We show recent progress of a high efficiency 100 kW IOT amplifier design operating at 1.3 GHz. The IOT is composed of a multi cavity interaction circuit with upward of 80% power efficiency. We present details of the electrical and thermal design of the IOT as well as testing plans.

The SLAC 75XP series klystron was developed in the late nineties / early 2000's as a 60% efficient, 75 MW permanent magnet-focused klystron at X-band for the Next Linear Collider (NLC). In 2023, SLAC is resurrecting the 75XP series for potential deployment in future science facilities like the Cool Copper Collider. Here, we discuss the history and lessons learned from the initial 75XP klystrons (75XP1 through 75XP3 series). Finally, an overview of SLAC's current efforts to finish the design, build, and test of an improved 75XP4 version will be presented.

The SLAC 75XP series klystron was developed in the late nineties / early 2000‘s as a 60% efficient, 75 MW permanent magnet-focused klystron at X-band for the Next Linear Collider (NLC). In 2023, SLAC is resurrecting the 75XP series for potential deployment in future science facilities like the Cool Copper Collider. Here, we discuss the history and lessons learned from the initial 75XP klystrons (75XP1 through 75XP3 series). Finally, an overview of SLAC's current efforts to finish the design, build, and test of an improved 75XP4 version will be presented.

SLAC is developing multiple compact linear accelerator systems, with a fully integrated modulator, RF sources, controls, and the linac structure itself. To minimize the size and cost of these systems, a series of relatively low voltage (~60 kV) high power klystrons have been developed. This presentation will provide an update on the build status and preliminary test results from these compact klystron prototypes.

We show results of 3D space-charge beam dynamics simulation of an L-Band inductive output tube (IOT) rf electron gun. These results are essential to optimize the interaction circuit potentially achieving 100 kW of average power with upward of 80% power efficiency.

We report on the design of a multi-cell mm-wave accelerator structure operating near 100 GHz. This frequency regime enables not only a high gradient due to higher breakdown thresholds, but also reduced fill times which decreases pulsed heating and allows for higher repetition rates. These advantages make mm-wave accelerators an attractive solution to applications requiring high gradients or ultra-compact structures. The cavity geometry and distributed RF coupling network for the reported linac design have been optimized using ANSYS-HFSS and SLAC's parallel electromagnetic code suite ACE3P, with simulated accelerating gradients reaching 169 MeV/m with 100 kW per cell.

We present the design for a rapid proton energy modulator with radiofrequency accelerator cavities, which can deliver the proton radiation dose to varied depth in human tissues much faster than traditional mechanical beam energy degraders. The proton energy modulator is designed as a multi-cell 1-m long accelerator working at 2.856 GHz. Each individual accelerator cavity is powered by a 400 kW compact klystron to provide an accelerating/decelerating gradient of 30 MV/m. The high gradient is enabled by the individual power coupling regime, which provides a high shunt impedance. Beam dynamics simulations were performed, showing that the energy modulator can provide ±30 MeV of beam energy change for a 150 MeV, 7 mm long (full length) proton bunch, and the total energy spread of 3 MeV is satisfactory to clinical needs. A prototype experiment of a single cell has been built and tested, and the low-power microwave measurement results agree very well with simulations. The energy modulator is optimized for the 150 MeV cyclotron proton beam, while this approach can work with different beam energies.

SLAC is pursuing high efficiency, low cost RF source designs that are suitable for mass production, in order to enable the next generation of linear accelerators. These efforts have driven the development of compact linac systems with integrated modulators, klystrons, and accelerator structures, based on a modular, low voltage klystron topology. There is substantial demand for compact linacs for X-ray radiography with security applications, and for new radiation therapy machines that reduce treatment times by orders of magnitude and may yield beneficial biological effects. In this presentation, details and challenges for these new compact accelerator systems and the corresponding RF sources are discussed.

In this article, the design, characterization, and manufacture of a circuit for a ${W}$ -band, ultracompact continuous wave sheet beam klystron are described. This klystron, aiming to produce more than 1.5-kW output power, operates at a comparatively low voltage benefiting the device size and extending its application space. However, a lower operation voltage adversely affects the beam-gap interaction efficiency. Two multigap output cavities have been employed to achieve a higher beam interaction efficiency without introducing additional competing modes or shrinking the interval between the operation mode and competing mode. Inspired by the resynchronization technology, the second output cavity can be optimized for the spent beam from the first output cavity. Comprising a single cavity output structure, the circuit efficiency of this double output cavity circuit is increased from 2.37% to 4.96%. In this circuit, electromagnetic modeling and design have been optimized to avoid complicated structures which are constrained by current mechanical design and manufacturing techniques. The MAGIC simulation shows that, driven by a 1-W RF signal, this circuit can extract more than 1.7-kW output power from a 35-kV, 1-A beam. A prototype single output cavity circuit has been designed and manufactured to determine the required machining tolerances for the final double output cavity circuit.

A braze technique is developed for high-gradient Wband accelerator structures. Thin spacers were used to set the final gap between blocks during the braze process and provide precise control of the operational frequency. To demonstrate the robustness of this technique, we show cold testing results after the various manufacturing steps to monitor and track frequency changes throughout the process, and we show excellent quality of the fabricated test mm-wave structures.

A W-band, ultra-compact continuous wave sheet beam klystron aiming to produce 2 kW power is being developed to demonstrate compactness and high efficiency. The design utilizes a comparatively low voltage electron beam benefiting the device size and extending its application space; however, the lower operation voltage significantly affects the beam-wave interaction efficiency. Two single port multi-gap output cavities have been employed to achieve a higher beam interaction efficiency without introducing additional competing modes. By applying the new design, the simulation output power has been improved to 1.7 kW from 1.0 kW and the interaction efficiency increased by 2.1%.

A Periodic Cusped Magnet (PCM) - Tunable Quadrupolar Magnet (TQM) is proposed to focus the sheet electron beam of a low voltage W-Band, ultra-compact continuous wave sheet beam klystron (LUWK). The design utilizes a PCM to prevent the vertical beam spreading, and a TQM to control transverse beam spreading the force. Employing the PCM-TQM not only benefits the compact size of the LUWK, but it also mitigates the Diocotron instability of the sheet beam. Using the PCM-QM focusing system, the MAGIC-3D simulations show that an elliptical sheet beam with transverse size 6.0 mm x 0.5 mm can achieve a 99% cold beam transmission, and a 97% hot beam transmission through a 140 mm beam tunnel.

L3 Electron Devices has developed a high-power multibeam inductive output tube (MBIOT) for the European Spallation Source. In July 2014, L3 was awarded a contract for the design, manufacture, and test of a 1.2 MW, 704.42 MHz, pulsed power MBIOT. In October 2016, factory acceptance testing of the L6200 1.2-MW MBIOT was completed at L3 Electron Devices. In October 2017, site acceptance testing was successfully completed at the European Organization for Nuclear Research (CERN). The ten-beam, permanent magnet-focused device meets the maximum 1.2-MW output power specification with a dc-to-RF efficiency of 70%; efficiency is maintained above 60% down to 650 kW. Factory testing at L3 was limited to a cathode voltage of 43.6 kV and a maximum pulse width of 200 mu s. The CERN MBIOT test stand enabled operation at the preferred cathode voltage of 45 kV and at the full pulsewidth of 4 ms. The CERN test stand also offered the ability to individually adjust the filament voltage and bias voltage to each gun, which was not possible with the L3 test configuration. The CERN test stand and MBIOT are currently being prepared to demonstrate long-duration operation.

DRIVEN ACCELERATORS D.C. Nguyen , C.E. Buechler, G.E. Dale, R.L. Fleming, M.A. Holloway, J.W. Lewellen, D. Patrick Los Alamos National Laboratory J. Neilson, V. Dolgashev, E.N. Jongewaard, E.A. Nanni, A. Sy and S. Tantawi SLAC National Accelerator Laboratory Abstract Small, lightweight, few-MeV electron accelerators that can operate with low-voltage power sources, e.g., solidstate transistors running on 50 VDC, instead of highvoltage klystrons, will provide a new tool to enhance existing applications of accelerators as well as to initiate new ones. Recent advances in gallium nitride (GaN) semiconductor technologies [1] have resulted in a new class of high-power RF solid-state devices called highelectron mobility transistors (HEMTs). These HEMTs are capable of generating a few hundred watts at S-, Cand X-bands at 10% duty factor. We have characterized a number of GaN HEMTs and verified they have suitable RF characteristics to power accelerator cavities. We have measured energy gain as a function of RF power in a single low- C-band cavity. The HEMT powered RF accelerators will be compact and efficient, and they can operate off the low-voltage DC power buses or batteries. These all-solid-state accelerators are also more robust, less likely to fail, and are easier to maintain and operate. In this poster, we present the design of a low-, 5.1-GHz cavity and beam dynamics simulations showing continuous energy gain in a ten-cavity C-band prototype.

The SLAC National Accelerator Laboratory Technology Innovation Directorate is developing a 2 kW, 95 GHz ultra-compact continuous wave sheet beam klystron aiming to demonstrate 50% efficiency and weight of under 15 lbs. The design utilizes high current density cathodes, periodic cusped magnetic focusing for 99% beam transmission, and a multi-stage depressed collector to achieve device metrics. Beam stick construction is currently underway with operation slated to occur early in the year, the results of which will be used to validate beam transmission simulations. Construction of the interaction cavities is proceeding in parallel to ensure appropriate resolution and surface roughness is achieved.

L-3 Electron Devices has successfully completed factory testing of the L6200 1.2 MW Multi-Beam Inductive Output Tube (MBIOT) for the European Spallation Source. The ten-beam, permanent magnet-focused device meets the maximum 1.2 MW output power specification at 704.42 MHz with a DC-to-RF efficiency of 68.4%; efficiency is maintained above 60% down to 650 kW. The gain at 1.2 MW exceeds 21 dB. No oscillations were detected at any point during operation. The MBIOT is currently being prepared for shipment, after which it will undergo long-duration life testing.

L-3 Communications Electron Devices (L-3 EDD) is developing a high power Multi-beam IOT (MBIOT) for the European Spallation Source (ESS). Extensive simulation work has been completed and, where possible, results were validated against experiment using a scaled single-beam prototype. The MBIOT design has been finalized; fabrication of the tube is in progress, and configuration of a factory test bay is underway.