vacuum birefringence/dichroism at the $3\sigma$ confidence level on the time scale of a few days at several upcoming laser facilities. \newline\noindent [1] Di Piazza et al., Rev. Mod. Phys. \textbf{84}, 1177 (2012)\newline\noindent [2] S. Bragin, SM, C. H. Keitel, A. Di Piazza, arXiv:1704.05234 (2017)