In this paper, the cold-fluid model theory of an intense sheet electron beam is developed to investigate the diocotron instability during its transport in a uniform magnetic field. The model shows that if the magnetic field strength and the beam filling factor are increased separately, the diocotron instability will be suppressed, which extends the sheet-beam transport distance effectively. To verify the above conclusion, a set of complicated instruments, the electron beam measuring and analyzing system (EBMAS), was developed to measure the beam cross section, beam current density, and the 3-D trajectory. The sheet electron beam transport process in a uniform magnetic field with its diocotron instability is investigated on the basis of EBMAS measurements. The measured results agree very well with theoretical calculation and numerical simulation. A sheet-electron-beam tube based on a uniform magnetic field was then manufactured to drive a future W-band sheet-beam klystron (WSBK). Tuning the sheet-beam parameters over a broad range, including a cathode voltage range of 20–82 kV, current range of 0.5–4.27 A, and with a beam cross section of about 10 mm $\times\,$ 0.5 mm, the experimental beam transmission rate is above 98% through a beam tunnel of 100 mm in length.

Theoretical and Experimental Investigation on Intense Sheet Electron Beam Transport With Its Diocotron Instability in a Uniform Magnetic Field

This paper reports the research work of an X-band sheet beam klystron with the aim of principle verification, which is fulfilled at the Institute of Electronics, Chinese Academy of Sciences. This paper includes two phases. The first phase is planned to build the sheet beam electron optics prototype tube using the closed periodically cusped magnetic focusing. The measured data shows that the rectangular beam with the cross section of 50 mm × 4 mm can propagate through ~ 300 mm from the cathode surface to the collector with the transmission of 92.4% at the voltage of 110 kV. This foregoing work successfully solves the beam formation and transportation and becomes the basis for developing the sheet beam klystron. In the second phase, the high-frequency interaction structure is designed and cold tested. The X-band sheet beam klystron is constructed and hot tested in the full voltage. The amplification characteristic can be observed near the voltage of 125 kV. For the drive power of 0.71 kW and the working frequency of 11.69 GHz, the output power of 2.8 MW is achieved with the 3 dB bandwidth of 30 MHz, and, correspondingly, the gain and the efficiency are 35.96 dB and 32.52%, respectively. At the same time, the beam transmission is 73.3%. An over 93% transmission is realized at the voltage of 135 kV, whereas the oscillation occurs in the tube. This research exhibits that the sheet beam klystron is a promising device for the high-power applications, whereas seeking the measures to overcome the oscillation is still an arduous task in the future.

Researches on an $X$ -Band Sheet Beam Klystron

Sheet electron beam transport under the magnetic field of a novel staggered closed periodic cusped magnet (PCM) has been investigated. In a staggered closed PCM, the horizontal and side magnets are placed in axially staggered positions and magnetized in the axial and transverse directions, respectively. Soft iron pole pieces are placed between two adjacent horizontal magnets of opposite polarity. Numerical analysis shows that effective root-mean-square value of the axial component of the magnetic field for the initial half cycle is higher for the proposed PCM than the conventional one by keeping the pole piece close to the magnets at first position. The magnetic period required to obtain desired peak value of the axial component of the magnetic field is shorter for the proposed PCM than for the conventional PCM for similar other parameters. Aforementioned advantages of the staggered closed PCM reduce the vertical displacement of the beam edge and stable transport of the sheet beam is possible through it. For a wide magnetic tunnel, required horizontal component of the magnetic field can be provided by extending the pole pieces between the horizontal magnets toward the beam surface and in the axial direction after the beam edges.

Staggered Closed PCM for Stable Rectangular Sheet Electron Beam Transport

The focusing and transmission of a sheet electron beam with periodic cusped magnetic (PCM) fields were studied for the terahertz traveling-wave tubes (TWT). Due to the limitation on the width of the sheet beam from the slow wave structure (SWS), the resulting transmission current density is inevitably high and the width of the sheet beam is highly disproportionate to the magnet system. This defines a situation different from before, which challenges the validity of the traditional PCM scheme. Thus, the focusing of a sheet beam in the above scenario was theoretically analyzed to evaluate the focusing conditions. A practical magnetic field system was then designed based on the offset-pole-piece PCM scheme with the shortened pole pieces, which can meet the focusing requirements, especially on the side focusing. The theoretical analysis and the modified PCM scheme lead to a significant improvement in the focusing/transmission of the sheet beam. The 3-D particle simulation showed that a stable long-distance transmission over 100 mm can be achieved with the two-plane PCM focusing where an elliptical-cross section sheet beam is assumed with dimensions of 0.6 mm $\times0.11$ mm, voltage of 24.5 kV, and current of 0.18 A.

Focusing of the Sheet Electron Beam With Two-Plane Periodic Cusped Magnetic System for Terahertz TWTs

The electron optical systems (EOS) play a vital role in vacuum electron devices (VED). This letter reported the design and experiments of the sheet EOS for the 1.03 THz traveling wave tube (TWT). A high-current-density sheet electron beam (SEB) electron gun generating up to 20.3-mA at 22.4-kV is designed, based on the requirements of this TWT. Due to the very high operating frequency, more than 1 THz, the miniature tunnel and beam waist with a dimension of 0.05 mm * 0.148 mm and 0.025 mm * 0.114 mm, respectively, will lead to a very high beam current density (906.9 A/cm 2 ). Therefore, a 1-Telsa uniform magnetic system is designed, to ensure such a high-current-density SEB passes through the 27-mm tunnel. Finally, the primary experiments indicate that 7.56-mA current reaches the collector.

Sheet Electron Optical System for a 1.03-THz Traveling-Wave Tube

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.

X-Band Sheet Beam Klystron (XSBK)

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X-Band Sheet Beam Klystron (XSBK)