PIP-II is an 800 MEV superconducting linac that is in the initial acceleration chain for the Fermilab accelerator complex. The RF system consists of a warm front-end with an ion source, RFQ and buncher cavities along with 25 superconducting cryo-modules comprised of five different acceleration \(\beta\). The LLRF system for the LINAC has to provide field and resonance control for a total of 125 RF cavities.The LLRF system design is in the final design review phase and will enter the production phase next year. The PIP-II project is an international collaboration with various partner labs contributing subsystems. The LLRF system design for the PIP-II Linac is presented and the specification requirements and system performance in various stages of testing are described in this paper.

Cold powered testing of all LCLS-II production cry-omodules at Fermilab is complete as of February 2021. A total of twenty-five tests on both 1.3 GHz and 3.9 GHz cry-omodules were conducted over a nearly five-year time span beginning in the summer of 2016. During this cam-paign cutting-edge results for cavity Q0 and gradient in continuous wave operation were achieved. A summary of all test results will be presented, with a comparison to established acceptance criteria, as well as overall test stand statistics and lessons learned.

The Cryomodule Test Stand (CMTS1) at Fermilab has been engaged with testing 8-cavity 1.3 GHz cryomodules designed and assembled for the LCLS-II project at SLAC National Accelerator Laboratory since 2016. Over these three years twenty cryomodules have been cooled to 2 K and power tested in continuous wave mode on a roughly once per month cycle. Test stand layout and testing procedures are presented together with results from the cry-omodules tested to date. Lessons learned and future plans will also be shared.

The SLAC National Accelerator Laboratory is building LCLS-II [1], a new 4 GeV CW superconducting (SCRF) Linac as a major upgrade of the existing LCLS. The LCLSII Low-Level Radio Frequency (LLRF) collaboration [2] is a multi-lab effort within the Department of Energy (DOE) accelerator complex. The necessity of high longitudinal beam stability of LCLS-II imposes tight amplitude and phase stability requirements on the LLRF system (up to 0.01% in amplitude and 0.01° in phase RMS) [3]. This is the first time such requirements are expected of superconducting cavities operating in continuous-wave (CW) mode. Initial measurements on the Cryomodule test stands at partner labs have shown that the early production units are able to meet the extrapolated hardware requirements to achieve such levels of performance [4]. A large effort is currently underway for system integration, Experimental Physics and Industrial Control System (EPICS) controls, transfer of knowledge from the partner labs to SLAC and the production and testing of 76 racks of LLRF equipment.

In this paper we present the design, simulation, and proof of principle results of an optimization based adaptive feed-forward algorithm for beam-loading compensation in a high impedance room temperature cavity. We begin with an overview of prior developments in beam loading compensation. Then we discuss different techniques for adaptive beam loading compensation and why the use of Newton's Method is of interest for this application. This is followed by simulation and initial experimental results of this method.

A new test stand dedicated to Superconducting Radiofrequency (SRF) cryomodule testing, CMTS1, has been commissioned and is now in operation at Fermilab. The first device to be cooled down and powered in this facility is the prototype 1.3 GHz cryomodule assembled at Fermilab for LCLS-II. We describe the demonstrated capabilities of CMTS1, report on steps taken during commissioning, provide an overview of first test results, and survey future plans.

LCLS-II 4 GeV linac consist of 35 (+5 spares) of 1.3 GHz cryomodules (CM) and three of 3.9 GHz CM’s including one spare. Fermilab responsible for the CM design and share responsibility with JLAB for module assembly and testing. CM production is almost on the middle of production stage. Paper will overview the performance of the CM’s tested at Fermilab, lessons learned and modifications in design to improve performance.

The Warm Front End (WFE) of the Proton Improvement Plan II Injector Test at Fermilab has been constructed to its full length. It includes a 15-mA DC, 30-keV H- ion source, a 2 m-long Low Energy Beam Transport (LEBT) with a switching dipole magnet, a 2.1 MeV CW RFQ, followed by a Medium Energy Beam Transport (MEBT) with various diagnostics and a dump. This report presents the commissioning status, focusing on beam measurements in the MEBT. In particular, a beam with the parameters required for injection into the Booster (5 mA, 0.55 ms macro-pulse at 20 Hz) was transported through the WFE.

For a free electron laser, the stability of injector is critical to the final electron beam parameters, e.g., beam energy, beam arrival time, and eventually it determines the photon quality. The LCLS-II project’s injector contains a VHF copper cavity as the gun and a two-cell L-band copper cavity as its buncher. The cavity designs are inherited from the APEX [1] design, but requires more field stability than demonstrated in APEX operation. The gun LLRF system design uses a connectorized RF front end and low noise digitizer, together with the same general purpose FPGA carrier board used in the LCLS-II SRF LLRF system. The buncher LLRF system directly adopts the SRF LLRF chassis design, but programs the controller to run the normal conducting cavities. In this paper, we describe the gun/buncher LLRF system design, including the hardware design, the firmware design and bench test.

The PIP-II accelerator is a proposed upgrade to the Fermilab accelerator complex that will replace the existing, 400 MeV room temperature LINAC with an 800 MeV superconducting LINAC. Part of this upgrade includes a new injection scheme into the booster that levies tight requirements on the LLRF control system for the cavities. In this paper we discuss the challenges of the PIP-II accelerator and the present status of the LLRF system for this project.

The LCLS-II is a CW superconducting linac under construction to drive an X-ray FEL. The energy and timing stability requirements of the FEL drive the need for very high precision RF control. This paper summarizes the design considerations and early demonstration of the performance of the modules and system we developed. INTRODUCTION LCLS-II is a project to generate high quality, high repetition rate soft X-ray beam for advanced science discovery. The project will construct a 4 GeV superconducting linac in the existing SLAC tunnel. The accelerated electrons will be sent through undulators to produce X-rays. LCLS-II requires electron beam jitter and energy spread better then 20 fs and 0.014% at the undulator to achieve its X-ray beam quality goals. That, in turn, requires 0.01◦ in phase and 0.01% amplitude stability for the RF field in each superconducting 1300 MHz cavity. [1] The superconducting linac will contain 35 cryomodules, each with eight 9-cell 1.3 GHz superconducting cavities. The machine layout is shown in Figure 1. CM01 CM02,03 CM04 CM15 CM16 CM35 BC1 E=250 MeV R56=-55 mm =1.6 % BC2 E=1.6 GeV R56=-37 mm =0.38 % GUN 750 keV LH E=100 MeV R56=-3.5 mm =0.05 % L0 = ** V0=100 MV Ipk=12 A z=1.02 mm L1 = 12.7 V0=211 MV Ipk = 12 A z=1.02 mm HL = 150 V0=64.7 MV L2 = 21 V0=1446 MV Ipk=80 A z=0.15 mm L3 = 10 V0=2437 MV Ipk=1.0 kA z=9.0 m BYP/LTU E=4.0 GeV R56 0 mm 0.014% 100-pC machine layout: Aug. 25, 2015; v21 ASTRA run, L3 10 deg. 3.9GHz Figure 1: LCLS-II Linac layout. A system design is a compromise of different parameters, including cost, robustness, noise etc. Series of project architectural choices made the high precision RF control possible for the machine. The low level RF collaboration team designed the low noise digital LLRF system to minimize the noise sources within the control bandwidth. Noise or disturbances outside the LLRF system’s control bandwidth must be either handled by a beam-based feedback system or be eliminated from the source. [2–4] ∗ Work supported by the LCLS-II Project and the U.S. Department of Energy, Contract DE-AC02-76SF00515 † ghuang@lbl.gov SYSTEM ARCHITECTURE SELECTION Digital Low Level RF Control System The low level RF control system measures the cavity pickup signal and compares it with the vector set point to generate an error signal. The error signal goes through a Proportional and Integral control loop and generates the correction to drive the high power RF system. With the development of ADC, DAC and FPGA technology, the system can be implemented digitally as shown in Figure 2. The flexibility and self-monitoring capability of a digital implementation is so advantageous that analog systems are no longer considered.

The unique properties of SRF cavities enable a new generation of X-ray light sources in XFEL and LCLS-II. The LCLS-II design calls for 280 L-band cavities to be operated in CW mode with a QL of 4 × 107, using Single-Source Single-Cavity control. The target RF field stability is 0.01% and 0.01◦ for the band above 1Hz. Hardware and software implementing a digital LLRF system has been constructed by a four-lab collaboration to minimize known contributors to cavity RF field fluctuation. Efforts include careful attachment to the phase reference line, and minimizing the effects of RF crosstalk by placing forward and reverse signals in chassis separate from the cavity measurement. A low-noise receiver/digitizer section will allow feedback to operate with high proportional gain without excessive noise being sent to the drive amplifier. Test results will show behavior on prototype cryomodules at FNAL and JLab, ahead of the 2018 final accelerator installation.

The PIP-II injector test radio frequency quadrupole (RFQ) arrived at Fermilab in the fall of 2015. The RFQ is a 162.5MHz Haccelerator with a nominal drive power of 100kW, which produces a bunched Hbeam at 2.1MeV. In this paper we discuss commissioning, operational performance, and improvements to the low level RF (LLRF) control system for the RFQ. We begin by describing the general system configuration and initial simulation results. We will then highlight temperature related issues in the high power RF system, which necessitate active control over the phase balance of the two amplifiers. Finally we demonstrate performance of the RF feedback and feed-forward compensation needed to meet specification during a 20-microsecond beam pulse.

The SLAC National Accelerator Laboratory is planning an upgrade (LCLS-II) to the Linear Coherent Light Source with a 4 GeV CW Superconducting Radio Frequency (SCRF) linac. The nature of the machine places stringent requirements in the Low-Level RF (LLRF) system, expected to control the cavity fields within 0.01◦ in phase and 0.01% in amplitude, which is equivalent to a longitudinal motion of the cavity structure in the nanometer range. This stability has been achieved in the past but never for hundreds of superconducting cavities in Continuous-Wave (CW) operation. The difficulty resides in providing the ability to reject disturbances from the cryomodule, which is incompletely known as it depends on the cryomodule structure itself (currently under development at JLab and Fermilab) and the harsh accelerator environment. Previous experience in the field and an extrapolation to the cavity design parameters (relatively high QL ≈ 4 × 107, implying a half-bandwidth of around 16 Hz) suggest the use of strong RF feedback to reject the projected noise disturbances, which in turn demands careful engineering of the entire system. The project has passed the conceptual design phase and is approaching the final design phase. LLRF prototypes are currently under construction and expected to be exercised in a cryomodule test stand at both JLab and Fermilab (using the LCLS-II TESLA-ILC type 1.3 GHz cavities) before the end of 2016 to validate the engineering design presented here. INTRODUCTION LCLS-II [1] adds a 4 GeV L-band (1.3 GHz) superconducting linac to the first 700 meters of the SLAC tunnel, increasing the repetition rate of LCLS from 120 Hz to 1 MHz (operating in CW) and an average beam current of 100μA. The SCRF linac includes 35 ILC-style cryomodules (eight 9-cell cavities each) for a total of 280 cavities. LCLS-II will become the world’s first X-ray Free Electron Laser (FEL) providing CW pulses, probing the motion of molecules, atoms and electrons on their natural time scales (femtoseconds and faster) enabling a broad range of new energy science. Accelerating cavity RF field stability affects the final FEL performance through its relationship with the longitudinal beam dynamics, namely on final beam energy, peak current and arrival time [2]. Energy stability in the order of 10−5 converts to different RF stability requirements in each Linac section; 0.01◦ in phase and 0.01% in amplitude represents ∗ This work was supported by the LCLS-II Project and the U.S. Department of Energy, Contract DE-AC02-76SF00515 † CSerrano@lbl.gov Figure 1: Noise requirements vs. zero-dB crossing. the tightest requirement, and that for which the LLRF system is engineered. The baseline LCLS-II project does not include fast beambased feedback, thus the RF stability requirement has to be satisfied by the LLRF system alone for noise integrated above 1 Hz, with beam currents up to 100 μA and a bunch repetition rate of up to 1 MHz. Enough RF power is made available by the 3.8 kW SSAs to operate the cavities at a nominal 16 MV/m, with RF budget allocated to beam loading and RF corrections (including ±10 Hz peak cavity detuning due to microphonics). The LLRF system design [3] is centered around a Precision Receiver Chassis (PRC) providing low-noise detection of the cavity probe signal, an RF Station (RFS) chassis implementing digital RF control in an FPGA, and two other chassis dedicated to cavity resonance control (driving fast piezos and slow stepper motors) and interlocks. FIELD CONTROL CONSIDERATIONS The very narrow bandwidth of the cavity makes it very sensitive to microphonics, where nanometers of mechanical deformation translates into tens of Hz of detuning. The nature of the disturbance (external noise sources, mechanical resonances, couplings, etc.) depends on the cryomodule structure itself and the accelerator environment, which are both imperfectly known at this stage of the design. Microphonics rejection typically requires a control bandwidth in the tens of kHz range, where the optimal controller settings will be measured in the accelerator environment. The combination of high sensitivity to microphonics and the tight field stability requirements makes it necessary for the LLRF design to support high feedback gain. Proceedings of IPAC2016, Busan, Korea WEPOR042 06 Beam Instrumentation, Controls, Feedback and Operational Aspects T27 Low Level RF ISBN 978-3-95450-147-2 2765 C op yr ig ht © 20 16 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s FERMILAB-CONF-16-320-AD Operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy Figure 2: Block diagram of the LCLS-II LLRF design, including controls for two 1.3-GHz superconducting cavities. The above considerations form the basis for the engineering of the LLRF system and present different trade-offs. Increasing the open-loop zero-dB crossing helps rejecting cavity disturbances, but (as an example) achieving a 20 kHz open-loop zero-dB crossing with a 16 Hz cavity bandwidth implies a proportional gain of about 1200. Any noise in the cavity field measurement chain will get amplified by the same factor and the amplified noise signal will be transferred directly into the input of the SSA. This places the critical point of the design in building a quiet sensor for the cavity field. Figure 1 shows the relationship between the noise tolerance in the cavity field detection (down-conversion, digitizing, etc.) in units of dBc/Hz and the choice of open-loop zero-dB crossing (and therefore proportional gain). The assumption is a 4% RMS (amplitude) allowed SSA drive noise, 1 μs hardware delay, 16 Hz cavity bandwidth, and 1/12T stability guideline. SYSTEM ENGINEERING DESIGN In order to build a quiet sensor, the cavity probe signals are downconverted and digitized in a separate, temperaturestabilized chassis (the Precision Receiver Chassis, PRC) described in detail later. The PRC has an FPGA board that acquires the digitized cavity signals and sends them over a digital optical-fiber link to the RF Station, implementing the feedback algorithm. Figure 2 shows a block diagram of the LLRF design where 4 out of the 8 cavities in a cryomodule are represented along with the controls for two cavities through one penetration. Note the physical separation of the forward and reflected signals from the cavity probe, responding to the engineering needs discussed above. PROTOTYPE CHARACTERIZATION Prototypes for all the components in the LCLS-II LLRF system [3] have been built and tests with the LCLS-II cryomodules are expected before the end of 2016. Figure 3 shows a block diagram of the characterization exercise for the up and downconverters (designed at Fermilab) and the digitizer boards (designed at LBNL). The FPGA board is the common platform used in the RF stations, precision receivers, interlocks and resonance control chassis illustrated in Figure 2. A 1320 MHz local oscillator provides a frequency reference as in the production system. A 203/264 (non-binary) NCO drives the DAC in the digitizer board at a 188.6 MHz clock (derived from the 1320 MHz LO with a divide-by-7). This produces 145 MHz, which is mixed up to 1300 MHz and then back down to 20 MHz (the Intermediate Frequency, IF). The ADCs receives 20 MHz with a clock of 94.3 MHz (derived using a divide-by-14 output of the same divider chip). Note that 20 MHz = 7/33 · the ADC clock. The use of two independent ADC channels allows for computing correlations and separating the noise contributions form the DAC + upconverter chain from the downconverter + ADC chain as shown in Figure 3 [4]. If we call the ADC signals x and y respectively, and we convert the time-series signals to frequency domain, we can compute the autocorWEPOR042 Proceedings of IPAC2016, Busan, Korea ISBN 978-3-95450-147-2 2766 C op yr ig ht © 20 16 C C -B Y3. 0 an d by th e re sp ec tiv e au th or s 06 Beam Instrumentation, Controls, Feedback and Operational Aspects

The SLAC National Accelerator Laboratory is planning an upgrade (LCLS-II) to the Linear Coherent Light Source with a 4 GeV CW Superconducting Radio Frequency (SCRF) linac. The nature of the machine places stringent requirements in the Low-Level RF (LLRF) system, expected to control the cavity fields within 0.01 degrees in phase and 0.01% in amplitude, which is equivalent to a longitudinal motion of the cavity structure in the nanometer range. This stability has been achieved in the past but never for hundreds of superconducting cavities in Continuous-Wave (CW) operation. The difficulty resides in providing the ability to reject disturbances from the cryomodule, which is incompletely known as it depends on the cryomodule structure itself (currently under development at JLab and Fermilab) and the harsh accelerator environment. Previous experience in the field and an extrapolation to the cavity design parameters (relatively high Q_{L}c≈ 4×10⁷ , implying a half-bandwidth of around 16 Hz) suggest the use of strong RF feedback to reject the projected noise disturbances, which in turn demands careful engineering of the entire system.

Fermilab is currently building the Project X Injector experiment (PXIE). The PXIE linac will accelerate a 1 mA H− beam up to 30 MeV and serve as a testbed for validation of Project X concepts and mitigation of technical risks. A cryomodule of eight superconducting RF Single Spoke Resonators of type 1 (SSR1) cavities operating at 325 MHz is an integral part of PXIE. Ten SSR1 cavities were manufactured in industry and delivered to Fermilab. We discuss tests of nine bare SSR1 cavities at the Fermilab Vertical Test Stand (VTS). Recently, one of the SSR1 cavities was welded inside a helium jacket. Results of the test of this cavity in the Fermilab Spoke Test Cryostat (STC) are shown. We report on the measured performance parameters of SSR1 cavities achieved during the tests.

The technique presented in this paper enables the regulation of both radio frequency amplitude and phase in narrow band devices such as a Superconducting RF (SRF) cavity driven by constant power output devices i.e. magnetrons [1]. The ability to use low cost high efficiency magnetrons for accelerator RF power systems, with tight vector regulation, presents a substantial cost savings in both construction and operating costs - compared to current RF power system technology. An operating CW system at 2.45 GHz has been experimentally developed. Vector control of an injection locked magnetron has been extensively tested and characterized with a SRF cavity as the load. Amplitude dynamic range of 30 dB, amplitude stability of 0.3% r.m.s, and phase stability of 0.26 degrees r.m.s. has been demonstrated.