Session JO7: Intense Beams and High Power Microwaves

Early Commissioning Results of the NDCX-II Accelerator Facility

The Neutralized Drift Compression Experiment-II (NDCX-II) will generate ion beam pulses for studies of Warm Dense Matter science and heavy-ion-driven Inertial Fusion Energy. The machine accelerates 20-50 nC of Li+ to 1.2-3 MeV energy, starting from a 10.9-cm alumino-silicate ion source. At the end of the accelerator the ions are focused to a sub-mm spot size onto a thin foil (planar) target. The pulse duration is compressed from $\sim $500 ns at the source to sub-ns at the target following beam transport in a neutralizing plasma. We report on the results of early commissioning studies that characterize beam quality and beam transport, acceleration waveform shaping and beam current evolution. We present measurements of time-resolved beam phase space density and variation in transverse beam centroid position. We present simulation results to benchmark against the experimental measurements, and to predict performance in subsequent sections of the accelerator.   

Optical Spectroscopy of High Intensity Electron Beam Plasmas$^{1}$

This talk will be an overview of spectroscopic results obtained on the RITS-6 accelerator at Sandia National Laboratories on the Self-Magnetic Pinch (SMP) electron beam diode. The SMP diode produces a focused ($<$3mm diameter), e-beam at 7MeV and 150kA, which is used as an intense, flash x-ray source. During the $\sim $45ns electron beam pulse, plasmas are generated on the electrode surfaces which propagate into the A-K vacuum gap, affecting the diode impedance, x-ray spectrum, and pulse-width. These plasmas are measured using a series of optical diagnostics including: streak cameras, ICCD cameras, and avalanche photodetectors. Visible spectroscopy is used to gather time and space information on these plasmas. Density and temperature calculations are made using detailed, time-dependent, collisional-radiative (CR) and radiation transport modelings. The results are then used in conjunction with hybrid PIC/fluid simulations to model the overall plasma behavior. Details regarding the data collection, system calibration, analyses, and interpretation of results will be presented. \\[4pt] $^{1}$Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Effects of two-stream instability on electron return current of ion beam propagating in background plasma

The current and charge neutralization of intense charged particle beams by background plasma enables ballistic beam propagation and has a wide range of applications in inertial fusion and high energy density physics. However, beam-plasma interactions can result in the development of collective instabilities that may have deleterious effects on the ballistic propagation of intense ion beams. In this paper we study an ion beam pulse propagating in a background plasma, which is subject to the two-stream instability between the beam ions and plasma electrons. Making use of the particle-in-cell code LSP we have simulated this interaction over a wide range of beam and plasma parameters. We show that depending on the beam and plasma parameters, there are two different regimes where the instability saturates due to nonlinear wave-trapping effects of either the beam ions or plasma electrons. The two regimes have different scalings for the self-electric and self-magnetic field of the ion beam pulse propagating in the background plasma.   

Mode Control Cathode Modeling and Experiments on a Recirculating Planar Magnetron

We present simulations and experimental results of a new class of crossed-field device: Recirculating Planar Magnetron (RPM) [1]. Experiments on a 12 cavity, 1 GHz, RPM are underway using MELBA accelerator at -300 kV, 1-10 kA and pulselengths of 0.3-1 microsecond. A mode control cathode (MCC) is proposed to address RPM mode competition and cross-oscillator coupling. The MCC is a periodically spaced conducting network designed to act as both an electron source and a resonant electromagnetic coupler between the two planar RPM oscillators. MCC simulations have verified such mechanisms, resulting in faster mode development and phase locking in the RPM. Manipulation of the cathode's geometry has also been analytically established to enhance mode separation of the cold slow wave structure. Experimental frequency and phase measurements using the MCC will be presented. \\[4pt] [1] Gilgenbach et al., IEEE Trans PS 39, 980 (2011); also, patent pending.   

Equilibrium and Stability of the Brillouin Flow in Inverted Magnetron

One embodiment of the novel recirculating planar magnetron, RPM [1] utilizes an inverted configuration for fast startup. While the negative mass behavior on the thin electron layer model [2] is well-known for the inverted magnetron, the corresponding behavior for the equilibrium Brillouin flow [3] is an open question. Simulations using the particle-in-cell codes ICEPIC and/or MAGIC will be performed and compared to the solution to the eigenvalue problem that governs the stability of Brillouin flow, leading to a fundamental study of the flow's negative, positive, and infinite mass properties. Research supported by AFOSR (grant{\#}: FA9550-10-1-0104), AFRL, and L-3 Communications Electron Devices. \\[4pt] [1] R. M. Gilgenbach, et.al., IEEE Trans. Plasma Sci. 39, 980 (2011); Also patent pending.\\[0pt] [2] D. M. French, et al., Appl. Phys. Lett. 97, 111501 (2010).\\[0pt] [3] D. Simon, et al., Phys. Plasmas 19, 043103 (2012).   

Dual Band Relativistic Backward Wave Oscillator with Gaussian Radiation

Generating dual-band signals using a relativistic backward wave oscillator (RBWO) with a two-section slow wave structure is attractive to many applications such as plasma diagnostics and sounding systems. Using different sections in the RBWO provides a change of the synchronism conditions and as a result two microwave frequencies at C-band and X-band have been produced at the output. The synchronism condition can be provided by variation of the corrugated waveguide period. In this abstract, a two-spiral corrugated and a sinusoidal structures have been used as two sections to produce the frequency bands. The slow wave structures have been designed analytically and the simulation results verified the analytics. The RBWO is generating a microwave signal that propagates backwards, so the two-spiral corrugated structure acts as a reflector. This reflector gives the ability to extract the signals azimuthally. Two dominant frequencies at 7GHz and 10GHz have been found with a microwave power of megawatts and with a Gaussian beam. The simulations show that the two beam-wave interaction regions work independently. The fully electromagnetic, fully relativistic particle-in-cell (PIC) code MAGIC was used to simulate the device with a voltage pulse 460kV and with a 2T axial magnetic field.   

Excitation of parasitic waves in forward-wave amplifiers with weak guiding fields

One of the issues critical for the development of high-power millimeter-wave amplifiers driven by electron beams is possible excitation of some parasitic oscillations. As a rule, the most dangerous are parasitic modes which can be excited at the ends of the passbands because such waves have low group velocities and, hence, can be strongly coupled to an electron beam. Excitation of parasitic waves near cutoff in forward-wave amplifiers was studied elsewhere [1] where the effect of the signal wave on the excitation conditions of such parasitic waves was analyzed. In Ref.1, it was assumed that electrons are guided by strong magnetic fields and, therefore, exhibit a 1D motion. In practice, it is often desirable to minimize the weight of the focusing systems, i.e. to operate in low focusing fields where electrons can exhibit 3D motion. This problem is analyzed in the present paper. Our study consists of two stages. First, we characterize the operation of a forward-wave amplifier in a weak magnetic field. This part of the study is a continuation of the work described in Ref.2. Next, we analyze the self-excitation of parasitic waves in the presence of forward waves and the effect of the signal wave on these excitation conditions.\\[4pt] [1] G. Nusinovich, O. Sinitsyn and T. Antonsen, Phys. Rev. E, \textbf{82}, 046404 (2010).\\[0pt] [2] T. M. Abu-elfadl, G. S. Nusinovich, A. G. Shkvarunets, Y. Carmel, T. M. Antonsen, Jr., and D. Goebel, Phys. Rev. E, \textbf{63}, 066501 (2001).   

Reduced Conductivity of Nano-Rough Copper Surfaces at 650 GHz

Effective design of powerful sources and efficient components for terahertz (THz) regime radiation requires knowledge of dissipation losses caused by conducting surfaces. However, theoretical predictions for the effect of roughness on the reflectivity of surfaces are untested in this frequency regime. Measurements of the electronic properties of metals and semiconductors are performed using a high quality factor quasi-optical (QO) hemispherical resonator operating at 650 GHz. Large area ($>$ 1 cm $\times$ 1 cm) copper surfaces with controlled nanoscale surface roughness are fabricated using either an abrasive process or a chemical etching process. Measurement of the reflectance of the samples shows the increased resistivity of the metal due to the surface features. These measurements are compared to approximate theoretical predictions developed by Hammerstad and Bekkadal, rigorous theoretical predictions developed by Tsang et al. and computational simulations. Comparisons show a deviation between measurement and theory when the average roughness of the surface is less than one skin depth in copper at 650 GHz. We suspect that the grain size of the copper metal could play an important role in the discrepancy.   

Role of Nottingham and Thomson effects in heating of micro-protrusion in high-gradient accelerating structures

It is widely accepted that one of the reasons for appearance of the RF breakdown which limits operation of high-gradient accelerating structures is the electron dark current [1]. This field emitted current, usually considered as a precursor of the breakdown, can be emitted from apexes of micro-protrusions on a structure surface. Therefore field and thermal processes in such protrusions deserve careful studies [2, 3]. The goal of our first study [3] was to analyze 2D process of RF field penetration inside protrusion of a metal with finite conductivity and to study corresponding Joule heating. In the current study, it is found that space charges can have a stabilizing effect on the electric field. We include a modification of the 1D model described in [4]. Moreover, we include into consideration, first, the Nottingham effect which may significantly change the protrusion heating. We also investigate the interplay between high temperature gradients and electric fields (Thomson heating).\\[4pt] [1] Wang and Loew, SLAC PUB 7684 October 1997.\\[0pt] [2] K.L. Jensen, Y.Y. Lau, D.W. Feldman, P.G. O'Shea, Phys. Rev. ST Accel. Beams 11, 081001(2008).\\[0pt] [3] Kashyn et al, AAC-2010.\\[0pt] [4] K.L. Jensen, J. LEbowitz, Y.Y. LAu, J. Luginsland, Journal of Applied Physics 111, 054917(2012).   

Spreading Resistance on Thin Film Contacts

Electrical contact [1] is important to wire-array z-pinches, metal-insulator-vacuum junctions, and high power microwave sources, etc. Contact problems account for 40 percent of all electrical failures, from small scale consumer electronics to large scale defense and aerospace systems. The crowding of the current lines at contacts leads to enhanced localized heating, a measure of which is the spreading resistance ($R_{s})$. For a microscopic area of contact (the ``$a$-spot'' [1]) on a thin film, we calculate $R_{s}$ in both Cartesian and cylindrical geometries [2]. In the limit of small film thickness, $h$, the normalized thin film spreading resistance converges to the finite values, 2.77 for the Cartesian case and 0.28 for the cylindrical case. These same finite limits are found to be applicable to the $a$-spot between bulk solids in the high frequency limit if the skin depth is identified with $h$. Extension to a general $a$-spot geometry is proposed [2]. \\[4pt] [1] R. Holm, Electric Contacts, 4th ed., Springer (1967). \\[0pt] [2] P. Zhang et al., IEEE Trans. Electron Devices \textbf{59}, 1936 (2012).   

Reduced Breakdown Delay via Memory and Penning Effects in High Power Microwave Dielectric Window Discharges

Development of high power microwave (HPM) distributed discharge limiters relies critically on minimizing the delay time between HPM incidence and diffuse plasma creation. Breakdown is achieved by illuminating a gas cell with a train of $\sim $25kW, $\sim $2 kV/cm, 800ns-long pulses at 41 HZ repetition rate. Using mixtures of neon with small concentrations of argon or xenon at pressures between 5-350 torr, we have observed breakdown in $<$100ns for particular choices of gas composition and pressure. Breakdown times predicted by published theoretical models\footnote{Y.Y. Lau, J.P. Verboncoeur, H.C. Kim, ``Scaling laws for dielectric window breakdown in vacuum and collisional regimes,'' Appl. Phys. Letters, Vol. 89, 261501-1.} are approximately 3-5 times longer than our experimental observations. Careful study of experimental trends suggest surface charge accumulation on the gas cell's polycarbonate window and Penning-like effects in mixtures of noble gases may explain the observation of breakdown times shorter than the theoretical models predict.   

Work Functions of Sc$_{2}$O$_{3}$ surfaces containing Ba/BaO for thermionic electron emitters using Density Functional Theory

The work functions of (001), (011) and (111) bixbyite Sc$_{2}$O$_{3}$ surfaces with adsorbed Ba atoms, Ba-O dimers, and rocksalt BaO films have been calculated using Density Functional Theory (DFT) to investigate the role Ba plays in producing the low experimental scandate cathode work functions of $\sim $1.1-1.4 eV. Our lowest calculated work function was 1.48 eV for a single rocksalt film layer of BaO (011) on Sc$_{2}$O$_{3}$ (011). Work functions for Ba atom and Ba-O dimer adsorption on Sc$_{2}$O$_{3}$ (011) and (111) surfaces ranged between 2.1 to 3.7 eV, and depended on the Sc$_{2}$O$_{3}$ surface termination, coverage density, and adsorption geometry employed. We found the total work function change of a surface with adsorbed species can be decomposed into two parts: the dipole component and Fermi level shift component. The dipole component is described by the Helmholtz equation, and takes into account charge transfer into valence band states and surface charge rearrangement. The Fermi level shift is a result of either mobile charge transfer into conduction band states, or a restructuring of the valence band, which can also modify the work function.   

Effects of Random Circuit Fabrication Errors on Small Signal Gain and on Output Phase in a Traveling Wave Tube

Random fabrication errors may have detrimental effects on the performance of traveling-wave tubes (TWTs) of all types. A new scaling law for the modification in the average small signal gain and in the output phase is derived from the third order ordinary differential equation that governs the forward wave interaction in a TWT in the presence of random error that is distributed along the axis of the tube. These scaling laws extend previous works in that they account for non-synchronous beam velocities and the inclusion of Pierce's ``space charge'' term. Analytical results compare favorably with numerical results in the absence of space charge, in both gain and phase modifications as a result of random error in the phase velocity of the slow wave circuit. Results on the effects of non-synchronous beam velocities and ac space charge are reported. Effects of internal reflections are investigated [1]. \\[4pt] [1] D. Chernin, et al., IEEE Trans. Electron Devices 59, 1542 (2012).   

Slow Wave Structure as a Radiator for Nonlinear Transmission Lines

Motivated by the potential in the generation of electromagnetic waves using the output pulses of a nonlinear transmission line (NLTL), we constructed the Green's function on a slow wave structure. A NLTL-based radiation source will not require an electron beam, and the key question is the conversion of the NLTL output of a general temporal pulse shape into radiation. A slow wave structure may be used as a radiator for the NLTL-generated pulses. We compare the analytic solution of the frequency response on the slow wave structure with that obtained from a particle-in-cell code. Favorable comparison is obtained for the first few lower order modes that are resonantly excited.