W-band Sheet Beam Klystron Research at SLAC

Abstract

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.

Developing Sheet Beam Klystron Simulation Capability in AJDISK

“…AJDISK VS MEASUREMENT Fig. 9 compares AJDISK with measured data of SLAC's W-band sheet beam klystron [2]. The results are in good agreement and it is believed that most of the error is due to the difficulty of maintaining tight mechanical tolerances at 95 GHz.…”

“…Drawbacks for sheet beam devices include lower cavity interaction impedance and possible mode competition, but 3D simulations can help provide insight for overcoming these challenges. The klystron group at Stanford Linear Accelerator Center (SLAC), for example, has recently designed and manufactured a working low power 95GHz sheet beam klystron [2].…”

Efficient terahertz oscillation using a half-period staggered grating resonator

“…At least one TE mode exceeds the cutoff frequency of the circuit; thus, over certain amount of current, TE oscillation prevents the RF growth of the designed TM mode. 26,27) In contrast, the sEIO, shown in Fig. 1(a), has a symmetric electric field along the beam propagation line, as shown in Figs.…”

Radiofrequency Accelerator R&D Strategy Report: DOE HEP General Accelerator R&D RF Research Roadmap Workshop, March 8-9, 2017

“…This is true for all types of sources, whether a klystron, magnetron, gyrotron, magnicon, etc. Creating efficient, high-power devices at higher frequency, where the device shrinks in size as the square of the wavelength [63] is even more difficult. New cathodes producing higher current densities, simplifying beam transport, have been developed, but need to be reliably integrated in RF sources [68].…”