Bruce Eric Carlsten

Although planar beams appears to be the ultimate solution for realizing high frequency vacuum electron devices, it is very challenging to form a sheet beam that exhibits high transport characteristics and low beam loss. Typically, a circular/elliptical beam from thermionic cathode is compressed to a high compression ratio using high magnetic focusing. The compression often causes rapid increase of transverse momentum, increasing emittance. The emittance growth in turn leads to beam interceptions at the narrow aperture of SWS/cavities. To mitigate this, we propose to use planar array of field emitters that will eventually provide a uniform planar electron beam. Field emitters are electron emitters capable of emitting electrons after the application of high voltage. The idea is to form a sheet beam from a planar array Field Emitter Arrays (FEAs), obviating the need for high magnetic compression of the beam, with the hypothesis that there will be no anomalous emittance growth. The emittance behavior of the Diamond FEAs has been analyzed and demonstrated at LANL before. This presentation will focus on beam coalescing effect of multiple beamlets emitted from field emitters and how they form sheet beam from a planar array of these beamlets.

We are exploring the amplification of W-band electromagnetic radiation using a dielectric-loaded traveling wave tube (TWT) by employing several particle-in-cell (PIC) codes. We are seeking to replicate recent results obtained by a Naval Research Laboratory's (NRL's) dielectric-loaded TWT design [1] consisting of a solid circular electron beam (26 kV, 100 mA and 0.185 mm beam radius) surrounded with dielectric material, \pmb\varepsilon_{\mathbf{r}}\pmb{=13.5} , and coupled to a \mathbf{TM}_{\pmb{01}} electromagnetic wave at a frequency of 94 GHz. NRL used a finite-difference-time-domain (FDTD) formulation in a 2-D cylindrical coordinate system to perform the dielectric-loaded TWT simulations. In our work, we have opted for PIC simulations comparing three different software tools-a 3-D Cartesian coordinate system 'FDTD-PIC method-based MAGIC', 'CST Electromagnetic and Multiphysics Simulation Studio Suite', and 'Improved Concurrent Electromagnetic Particle-In-Cell (ICEPIC)'. This paper summarizes our results from these studies.

The dielectric constant varies with frequency so that the known dielectric constant at low-frequency is no longer valid at high-frequency regions. When the frequency increases, experimental imperfection such as air gap between a sample and a resonator cannot be ignored anymore in the dielectric constant measurement by means of the resonator method. In contrast to the existing resonator method using a fundamental TE 10 mode in a rectangular waveguide, we propose to use a TE 01 mode in a circular waveguide which is less affected by the air gap between the surface of the waveguide and the dielectric material.

We have designed, fabricated, and are currently testing a Ka-band dielectric-loaded traveling-wave tube with a very high bandwidth. Applications for high power mm-wave sources span biology, medicine, communications, national security, and other areas. One of these applications, high resolution imaging using synthetic aperture radar techniques, requires a power source with an average power level of 1 kW and the bandwidth of 10 GHz. We present a novel Ka-band TWT architecture that approaches these requirements.

We have fabricated a Ka-band dielectric loaded traveling-wave tube with high instantaneous bandwidth of 10GHz centered at 33.25GHz. The process of fabrication involves copper and ceramics machining and several hierarchical assembly steps. We report on the fabrication steps and RF system design. We are currently doing cold-test of the TWT using network analyzer and Ka band signal source.

We demonstrate an alternative realization of a bunch compressor (specifically, the second bunch compressor for the MaRIE XFEL beamline, 1GeV electron energy) using a double emittance exchanger (EEX) and a telescope in the transverse phase space. We compare our results with a traditional bunch compressor realized via a chicane, taking into account the nonlinear dynamics, Coherent Synchrotron Radiation (CSR) and Space Charge (SC) effects. In particular, we use the Elegant code for tracking particles through the beamline, and analyze the evolution of the eigen-emittances to separate the influence of the CSR/SC effects from the nonlinear dynamics effects. We optimize the scheme parameters to reach a desirable compression factor and minimize the emittance growth. We observe dominant CSR effects in our scheme, resulting in critical emittance growth, and introduce an alternative version of an emittance exchanger with a reduced number of bending magnets to minimize the impact of CSR effects.

This paper summarizes the activities and presentations of Working Group 7 (WG7) on Radiation and Advanced Concepts of the Advanced Accelerator Concepts Workshop held at the National Harbor, Maryland, from July 31 to August 5, 2016. Material presented at WG7 was wide-ranging and included various beam-based radiation mechanisms, applications using these novel radiation schemes, and novel beam optics.

We present the results of our investigations of fabrication technologies for ceramic photonic band gap (PBG) structures for mm-wave traveling-wave tubes (TWTs). There is a need for a high-bandwidth, high output power TWT operating at relatively low electron-beam voltages (20 keV). The key advance needed for this technology is the development of a novel high-frequency TWT structure. We proposed to put together a TWT with a sheet electron beam in an elliptical wide-bandwidth dielectric RF structure. PBG structures can be designed to be mode-selective and enable the required wide bandwidth and output power for the TWT. We conducted a feasibility study on fabrication of dielectric PBG structures in a high-epsilon ceramic material, designed suitable RF structure, drilled holes in high-dielectric ceramic blanks, measured them and studied the effect of hole's size variations and misalignments on the RF mode.