Session TO7: AB: Coherent Sources of Radiation and Plasma Optics

Investigation of UHF multipactor onset and suppression in a 50 ohm microstripline test cell

Multipactor is a cascade avalanche of secondary electrons in resonance with an oscillating RF electric field in vacuum conditions [1]. It can disrupt transmission of high power electromagnetic fields and even cause surface damage in waveguides or RF components. We have developed a broadband microstripline test cell to investigate two-surface multipactor susceptibility and suppression. The input and output vacuum-sealed, high power, coaxial couplers are well-matched to the \textasciitilde 50 ohm, low-loss test-cell impedance, with excellent transmission from 0.1 -- 1.5 GHz. The top surface of the section where multipactor occurs is replaceable, enabling studies with surfaces having different secondary emission coefficients. Photoelectric electron seeding by a UV source ensures statistically reliable multipactor initiation. We will describe the results from initial experiments, including the effects of varying UV illumination intensity, surface conditions, and multi-tone RF field excitation. [1] J. R. M. Vaughan, "Multipactor,"IEEE Trans Elec Dev, vol. 35, no. 7, pp. 1172-1180, July 1988.   

Simulation, Design, and Testing of a Coaxial Multipactor Test Cell

Multipactor is a discharge phenomenon that occurs in vacuum microwave electronics [1,2]. Secondary emission of electrons colliding with electrode surfaces develop into large clouds of space charge that can cause severe degradation of signal quality and potentially result in catastrophic failure of the device. While there is an extensive theoretical background for planar two-surface multipactor [1,2], the phenomenon is less well understood in the coaxial geometry. Previous experiments have been performed that provide insight into coaxial multipactor at low frequencies [3,4]. At the University of Michigan, we expand on these works by developing a standardized test cell for studying multipactor in a coaxial geometry at 2-3 GHz. This paper will present simulations that have been performed in support of the design process and compare to our experimental data. [1] Vaughan, JRM. IEEE T Electron Dev 35: 1172-1180, 1988. [2] Kishek RA, Lau YY, Ang LK, Valfells A, and Gilgenbach RM. Phys Plasmas 5: 2120-2126, 1998. [3] Woo R. J Appl Phys 39: 1528–1533, 1968. [4] Graves T. (PhD thesis). Cambridge, MA: Massachusetts Institute of Technology, 2006.   

Experimental Results on the Harmonic Recirculating Planar Magnetron with All Cavity Extraction

Harmonic-frequency locking was originally observed on the Multi-Frequency Recirculating Planar Magnetron (MFRPM) [1], which is a high power microwave source that can generate multi-MW power levels at two frequencies simultaneously. To understand harmonic-frequency locking, the Harmonic Recirculating Planar Magnetron (HRPM) has been designed with oscillators at L-Band and S-Band (LBO and SBO, respectively). In the harmonic locked state, the SBO frequency locks to the harmonic of the LBO's frequency. Power is extracted from the SBO using coaxial-all-cavity-extraction [2]. Under certain operating parameters, the SBO has been observed to generate 9.3 $+$/- 1.4 MW when driven by MELBA-C, which applies -300 kV and 1-5 kA for 0.3-1.0 $\mu $s. The completed design, relevant simulations, and experimental results will be presented. [1] Greening et al, ``Harmonic Frequency Locking in the Multi-Frequency Recirculating Planar Magnetron'', IEEE T-ED, vol. 65, 2347, (2018). [2] Franzi et al, ``Coaxial All Cavity Extraction in the Recirculating Planar Magnetron,'' IEEE International Vacuum Electronics Conference, 2014.   

\textbf{Multi-MW Output from the Recirculating Planar Crossed-Field Amplifier}

Amplification to peak output powers up to 5-6 MW with approximately 9 dB gain has been demonstrated on the Recirculating Planar Crossed-Field Amplifier (RPCFA), with input of 100's kW, S-band microwave signals. The RPCFA is based on the Recirculating Planar Magnetron [1], which has been the focus of research at the University of Michigan. The performance of the RPCFA was predicted in simulation using the particle-in-cell code MAGIC [2], and the finite element frequency domain code ANSYS HFSS. Experiments on a prototype RPCFA showed generally good agreement with simulation. The device demonstrated zero drive stability, and approximately 15{\%} bandwidth over the range of design frequencies, 2.63 to 3.05 GHz. Amplification was observed at input RF drive powers below 150 kW, however, the amplification gain in this regime was highly variable ($\sigma \quad =$ 2.74 dB). Increasing the input signal power beyond 150 kW dramatically decreases the variability of gain ($\sigma \quad =$ 0.69 dB). The peak output power in this experiment is limited by RF breakdown of the structure. Future experiments will be focused on extending the peak power and bandwidth generated by RPCFAs. [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   

Secondary Electron Yield of Elemental Metals: First Principles Parameterization of Monte Carlo Simulations

Suppressing multipactor is crucial in designing efficient radio frequency (RF) systems. The use of low secondary electron yield (SEY) materials helps mitigate multipactor, hence, accurate SEY data is needed. In this work, the SEY of 25 elemental metals in face-centered cubic (FCC) and body-centered cubic (BCC) lattices was calculated using Monte Carlo (MC) simulations. The inelastic electron scattering was described by differential inverse inelastic mean free paths obtained with the use of the Penn approximation applied to dielectric functions obtained through full-potential and pseudopotential density functional theory (DFT) calculations, which was also used to calculate work functions. Comparison with the available experimental data shows high predictive capability of using first-principle data in MC simulations of SEY. This study is the first step in building a larger SEY database, which in turn will aid in high-throughput search for low SEY materials through machine learning.   

A study of the physics of Miram curves

A Miram curve [1] is a plot of emitted current versus temperature for thermionically emitting cathodes. It has a knee where the current transitions from temperature- to space-charge-limited emission. A physics-based model to predict the Miram curve could help elucidate cathode surface physics during emission as well as assist microwave vacuum electronic device design and testing. This work shows that nonuniform emission from the edges of low(est) work function grains on the surface of a polycrystalline inhomogeneous cathode is likely responsible for the details of the smooth temperature-to-space-charge-limited emission transition. This insight is derived from a collaborative combination of experimental, theoretical, and computational modeling of the surfaces of tungsten dispenser cathodes [2,3]. [1] M. Cattelino, et al, 1982 International Electron Devices Meeting (IEEE, 1982) pp. 36-39. [2] D. Chen, et al, 2019 IEEE Int'l Vacuum Elec Conf (IEEE, 2019). [3] J. Petillo, et al, paper 8B3, IEEE Pulsed Power Plasma Sci Conf, Orlando, FL June 23-28, 2019.   

Radiation sources based on laser-driven micro-scale plasma waveguide.

When a high-power laser propagating in a micro-scale plasma waveguide (MPW), the electromagnetic field takes the form of waveguide modes. New features emerge in the relativistic laser-solid interaction process that can be utilized to develop novel radiation sources from THz to X-rays. We show that high charge (10 nC) electron bunches can be produced in the MPW and accelerated to around 100 MeV by the transverse magnetic modes. As the beam co-propagates with the laser (waveguide modes), they are constantly wiggled by a transverse force that gives rise to bright synchrotron-like X-ray emission. In addition, as the electron beam exits the MPW, a substantial part of the electron energy is transferred to coherent diffraction radiation, whose wavelength is directly controlled by the duration of incident laser pulse. Thus, 100-mJ-strong, relativistic intense radiation from sub-THz to infra-red frequency range can be obtained in this stage. The underlying physical process depends strongly on the waveguide mode structure, which can be tailored by the micro-engineering of MPW. This enables a new degree of freedom to control over laser-plasma dynamics, which can be harnessed to reach radiation generation capability beyond the state of the art.   

Frequency Conversion and Intensification of Laser Pulses Reflected from Ionization Waves of Arbitrary Velocity

A recently pioneered optical technique called the flying focus allows for the position of maximum laser intensity in a chromatically focused, chirped laser pulse to be propagated at any velocity over long distances. An ionization front that moves at the flying focus velocity has been demonstrated when the instantaneous intensity is above the ionization threshold of a background material. These ionization waves of arbitrary velocity (IWAV's) can be propagated backward with respect to the laser group velocity, eliminating the effect of ionization refraction and allowing for the production of high-density IWAV's moving arbitrarily close to, or even exceeding the speed of light. When a second laser pulse is reflected from an IWAV, it can undergo extreme shifts in frequency because of the double-Doppler effect, as well as intensification because of pulse compression. Calculations show that even for diffuse ionization fronts, a frequency upshift of nearly 20 can be achieved beginning with infrared laser pulses. The output pulse duration and intensity can be tuned by chirping the input pulse, and the output frequency can be tuned by adjusting the flying focus velocity. Preliminary results suggest that this process could enable high-intensity, ultrashort, tunable frequency laser pulses in the extreme ultraviolet to soft x-ray regime. This material is based upon work supported by the Department of Energy grant DE-SC0019135 and the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Fully-kinetic simulations of coherent radio emission in nonrelativistic plasmas

Coherent radio emission from space plasmas is detected in multiple astrophysical environments. Possible sources of fast radio bursts were only very recently identified as distant galaxies, pulsars, or magnetars. In nonrelativistic plasmas, such emission is detected in Type III radio bursts from solar flares. In any such scenarios the mechanism behind the sudden release of large amounts of energy in the form of coherent radiation is poorly understood. Beam-plasma instabilities driven by the interaction of energetic particles (e.g. fast electrons from flares) with a thermal background (e.g. the solar wind) are among the best candidates to explain this so-called "plasma emission". Such models have been mainly explored with analytic quasi-linear theoretical approaches. Here we approach the problem with fully kinetic Particle-in-Cell (PiC) simulations. We study beam-plasma instabilities in nonrelativistic environments with implicit energy-conserving PiC methods. These allow for stable simulations over unprecedented long times, accurately modelling the energy exchange between particle populations.   

Manipulating the polarization state of an intense laser beam in a plasma using a less intense auxiliary laser.

Manipulating the polarization of intense laser beams in plasmas was recently proposed [1] and subsequently achieved in proof-of-principle laboratory experiments [2,3] that demonstrated the feasibility of plasma-based photonics devices such as plasma-Pockels cells or polarizers. However, both the theory and experiments were carried out in the linear regime of polarization mixing, whereby the ``pump'' beam that was used to introduce birefringence in the plasma was much more intense than the ``probe'' beam whose polarization was being manipulated. The absence of a means of surpassing the linear regime is the major hurdle that has to date prevented the practical applications of these concepts. In this presentation, we propose a novel solution to this fundamental problem. Our method enables the practical application of plasma-based photonic devices in a regime where the intensity of the probe beam significantly exceeds that of the pump. This is achieved by taking advantage of a particular geometrical arrangement that preserves the polarization state of the pump while allowing the polarization of the probe to be manipulated arbitrarily. We present a non-linear, two-dimensional analytical solution for this interaction geometry, and discuss the implications of this non-linear regime for the plasma-polarizer and plasma-Pockels cell concepts. [1] P. Michel et al., PRL 113, 205001 (2014). [2] D. Turnbull et al., PRL 116, 205001 (2016). [3] D. Turnbull et al., PRL 118, 015001 (2017).   

Self-Focusing of a Flying Focus Pulse

The chromatic focusing of a chirped laser pulse creates a flying focus---a moving focal point that can travel at any velocity. The intensity peak formed by the focal point propagates with a self-similar profile over a distance determined by the focal positions of the minimum and maximum frequencies composing the pulse. In a nonlinear medium, weakened diffraction resulting from self-focusing modifies the propagation throughout this focal region. Here we will present theory and simulations exploring the nonlinear self-focusing of flying focus pulses and its dependence on the focal velocity.   

Direct Laser Acceleration in a Flying Focus

A planar laser pulse propagating in vacuum imparts no net energy to an electron. At its rising edge, the pulse ponderomotively accelerates the electron in the direction of propagation, but as the pulse overtakes and outruns the electron, its trailing edge imparts an equal and opposite ponderomotive impulse, bringing the electron to rest. Planar-like ``flying'' focus pulses can break the symmetry of this fundamental laser--matter interaction, imparting net energy to an electron. The flying focus---a moving focal point resulting from a chirped laser pulse focused by a chromatic lens---creates an intensity peak that can travel at any velocity. When this velocity is sufficiently slow, the electron gains enough momentum during its initial ponderomotive acceleration to outrun~the intensity peak. Here we will present theory and simulations describing the energy gain and dynamics of electrons accelerated by flying focus laser pulses.   

Angular momentum conversion of light in laser-plasma interaction

It is well-known the spin angular momentum of light is associated with the polarization of the light. During the laser-plasma interaction by a circularly polarized pump laser, the angular momentum from the pump beam can transfer to the plasma or radiations generated in the plasma. In this work, the angular momentum properties of nonlinear harmonic generation by an intense laser in plasmas have been studied. Even harmonics are excited when plasmas have transverse density gradients produced either by ponderomotive force of the laser or in the boundary region of an optical-field ionized (OFI) plasma. We experimentally observe the conversion of spin to orbital angular momentum by measuring the twisted wavefront of the second harmonic light generated from an OFI helium plasma produced by an intense circularly polarized pulse.   

Generation of RF Radiation by Femtosecond Atmospheric Filaments

Atmospheric filaments from femtosecond scale laser pulses are famous for generating THz radiation, and theories have been developed that explain its production by both one and two color pulses. Recent experimental work at the AFRL has characterized the distinct GHz radiation that is also generated by filaments, and has determined that a different physical mechanism is needed for its production. We have developed particle-in-cell simulations that show that the expansion of a hot outer shell of electrons from the plasma column is responsible for the longitudinal currents that drive the RF radiation. The frequency profile, field amplitude, angular distribution and pressure dependence of the simulated RF radiation all closely match the measured results from the lab.