Status and Future Strategy for Advanced High Power Microwave Sources for Accelerators
Year: 2018

Energy efficient conversion of electrical grid power into radio frequency power is becoming one of the key aspects of future accelerators. A significant part of the initial investment and running costs of these machines will be determined by the cost and efficiency of their RF systems. For largescale accelerators, which are proposed for instance by the International Linear Collider Study or by the Future Circular Collider Study, RF efficiency will likely be a determining factor in the approval process. Efforts are already in place to stretch the efficiency of existing RF sources to higher levels. Well known technologies such as klystrons are being reinvented with modern day beam physics, inductive output tubes are being combined for higher power output, and modular solid state amplifiers are becoming more popular as RF sources for accelerators. This talk will give an overview of recent advances and trends in RF source developments. It will discuss future needs and the strategy towards higher efficiency devices for the benefit of the accelerator community. THE NEED FOR HIGHER EFFICIENCY RF SOURCES The electrical power consumption of future accelerators will be driven to a large part by their RF systems. This is particularly true for electron colliders, circular (e.g. FCC) or linear (e.g. ILC or CLIC) which need RF systems in the order of 100 MW or above (see Table 1, [1 6] Other examples with significant RF power consumption are high power hadron linacs (e.g. ESS) and cyclotrons (PSI) for neutron science or Accelerator Driven Systems. In the case of hadron colliders, magnet systems dominate electrical power consumption while RF systems only use a few percent. At CERN, where most accelerators are circular, only 6% of the annual consumption 1.1 TWh in 2017) are caused by the RF systems. For a 3 TeV CLIC machine, around 50% of the facility power consumption is due to the RF system. With an estimated yearly total of 2.74 TWh [4] and a European average price for non household consumption of 0.1e/kWh in 2017, this translates into 187 Me of electricity costs/year for the RF system alone. Therefore every percentage point in RF efficiency has a significant impact on the running cost. For Accelerator Driven Systems (ADS) the power to run the accelerator complex (including cooling, ventilation, cryogenics, offices, etc. must be small (5 10% compared to the power, which is produced in the ADS core. To achieve these values the accelerator complex must have a total efficiency between 0.2 eacc 0.4 [7] which presents a challenge for today's accelerators. If not achieved, the idea frank.gerigk@cern.ch Table 1: RF Power for Various Future Accelerators Project Ptotal [MW] PRF [MW] Pbeam [MW] PSI* 10 4 1.3 LEP2+ 120 42 19 FCC ee+ tbd 165 100 ESS* 35 15 5 CLIC 500* 272 109 9.8 ILC 500* 164 68 9.4 CLIC 3000* 582 289 28 total power is for the whole facility with experiments, +without injector of using ADS for energy production is unlikely to become economically viable  TODAY'S TECHNOLOGIES NEW DEVELOPMENTS In the following we will review today's state of the art of high power RF sources  and point to developments, which may change their efficiencies or power reach in the near future. As pulsed and Continuous Wave (CW) operation pose different challenges to the RF sources both modes of operation will be treated. Modulator Efficiency Most high power RF sources need a High voltage (HV) modulator, which transforms the voltage of the electrical grid to the HV and pulse pattern needed by the RF source. For CW operation in general but also for pulsed operation of gridded tubes (tetrodes, IOTs) the modulator is basically a HV power supply. Pulsed power for gridded tubes is obtained by simply pulsing the grid using the input RF signal, whereas HV pulses for the operation of non gridded tubes (klystrons, magnetrons) have to be formed by the modulator itself. Various topologies have been developed in recent years, which are adapted to different pulse lengths and voltage needs [9] i) Traditional pulse transformer based modulators with efficiencies between 85% and 90% and rise times in the range of a few hundred microseconds. ii) High frequency transformer based modulators such as the resonant polyphase design employed at SNS [10] or the stacked multi level (SML) design developed and used by the European Spallation Source (ESS) with efficiencies up to 92% and with a very short rise time in the range of 100 ms. iii) Transformerless modulators such as the Marx generator [11] which have the potential for even higher efficiency, or direct switch designs. 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978 3 95450 184 7 doi:10.18429/JACoW IPAC2018 MOYGB1