Showing papers by "Brian Chase published in 2021"

High Efficiency, Low Cost RF Sources for Accelerators and Colliders

Abstract: Calabazas Creek Research, Inc. (CCR) is collaborating with a number of institutions to develop new RF sources for accelerators and colliders. An overriding focus is on reducing acquisition cost as well as the continuing cost by increasing efficiency. Consequently, the goal of these programs is efficiencies exceeding 80% at power levels above 100 kW CW. Two research efforts involve modifying or controlling magnetrons to allow fast control of the frequency, amplitude and phase for beam loading compensation. Two techniques are described. Research is also investigating two multiple beam sources that achieve high efficiency through Class C operation. A 350 MHz RF source, nearing completion, uses power grid tubes, and a 700 GHz source will use an inductive output tube. An L-Band klystron is also nearing completion and is designed to produce 100 kW at 80% efficiency. These programs will be briefly described. 100 KW MAGNETRON SYSTEM Figure 1 shows a photograph of the magnetron system designed to deliver RF power with output power and phase control by an active feedback circuit compensating for beam loading. The system was designed for high Q, superconducting cavities. The system uses an approach developed by Fermilab in which power is controlled by modulating the phase of an RF locking signal directed into the magnetron output [1]. By modulating the phase of the locking signal one can shift power into sidebands. Power in these sidebands is reflected from high Q cavities, thereby reducing power into the cavity. Principal components include the magnetron, a four-port circulator, an RF driver, and control circuitry. These are integrated into the system shown in Fig. 1, including water cooling, interlocks, diagnostic instruments, and power supply for the * Work supported by U.S. Department of Energy Grants DE-SC0011229, DE-SC0017789, DE-SC0018838, DE-SC0019800, and U.S. Navy contract N6833520C0822. † RLI@Calcreek.com driver. The high-power supply for the magnetron in not included in this package. Figure 2 shows the relative amplitude at the fundamental frequency as a function of phase modulation. Power in side bands is reflected and absorbed in the circulator. The system was tested to full peak power at Fermilab, where the data in Fig. 2 was obtained. The system efficiency exceeded 80% for all modes of operation. The system shown in Fig. 2 used a 5 kW klystron as the driver, but only 316 W was required. This power is readily available from solid state sources. Estimated cost for the system as shown is approximately $100K, which is $1/Watt. This is approximately 25% of the cost for a comparable klystron or solid-state source at this frequency and power level. Figure 1: 1.3 GHz, 100 KW, Magnetron System with fast phase and amplitude control. 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-TUPAB349 TUPAB349 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I 2322 MC7: Accelerator Technology T08 RF Power Sources Figure 2: Power at fundamental frequency as function of driver phase modulation. COMPACT MAGNETRON WITH FAST FREQUENCY AND PHASE CONTROL CCR is teamed with SLAC National Accelerator Laboratory (SLAC) and Communications & Power Industries LLC to develop a compact magnetron RF source with fast phase and frequency control. This approach eliminates the circulator by using varactor diodes to modulate the capacitance, and thus the frequency, of a magnetron cavity. A 0.5 pF capacitance change shifts the magnetron frequency by 5 MHz. Figure 3 shows a MAGIC simulation of a magnetron where capacitance in controlled using a varactor. Figure 4 shows a suitable diode for this application. Figure 3: 3D MAGIC simulation of pi mode fields in a magnetron where varactors modulate capacitance of a cavity. Figure 4: Microsemi GC1712 varactor diode. This program integrates several innovative features to provide an extremely compact, low-cost configuration. The system can incorporate adaptive feed forward to cancel the high frequency resonances of the switching power supply. Optionally, an analog loop can be used for fast, high bandwidth control. The Navy is funding development of as S-Band, 5 kW prototype system for a communications application. CCR and SLAC are pursuing a higher power version for accelerator applications. 1.3 GHZ, ULTRA-HIGH EFFICIENCY KLYSTRON Interest has increased recently in klystron due to new circuit design techniques predicting significantly higher efficiency than available in conventional klystrons. These circuit design approaches facilitate more efficient bunching of electrons prior to power extraction in the output cavity. CCR and Leidos are using the Core Oscillation Method (COM) to achieve more than 80% efficiency in a 1.3 GHz, 100 kW klystron. Figure 5 shows a model of the klystron, which is currently being assembled. The COM approach achieves more efficient bunching by providing increased length. The circuit length for the klystron being built is 220 cm. This increases the tube length but uses fewer cavities than other approaches, such as the Bunch, Align, and Collect (BAC) method. Figure 5: Solid model of 1.3 GHz klystron designed to produce 100 kW at more than 80% efficiency. The solenoid was received, and the cavities are currently being cold tested. Other major subassemblies, including the gun, collector, and output window are complete, and the test set is currently being reconfigured to accommodate the increased length. High power tests are scheduled for summer 2021. MULTIPLE BEAM POWER GRID TUBES Power grid tubes have been in use for more than fifty years and have produced RF power at efficiencies approaching, and sometimes exceeding 90%. These tubes provide beam power for sources producing tens of kilowatts and frequencies from 300 MHz to 1 GHz. The high efficiency results from Class C operation, where the pulsed electron beam is generated by RF modulation of a grid. Gain is typically 14 dB. This program is developing multiple beam grid tubes to provide sufficient beam power to produce 200 kW or more of RF power. Figure 6 shows the array of eight grid-cathode assemblies powering the prototype tube. These assemblies use flat cathodes and grids cut from commercial tungsten screen. Consequently, the cost is extremely low 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-TUPAB349 MC7: Accelerator Technology T08 RF Power Sources TUPAB349 2323 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I

Resonance Control System for the PIP-II IT HWR Cryomodule

Abstract: The HWR (half-wave-resonator) cryomodule is the first one in the superconducting section of the PIP-II LINAC project at Fermilab. PIP-II IT is a test facility for the project where the injector, warm front-end and the first two superconducting cryomodules are being tested. The HWR cryomodule comprises of 8 cavities operating at a frequency of 162.5 MHz and accelerating beam upto 10 MeV. Resonance control of the cavities is performed with a pneumatically operated slow tuner which compresses the cavity at the beam ports. Helium gas pressure in a bellows mounted to an end wall of the cavity is controlled by two solenoid valves, one on the pressure side and one on the vacuum side. There is a pressure transducer that provides an electrical voltage as an indicator for tuner pressure for monitoring and feedback purposes. A simplified schematic of the tuner system is shown in Fig. 1. The resonant frequency of the cavity can be controlled in one of two modes. A pressure feedback control loop can hold the cavity tuner pressure at a fixed value for the desired resonant frequency this is referred to as the ’Pressure Mode’ operation. Alternately, the feedback loop can regulate the cavity tuner pressure to bring the RF detuning error to zero which is referred to as the ’RF Mode’. In the latter case, the tuner can compensate for slow drifts in resonant frequency. In the GDR (generator driven) mode of operation, all cavities are run at the same reference frequency. The RF Mode is neccessary for the GDR mode. A state machine based system controller can provide an automatic transition between the modes of operation according to the machine state. The tuner implementation is done with a signal conditioning module and the same LLRF SOCFPGA controller used for cavity field control. The control system design, implementation and performance are described in this paper.

Performance of the LLRF System for the Fermilab PIP-II Injector Test

Abstract: PIP-II IT is a test facility for the PIP-II project where the injector, warm front-end and the first two superconducting cryomodules are being tested. The 8-cavity half-waveresonator (HWR) cryomodule operating at 162.5 MHz is followed by the 8-cavity single-spoke resonator (SSR1) cryomodule operating at 325 MHz. The LLRF systems for both cryomodules are based on a common SOC FPGA based hardware platform. The resonance control systems for the two cryomodules are quite different, the first being a pneumatic system based on helium pressure and the latter a piezo/stepper motor type control. The data acquisition and control system can support both CW and Pulsed mode operation. Beam loading compensation is available which can be used for both manual/automatic control in the LLRF system. The user interfaces include EPICS, Labview and ACNET. Testing of the RF system has progressed to the point of being ready for 2 mA beam to be accelerated to 20 MeV. The design and performance of the field control and resonance control system operation with beam are presented in this paper.

LCLS-II-HE verification cryomodule high gradient performance and quench behavior

Abstract: An 8-cavity, 1.3 GHz, LCLS-II-HE cryomodule was assembled and tested at Fermilab to verify performance before the start of production. Its cavities were processed with a novel nitrogen doping treatment to improve gradient performance. The cryomodule was tested with a modified protocol to process sporadic quenches, which were observed in LCLS-II production cryomodules and are attributed to multipacting. Dedicated vertical test experiments support the attribution to multipacting. The verification cryomodule achieved an acceleration voltage of 200 MV in continuous wave mode, corresponding to an average accelerating gradient of 24.1 MV/m, significantly exceeding the specification of 173 MV. The average Q0 (3.0x10^10) also exceeded its specification (2.7x10^10). After processing, no field emission was observed up to the maximum gradient of each cavity. This paper reviews the cryomodule performance and discusses operational issues and mitigations implemented during the several month program.