A model of a thermionic cathode in a planar diode in which the Poisson and Vlasov equations are solved in 3-D assuming an infinite magnetic field is presented. We explore how 2-D work function variations across the cathode surface may affect the transition between temperature-limited and spacecharge-limited flow, commonly known as the "knee" of the Miram curve. We study a variety of work function distributions, both realistic and idealized, and demonstrate how emission from the lowest work function regions dominates the total anode current even when such regions make up a relatively small fraction of cathode area. Our model also illustrates the ability of cathodes to reach the full Child-Langmuir current despite the presence of a sizeable nonemitting region. We find that as the length scale of these work function variations decreases, the Miram knee grows sharper, indicating improved cathode performance.
AJDISK is a 1D large signal klystron simulator developed at SLAC. A brief discussion of AJDISK's simulation algorithm is given and [1] is fully developed to show how AJDISK was extended to enable sheet beam klystron simulations. The primary requirement for extending "disk" simulators to sheet beam simulators is a space charge equation. Therefore, the electric field due to an infinitely thin plate of charge (infinitely thin in the direction of propagation) in a rectangular drift tube is derived. The derivation is extended to a "2D" space charge equation in which the plate is split into a series of rods with varying positions in y. The derived equations are compared with numerical simulations and measurement.
The Sheet Beam Klystron (SBK) is characterized by a large drift tube, which allows the use of high beam current at a low voltage, resulting in low beam current density, high efficiency and the possibility of PPM focusing. CPI has designed, manufactured and is currently testing an X-Band SBK capable of 5 MW peak, 20 kW average output power This paper discusses the general design, manufacturing and performance to date of CPI's X-Band SBK.
SLAC is developing sheet beam klystron technology for narrow bandwidth, high peak and average power applications from L-band to W-band. Sheet beam devices are advantageous for several reasons. The primary advantage is the increased surface area in the RF circuit which significantly increases the dissipated heat that can be transferred through the circuit. The reduced charge density in the beam decreases the magnetic field required for beam transport and increases the achievable efficiency compared to a pencil-beam tube with the same beam voltage and current. Finally, both the RF circuit and the magnetic focusing system are simpler and less expensive to fabricate. The combination of features provided by a sheet beam klystron make it an ideal source for linear accelerator and high average power applications.SLAC designed, built and tested a proof of principle, W-band sheet beam klystron (WSBK). The goal of the design was to answer several basic questions regarding sheet beam klystron operation. First, does the cylindrical cathode produce an acceptable, elliptic cross Figure 2 One half of 7 cavity, single-gap, WSBK circuit.section beam. Second, can this beam be transported over the 9 cm. of RF circuit. Third, given that the rectangular drift tube is not cut off to TE modes, will the device be free of oscillations and spurious modes. Finally, does the measured performance of the device agree with the modeling codes used in its design. The test results for the WSBK answered all of the above questions. The 74 kV, 3.6 A beam produced 91% transmission with a 1.1 kG PPM field. There was no evidence of oscillation or beam breakup from TE modes in the circuit. The small signal RF performance agreed well with two different simulation programs. Full power output requires a multigap output cavity to achieve the required impedance. A multi-cell cavity is currently being designed. Progress on the design and fabrication of the 100 kW peak power, 2 kW average power WSBK will also be reported.
A methodology of modeling nonplanar surfaces, in which the microscale features of the emission sites can be orders of magnitude smaller than the mesoscale features defining the active emission area, has been developed and applied to both ordered arrays of identical emitters and random variations characteristic of a roughened surface. The methodology combines a general thermal-field-photoemission model for electron emission, a point charge model for the evaluation of field enhancement factors and surface geometry, and a Ballistic-Impulse model to account for the trajectories of electrons close to the cathode surface. How microscale and mesoscale features can both undermine the estimation of thermal-field emission parameters, such as characteristic field enhancement and total current predictions, as well as give rise to changes in the distribution of transverse velocity components used to estimate beam quality features such as emittance that are important to photocathodes, is quantified. The methodology is designed to enable both the proper characterization of emitters based on experimental current-voltage data and the development of a unit cell model of emission regions that will ease the emission model demands in beam optics codes.
The Compact Linear Collider (CLIC) is a proposed future electron-positron collider, designed to perform collisions at energies from 0.5 to 5 TeV, with a nominal design optimized for 3 TeV (Dannheim, 2012). CLIC generates three beams: the drive beam, the main electron beam and the positron beam. The drive beam is a high current electron beam that is accelerated in an S-band linac and then decelerated in 12 GHz structures to generate the RF for accelerating the colliding main electron and positron beams. The drive beam employs a high current thermionic DC electron gun. This report explores the optimal anode-cathode geometry as well as analyzes the mechanical and electrical integrity of the gun structure designed by CERN.
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