Session GO7: Electron Sources and Terahertz Generation

Effect of LaB6 films on field emission by knife edge cathodes

We present experimental and modeling results of electron field emission from metallic knife edge cathodes coated with LaB6 films. The cathodes are machined from copper, coated with Ti, and then sputter-coated with LaB6. The LaB6 work function (2.3 eV) is significantly lower than that of copper (4.7 eV) or Ti (4.3 eV). However, thick LaB6 films (100-300 nm) significantly reduce the field emission currents compared with the bare copper or Ti-coated cathodes. In contrast, field emission with thin (5-10 nm) LaB6 films significantly increase the emission currents. Modeling studies are investigating a hypothesis that explains these observations as resulting from a quantum transport barrier at the metal-LaB6 interface. It is hypothesized that penetration of the applied field reduces the interfacial barrier for the thinnest films.   

2D and 3D multipactor modeling in dielectric-loaded accelerator structures

Multipactor (MP) is known as the avalanche growth of the number of secondary electrons emitted from a solid surface exposed to an RF electric field under vacuum conditions. MP is a severe problem in modern rf systems and, therefore, theoretical and experimental studies of MP are of great interest to the researchers working in various areas of physics and engineering. In this work we present results of MP studies in dielectric-loaded accelerator (DLA) structures. First, we show simulation results obtained with the use of the 2D self-consistent MP model (O. V. Sinitsyn, et. al., Phys. Plasmas, vol. 16, 073102 (2009)) and compare those to experimental ones obtained during recent extensive studies of DLA structures performed by Argonne National Laboratory, Naval Research Laboratory, SLAC National Accelerator Laboratory and Euclid TechLabs (C. Jing, et al., IEEE Trans. Plasma Sci., vol. 38, pp. 1354-1360 (2010)). Then we present some new results of 3D analysis of MP which include studies of particle trajectories and studies of MP development at the early stage.   

On the role of microprotrusions in high gradient structures

The RF breakdown is a phenomenon critical for operation of high-gradient accelerating structures. One of possible reasons for the breakdown is the ohmic heating of microprotrusions on structure surfaces due to the field emitted electron current. In long enough pulses this heating may result in the protrusion melting. In this work we present the study of the heating of protrusion. We use point charge model model in connection with Fowler Nordheim equation to obtain the solution to the electric field potential and current density outside the protrusion and then linking those quanitities to obtain solution for electric field and current density inside the asperity. These quantities were used for studying the heat propagation inside the protrusion and calculating the temperature rise in the structure. The shape of the protrusion and the material of which the RF cavity is made are the parameters that can be varied during the simulation. Strong dependence of the time required to heat up protrusion to melting point on the amplification factor is reported.   

Development of sub-THz gyrotrons for detecting concealed radioactive materials

One of the topics of a recently formed Center for Applied Electromagnetics at the University of Maryland is the development of high-power, THz-range gyrotrons with pulsed solenoids. It is planned that radiation of such gyrotrons can be focused in small spots in atmospheric air where the amplitude of the wave field exceeds the breakdown threshold. However, the volume where this radiation is focused is so small that at the ambient electron density there is a very small probability during the THz pulse that there will be any free electrons to initiate this avalanche breakdown process. Thus, if breakdown does occur, it may be attributed to the presence in the vicinity of this volume of some concealed radioactive materials which have greatly increased the background ionization level. In this paper, an overview of the 300 kW, 670 GHz gyrotron development and discussion of various issues important for given application will be presented.   

Study on Low-Frequency Oscillations in a Gyrotron Using a 3D CFDTD PIC Method

Low-frequency oscillations (LFOs) have been observed in a high average power gyrotron and the trapped electron population contributing to the oscillation has been measured. As high average power gyrotrons are the most promising millimeter wave source for thermonuclear fusion research, it is important to get a better understanding of this parasitic phenomenon to avoid any deterioration of the electron beam quality thus reducing the gyrotron efficiency. 2D Particle-in-cell simulations quasi-statically model the development of oscillations of the space charge in the adiabatic trap, but the physics of the electron dynamics in the adiabatic trap is only partially understood. Therefore, understanding of the LFOs remains incomplete and a full picture of this parasitic phenomenon has not been seen yet. In this work, we use a 3D conformal finite-difference time-domain (CFDTD) particle-in-cell (PIC) method to accurately and efficiently study the LFOs in a high average power gyrotron. As the CFDTD method exhibits a second order accuracy, complicated structures, such as a magnetron injection gun, can be well described. Employing a highly parallelized computation, the model can be simulated in time domain more realistically.   

Preliminary design of a magnetron injection gun for a 670 GHz gyrotron

At present, our research team is working on the design of a 300 kW, 670 GHz gyrotron with a pulsed solenoid. The gyrotron will operate at the fundamental cyclotron resonance. The pulsed solenoid will provide magnetic fields in the range of 27-28 T. Design of a magnetron-type electron gun for such a gyrotron is done by using available numerical codes EGN2W and MICHELLE. Magnetic compression of a beam in the region between the gun and a resonator is about 100; the beam voltage and current are 70 kV and 15 A, respectively. Simulation results will be presented.   

Simulation of THz Generation in Corrugated Plasma Waveguides

It has been suggested [1] that a laser pulse passing through a corrugated plasma channel may generate THz radiation. These channels support EM modes that have a Floquet-type dispersion relation, which allows for phase matching with the ponderomotive force of the pulse. We simulate the response of a channel to a non-evolving laser pulse. The channel density is axially sinusoidal and radially parabolic. We impose a cutoff at fixed radius, resulting in a channel that supports a finite number of quasi-bound modes. This approximates experimentally produced channels [2]. We measure the size and frequency structure of the radial Poynting flux outside the channel. For a 0.25 J pulse with a 15 micron spot size in a channel with density 10$^{18}$ cm$^{-3}$, the average radial THz output is 10$^{5}$ W. We estimate that around ten percent of laser energy is available for conversion into THz radiation. This implies that several hundred centimeters of channel are required to deplete the excited modes through side coupling. This distance can be reduced by decreasing the channel barrier width and peak density.\\[4pt] [1] T. M. Antonsen et. al., Phys. Plasmas 14, 033107 (2007)\\[0pt] [2] B.D. Layer et. al., Phys. Rev. Lett. 99, 035001 (2007)   

Observations of laser plasma induced refractive index variations

Intense laser light focused in air is well known to generate a plasma spark. The density of liberated electrons from the plasma creates a change to the index of refraction. By propagating a laser probe perpendicular to the plasma generating pump laser beam, we can measure the effects of the change to the index of refraction. Similar work in Ref 1 shows a large increase of the index of refraction outside the plasma region generated by a nanosecond laser pulse. The index increase was attributed to a probe-induced macroscopic electric polarization in the sheath resulting in a higher sheath region susceptibility. We made similar observations closer into the center of the plasma region to map out the radial variation of the index of refraction with temporal resolution. Our observations show a less remarkable change to the index of refraction. This will have an impact on optical probing of atmospheric plasma phenomena such as plasma filaments. \\ 1. A. Robledo-Martinez, $et al$. Phys. Plasma 15, 093510 (2008)   

Generation of THz Radiation by Beating of Two Gaussian Lasers

The THz radiation generation has diverse applications in the field of material characterization, imaging, topography and remote sensing, chemical and security identification$^{ }$etc. In order to develop high-power and efficient THz sources, several schemes have been proposed. For example, tunable THz radiation can be generated by superluminous laser pulse interaction with large band gap semiconductors and electro-optic crystals, by nonlinear interaction of an intense short pulse laser with a dielectric, on synchrotron radiation from bunched electron beams etc. In the present investigation, an analytical study is made for the THz radiation generation based on beating of two spatial Gaussian laser beams having same electric field amplitude but different frequencies and wave numbers in a spatially periodic density plasma. In this situation, quasistatic nonlinear ponderomotive force is obtained along the direction of propagation and in the transverse direction due to beating and the spatial variation of the laser electric field. The ponderomotive force in the presence of periodic density structure gives rise to a transverse competent of the current, which results in resonant excitation of THz radiation. The efficiency of conversion in this scheme is $\sim $ 10$^{-4}$ and the emitted THz radiation power scales as the square of the density ripple amplitude.   

Electron Flow Stability in Magnetically Insulated Vacuum Transmission Lines

We evaluate the stability of electron current flow in high power magnetically insulated transmission lines (MITLs). A detailed model of electron flow in cross-field gaps yields a dispersion relation for electromagnetic TM waves [1] which is solved numerically to obtain growth rates for unstable modes in various density profiles. These results are compared with 2D particle-in-cell (PIC) simulations of electron flow in high power MITLs. We find that the macroscopic properties (charge and current density, self-fields) of the equilibrium profiles observed in the simulations are well represented by the laminar flow model of [1]. Idealized simulations of sheared flow in electron sheaths show that unstable, sharp-cutoff radial density profiles rapidly evolve into stable flows with a more gradual density gradient. Growth rates for both long (diocotron) and short (magnetron) wavelength instabilities observed in the simulations agree well with the dispersion analysis. We conclude that electron sheaths in high power 2D MITL flows form stable profiles and that sheath expansion is not caused by flow instability. Subsequently, we investigate the impact of electrode plasma formation and evolution on sheath stability and gap closure using PIC simulations. We compare MITL simulations with experimental measurements. [1] R. C. Davidson, et al., Phys. Fluids 27, 2332 (1984).   

Experimental Investigation of a Pseudospark-Produced High-Brightness Electron Beam for X-ray Applications

This paper presents the progress in the experimental investigation of pseudospark-produced electron beams to drive x-ray emission for medical devices. Experiments are performed to determine the discharge behaviors (breakdown characteristics, electron beam generation efficiency) and the qualities of electron beams (peak current, energy spread, and transverse emittance) produced by a thirty-gap pseudospark device. The system is operated at 70kV. The breakdown voltage and discharge current are measured by a high voltage probe and Rogowski coil. A movable multi-probe array consists of 4 electrostatic probes to measure the beam currents profile at different radial and axial locations. Thus the energy spread and rms emittance of electron beams can be determined radially and axially. A discussion of the measured discharge characteristics and beam parameters to drive x-ray emission are also presented.   

Anode plasma dynamics in the self-magnetic-pinch diode

The self-magnetic-pinch diode is being developed as an intense electron beam source for pulsed-power driven x-ray radiography. In ideal high-power operation, the beam electrons desorb contaminants from the anode surface from which positive ions are drawn to the cathode. The counterstreaming electrons and ions establish an equilibrium current. It has long been recognized, however, that expanding electrode plasmas can disrupt this ideal behavior and cause rapid loss of the diode impedance and the radiation pulse. Recently developed numerical techniques are applied to a model of the SMP diode which includes the formation and evolution of anode surface plasmas. Two mechanisms are shown to cause rapid impedance loss, anode plasma expansion into the anode-cathode (A-K) gap and increased ion space-charge along the cathode. The former mechanism dominates for shorter A-K gaps, while the latter dominates for longer gaps. Model results qualitatively reproduce the time-dependent impedances measured for this diode.