Session GO1: Beams and Coherent Radiation I

Demonstration of Guiding in a Preformed Periodically Modulated Plasma Waveguide for Quasi-Phase Matching

We report the generation of a periodically modulated plasma channel for the quasi-phase matched guiding of intense laser pulses, as proposed by Milchberg et al [1]. By varying system parameters, we can control the depth and period of the waveguide modulations, as well as the plasma ionization stage, electron density and guided spot size. Our method is highly tunable and extremely stable, and has shown remarkably reproducible modulation periods as short as 35 micrometers and as long as 3 mm. We have demonstrated single-mode guiding in these channels using Argon, Nitrogen, and Hydrogen with exit mode sizes as low as 15 micrometers FWHM and intensities of up to 6*10$^{17}$ W/cm$^{2}$, limited by our current Ti:Sapphire laser system. This method has a high degree of shot-to-shot consistency previously unattained with gas jets, resulting from improved gas jet design, leading to extremely uniform cluster formation. This technique has applications in direct electron acceleration, wakefield acceleration, high harmonic generation, and terahertz generation. \newline \newline [1] H.M. Milchberg et al., Phys. Plasmas 3, 2149 (1996).   

Electromagnetic properties of miniature corrugated plasma channels

Miniature corrugated plasma waveguides [1] are of interest in applications such as plasma based particle acceleration, high harmonic generation, and THz generation [2]. These corrugated waveguides support slow electromagnetic waves that can couple to ponderomotively driven currents due to the inhomogeneity of the plasma. Here we calculate the dispersive properties of such waveguides. First we use a simple model that allows for an analytic solution and gives a dispersion relation that predicts mode phase speeds and the location of stop-bands In addition, permits calculation of the mode coupling impedance, which is import for the above mentioned applications. Second, we use a modified version of the CEM field solver CTLSS to calculate the mode properties for sample electron density profiles present in experiments. Comparison between models will be given, and the implications for the various applications will be detailed. [1] H.M. Milchberg et al., Phys. Plasmas 3, 2149 (1996) [2] T. M. Antonsen et al. Bull. Am. Phys. Soc. 50, 186 (2005)   

Applications of the Corrugated Plasma Waveguide

The corrugated plasma channel [1] allows micron-scale control of the instantaneous intensity and phase velocity of a guided femtosecond laser pulse, at focused intensity, with no damage threshold. We review several applications of our new device. Extremely efficient direct electron acceleration by a radially polarized laser pulse can be achieved by quasi-phase matching in a fully ionized plasma. Since this acceleration is a linear process, very modest pulse energies ($\sim $1 mJ) could potentially give MeV-level acceleration. A similar setup could quasi-phase match guided high-harmonic generation in deeply ionized gas [2]. The short, tunable corrugation periods ($<$35 $\mu m)$ produced by our method are well suited for quasi-phase matching extremely high harmonic orders, potentially allowing us to reach the predicted atomic cutoff. Finally, recent theoretical work [3] shows that these channels could convert multi-millijoule femtosecond laser pulses into terahertz radiation with high efficiency by spatial control of ponderomotively driven electron currents. [1] Brian Layer, Advanced Accelerator Concepts Conference 2006 [2] H. M. Milchberg et. al., Phys. Plasmas 3, 2149 (1996) [3] T. Antonsen and H. M. Milchberg, submitted for publication.   

Energy gain scaling in the Plasma Wakefield Accelerator

We recently demonstrated the acceleration of electrons by 38GeV in a 90cm long plasma. The experiment was performed in the nonlinear or blowout regime of the PWFA where the beam density is larger that the plasma density. In this paper we show that the plasma density can be optimized for maximum energy gain. The largest energy gain is obtained at a plasma density of 2.7e17/cc with an average accelerating gradient of 37 GV/m. The energy gain scales linearly with plasma length, an important characteristics for the possible application of the PWFA to double the energy of a future linear collider. Details of the experimental results will be discussed.   

Design of higher-harmonic gyrotron oscillators with frequency-multiplied pre-bunched beams

There is currently considerable interest in operating gyrotrons at higher cyclotron harmonics in order to access the near-THz regime and reduce magnetic field requirements. Bandurkin and Savilov\footnote{I.V. Bandurkin and A.V. Savilov, Phys. Rev. ST 8, 010702 (2005).} recently proposed a scheme for bunching of gyrating electron beams at a multiple of the signal frequency based on Bragg cavities. In this work we investigate the use of this bunching technique to enhance higher-harmonic operation in gyrotron oscillators with annular beams. We compute the frequency-multiplied prebunching for given Bragg cavity parameters and use a large-signal, multimode, multi-harmonic gyrotron oscillator code to predict the parameter regions of stable higher-harmonic operation for several point designs.   

Breakdown Spot Size and Surface Temperature Studies in High Power Waveguide Simulations

Transmission of high power microwaves in room-temperature, normal-conducting, metallic waveguides is limited by breakdown of the metallic surfaces. A typical breakdown event results in plasma density in the waveguide high enough to result in zero transmitted power. The exact physics of waveguide breakdown is not fully understood, but secondary electron emission, ion-induced electron emission, surface heating, sputtering, gas desorption and ionization are all suspected of playing a role. We show simulation results for breakdown spots of 10-100 microns and how the breakdown rate changes from 100 nanoseconds to less than 10 nanoseconds as the spot size increases. We also show results for waveguide surface temperature for varying surface emission algorithms and discuss whether the temperature increase is sufficient to melt the waveguide structure.   

Undulator Induced Transparency in 3D: Theory and Simulation

Recent theoretical studies have found that the phenomenon of Electromagnetically Induced Transparency (EIT) has a classical counterpart in plasmas. A right hand circularly polarized electromagnetic field propagating along the magnetic field in a magnetized plasma is strongly absorbed when the field frequency matches the electron cyclotron frequency. However, the absorption is canceled by the presence of a second, ``drive'' field, when the frequency of the drive matches the difference between the electron cyclotron frequency and plasma frequency. When the electron plasma and cyclotron frequencies are identical, the zero frequency electromagnetic field can be replaced by a static helical undulator, yielding the so-called Undulator Induced Transparency (UIT). The theory of UIT in 1D has been extensively studied. It was shown that various exotic propagation modes can be obtained in the UIT regime, e.g., a dramatic slowdown of the group velocity of the wave, resulting in efficient compression of the wave energy, which in turn can be used for electron or ion acceleration. However, the 1D theory does not allow for any transverse variation in the problem, and the extension of the theory to a more realistic 2D or 3D geometry is not straightforward. We present here a full 3D theory of UIT, and compare it with the more intuitive 1D theory. We then apply the theory to study numerically the properties of UIT inside a waveguide.   

Random Dielectric Photonic Structures for Accelerating Particles

We investigate the applicability of random dielectric photonic structures to particle acceleration. Electromagnetic waves trapped within photonic crystal cavities can be used to accelerate charged particles. Whereas metallic cavities support higher harmonics, cavities in dielectric photonic crystals limit trapped oscillations to a frequency range within the bandgap of the surrounding crystal. This allows the cavity to contain only one frequency mode, and a resulting simplified field distribution within the cavity. In two dimensions, certain periodic arrangements of cylindrical rods (square and triangular lattices, for example) provide well-defined bandgaps, and are relatively easy to fabricate. Their 3D counterparts, however, pose an experimental challenge on the sub-micrometer scale. A recent simulation shows that an arrangement of randomly placed dielectric rods that are required to lie a minimum distance from any other rod, will also have a well-defined bandgap in two dimensions. If an analogous three dimensional structure exists, an improved accelerating cavity is feasible and may be easier to fabricate than regular 3D photonic crystals. Using FDTD simulation methods, we investigate the electromagnetic fields trapped in cavities inside 1D and 2D realizations of this random dielectric photonic structure toward particle acceleration applications and insights into a three dimensional counterpart. This work was supported by the U.S.DOE grant DE-FG02-04ER41317.   

Photon Acceleration by Laser Wakefields

In recent times, theory and simulation have been able to reproduce photon acceleration from wakefields where light is up- shifted in frequency by a moving refractive index gradient. The effect has been considered as a possible method of diagnosis of a wakefield such as the one postulated for a compact electron accelerator. Here we present further observations of spectral modulation caused by a laser-produced wakefield. In our experiments, the Astra laser at the Rutherford Appleton Laboratory was focused into jet of helium. Between 300mJ and 600mJ of laser energy was delivered into a 25 micron focal spot in pulses between 50fs and 200fs. The spectrum of the transmitted light was measured. The lower intensity laser pulse allowed a linear regime to be investigated. A photon kinetic code has been implemented to model the experiment. The feasibility of using this effect as a wakefield diagnostic will be discussed.   

Development of an Efficient Nanotube-Driven Amplifier

We are developing a compact two-cavity amplifier that uses a rugged carbon-nanotube cold-cathode to produce microwave radiation with very high efficiency. The entire RF circuit and electron gun have already been built and are currently under testing. We will present preliminary experimental results of electron beam generation and transport along the C-band RF circuit. This new amplifier should be capable of replacing conventional traveling-wave tubes in future radar and communication systems by offering substantial improvements in size, weight, and especially efficiency over its counterparts.   

Phase Space Tomography: A Simple, Portable and Accurate Technique to Map Phase Spaces of Beams with Space Charge

Charged particle beams can be viewed as a one component non-neutral plasma where the averaged focusing force takes the place of a fixed neutralizing background. In order to understand the charged particle dynamics, e.g. the halo formation, density waves, emittance growth, x-y energy transfer and coupling, knowledge of the actual phase space is needed. Other the past decade there is an increasing number of articles who use tomography in order to map the beam phase space and measure the beam emittance. These works where performed at high energy facilities where the effect of space charge was neglible and therefore not considered in the analysis. This work extends the tomography technique to beams with space charge. In order to simplify the analysis linear forces where assumed. By carefully modeling the tomography process using the particle-in-cell code WARP we test the validity of our assumptions and the accuracy of the reconstructed phase space. Finally, we report experimental results of phase space mapping at the University of Maryland Electron Ring (UMER) using tomography (This work was supported by the U.S Department of Energy, the Office of Naval Research and the Joint Technology Office).   

Initial Experimental Results on a Pulse Line Ion Accelerator

A new method of accelerating intense ion bunches has been investigated. The Pulse Line Ion Accelerator (PLIA) is best suited as an accelerator for intense bunches with pulse lengths of tens of cm. In a first beam dynamics validation experiment for the new PLIA concept, the predicted energy amplification and beam bunching were experimentally observed. Beam energy modulation of -80 keV to +150 keV was measured using a PLIA input voltage waveform of -21 kV to +12 kV. Ion pulses accelerated by 150 keV, and bunching by a factor of four were simultaneously achieved. The measured longitudinal phase space and current waveform of the accelerated beam are in good agreement with 3-D particle-in-cell simulations. Here we present initial experimental results of the PLIA as a proof-of-principle (POP) of the concept.   

Solenoid Transport of an Intense Ion Beam

Future WDM and HEDP experiments may use solenoids for transverse focusing of low energy, space-charge dominated ion beams during acceleration. An experiment to transport a 10 $\mu $s long, singly charged potassium ion bunch at an ion energy of 0.3 MeV and current of 45 mA through a solenoid lattice (STX) has been commissioned at LBNL. The beam should establish a Brillouin-flow condition, particle rotation at the Larmor frequency, with fields greater than 2T. The principal objectives of the STX are to match and transport the space-charge dominated ion beam and to study mechanisms that would degrade beam quality such as focusing-field aberrations, beam halo, spacing of lattice elements, and electron-cloud and gas effects. A qualitative comparison of experimental and calculated results are presented, which include time resolved transverse phase-space of the beam at different diagnostic planes throughout the focusing lattice, beam current density and beam-induced gas desorption, ionization and electron effects. (This work was supported by the U.S. D.O.E. under DE-AC02-05H11231)   

Thermal equilibrium of intense beam propagation through a periodic solenoidal focusing field

A thermal Vlasov equilibrium is obtained for an axisymmetric charged-particle beam with nonuniform density in the radial direction propagating through a periodic solenoidal magnetic focusing field. The thermal beam distribution function is constructed and the beam envelope equation which is consistent with the thermal density distribution is derived. Examples of periodically focused rigid-rotor thermal equilibria are presented. Statistical properties and possible applications of the thermal beam equilibrium are also discussed. Effects of the thermal temperature and nonlinear forces are being studied, and results will be presented.   

Low work function HPM graphite cathodes coated with alkali and alkaline-earth elements and compounds

Cesium-iodide (CsI) coated graphite cathodes have demonstrated superior emission characteristics particularly attractive for High-Power-Microwave (HPM) applications [1]. While the CsI layer appears to enhance cathode performance, its role at the nano-scale level remains unknown. In order to understand the electron emission physics of this system, an ab initio molecular physics study has been carried out on the surface work function modification of a clean graphite basal surface coated by thin adsorbent layers. The effects of both Cs and Ba monolayers on a semi-metallic (graphite) surface have been studied, as well as their ionic compounds CsI and BaO. It is shown that both pure Cs and Ba over-layers interact with and consequently lower the surface work-function of graphite in a way analogous to what has been observed for alkali metals chemisorbed on metallic surfaces [2]. On the other hand, CsI and BaO ionic compounds interact with the graphite surface in different ways depending whether the cation or anion is relaxed closest to the surface. The underlying fundamental mechanism responsible for lowering the work function appears to be essentially identical for all configurations. [1] D. Shiffler, et al., Phys. Plasmas Vol. 11 (2004) 1680. [2] P. Soukiassian, et al., Phys. Rev. B. Vol. 31 (1985) 4911.