3D - Low Level RF

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WE105 RF Control of High QL Superconducting Cavities 704
  • C. Hovater
    JLAB, Newport News, Virginia

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
In the last 20 years the requirements for rf control has increased as the target use has broadened from electron/ion accelerators for Nuclear and Particle Physics to light sources such as Free Electron Lasers. The increasing requirement of cavity field control to meet the spectral and jitter performance specifications for light sources has led system designers to a more rigorous approach in designing the rf controls. Design attention must be applied not only to the hardware and control algorithms but also to the overall accelerating system to meet performance and cost requirements. As an example, cavity QL in Energy Recovery Linacs (ERL) must be optimized such that the rf controls can accommodate the lowest possible rf power given the background cavity microphonics. This paper presents the status and future directions of high QL superconducting rf control systems.


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THP041 Analysis of Electronic Damping of Microphonics in Superconducting Cavities 876
  • J.R. Delayen
    JLAB, Newport News, Virginia
  • S.U. De Silva
    ODU, Norfolk, Virginia

Funding: Supported by US DOE Contract No. DE-AC05-06OR23177
In low current applications superconducting cavities have a high susceptibility to microphonics induced by external vibrations and pressure fluctuations. Due to the narrow bandwidth of the cavities, the amount of rf power required to stabilize the phase and amplitude of the cavity field is dictated by the amount of microphonics that need to be compensated. Electronic damping of microphonics is investigated as a method to reduce the level of microphonics and of the amount of rf power required. The current work presents a detailed analysis of electronic damping and of the residual cavity field amplitude and phase errors due to the fluctuations of cavity frequency and beam current.

THP100 Self Tuning Regulator for ISAC 2 Superconducting RF Cavity Tuner Control 1024
  • K. Fong, M.P. Laverty, Q. Zheng
    TRIUMF, Vancouver

The ISAC 2 superconducting rf cavities use self-excited, phase-locked mode of operation. As such the microphonics are sensitive to the alignment of the phase control loop. Although initial alignments can minimize the effect of microphonics, long term drifts, particularly in the power amplifiers, can cause the control loop to misalign and an increase in sensitivity to microphonics. The ISAC 2 control system monitors several points in the control loop to determine the phase alignment of the power amplifiers as well as the rf resonant cavities. Online adaptive feedbacks using Self Tuning Regulators are employed to bring the different components back into alignment.

THP101 AM-PM Conversion Induced Instability in I/Q Feedback Control Loop 1027
  • K. Fong, M.P. Laverty, Q. Zheng
    TRIUMF, Vancouver

Most rf feedback control systems today uses the I/Q demodulation and modulation scheme because of its simplicity. Its performance, however, depends on the alignment of the feedback loops. If the loop contains elements that have a high AM-PM conversion such as a class C amplifier, then the misalignment is dynamic and power dependent. In the extreme case the I/Q loops can become unstable and the system settled into a limit-cycle oscillation.

THP102 Evaluation of Fast ADCs for Direct Sampling RF Field Detection for the European XFEL and ILC 1030
  • Z. Geng, S. Simrock
    DESY, Hamburg

For the LLRF system of superconducting linacs, precision measurements of the rf phase and amplitude are critical for the achievable field stability. In this paper, a fast ADC (ADS5474) has been evaluated for the measurement of a 1.3 GHz rf signal directly without frequency down conversion. The ADC clock frequency is synchronized with the rf frequency and chosen for non-IQ demodulation. In the laboratory, the Signal to Noise Ratio (SNR) of the ADC was studied for different clock and rf input levels, and the temperature sensitivity of the ADC has been determined. A full bandwidth phase jitter of 0.2 degree (RMS) and amplitude jitter of 0.32% (RMS) was measured. For field control of superconducting cavities with a closed loop bandwidth up to 100 KHz, one can expect to achieve a phase stability close to 0.01 degree. The main limitation will be the jitter of the external clock. We present a measurements at the cavities at FLASH and compare the result with the existing system.

THP103 LLRF System Requirement Engineering for the European XFEL 1033
  • S. Simrock, G. Ayvazyan, Z. Geng, M.K. Grecki
    DESY, Hamburg
  • B. Aminov
    CRE, Wuppertal

The LLRF system of the European XFEL must fulfill the requirements of various stakeholders: Photon beam users, accelerator operators, rf experts, controls system, beam diagnostics and many others. Besides stabilizing the accelerating fields the system must be easy to operate, to maintain, and to upgrade. Furthermore it must guarantee high availability and it must be well understood. The development, construction, commissioning and operation with an international team requires excellent documentation of the requirements, designs and acceptance test. For the rf control system of the XFEL the new system modeling language SySML has been chosen to facilitate the system engineering and to document the system. SysML uses 9 diagram types to describe the structure and behavior of the system. The hierarchy of the diagrams allows individual task managers to develop detailed subsystem descriptions in a consistent framework. We present the description of functional and non-functional requirements, the system design and the test cases. An attempt of costing the software effort based on the use case point analysis is also presented.

THP104 Low Level RF and Timing System for XFEL/SPring-8 1036
  • T. Ohshima, N. Hosoda, H. Maesaka, Y. Otake
    RIKEN/SPring-8, Hyogo
  • M. Musha
    University of electro-communications, Tokyo
  • K. Tamasaku
    RIKEN Spring-8 Harima, Hyogo

Requirement on a Low Level rf (LLRF) system is very tight and allowable jitter is less than several tens femto seconds for the XFEL/SPring-8. To satisfy this requirement, we have developed special components; a low-noise master oscillator, a high precision IQ modulator/demodulator, a high speed DAC/ADC, and a delayed pulse generator with 700 fs jitter to a 5712 MHz reference clock. These components were installed in the SCSS test accelerator and their performance was checked. The standard deviations of the phase and amplitude were less than 0.02 degree and 0.03% for a 238 MHz SHB acceleration cavity. Measured rms jitter of the beam arrival time relative to the reference rf signal was 50 fs, which demonstrated the high performance of the total LLRF system. For the XFEL, the length of reference signal transmission line is long, about 1 km. Therefore an optical system is adopted because of low transmission loss and an ability to keep precise time accuracy using fiber length control, which has 0.2 um/sqrt(Hz) noise floor. Achieved performance of the LLRF and timing system, and development status on the optical transmission system will be presented in this paper.


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THP105 LLRF Control System of the J-PARC LINAC 1039
  • Z. Fang, S. Anami, S. Michizono, S. Yamaguchi
    KEK, Ibaraki
  • T. Kobayashi
    JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
  • H. Suzuki
    JAEA, Ibaraki-ken

At the J-PARC 181 MeV proton linac, the rf sources consist of 4 solid-state amplifiers and 20 klystrons with operation frequency of 324 MHz. The rf fields of each rf source are controlled by a digital feedback system installed in a compact PCI (cPCI). A very good stability of the accelerating fields has been successfully achieved about ±0.2% in amplitude and ±0.2 degree in phase, much better than the requirements of ±1% in amplitude and ±1 degree in phase. Besides, the tuning of each accelerator cavity including 3 DTL and 15 SDTL is also controlled by this LLRF system through a cavity tuner. We pre-defined the cavity resonance states with the tuner adjusted to obtain a flat phase during the cavity field decay. The cavity auto-tuning is well controlled to keep the phase of rf fields within ±1 degree. Furthermore, from the amplitude waveform during the cavity field decay, the Q-value of each cavity is calculated in real-time and displayed in the PLC TP of the LLRF control system.

THP106 High Speed Data Acquisition System Using FPGA for LLRF Measurement and Control 1042
  • H. Katagiri, S. Fukuda, T. Matsumoto, S. Michizono, T. Miura, Y. Yano, M. Yoshida
    KEK, Ibaraki

Recently, FPGA technology is widely used for the accelerator control owing to its fast digital processing. We have been developing several applications for LLRF control and measurement using commercial and custom-made FPGA board. XtremeDSP(the commercial FPGA board equipped two ADCs and two DACs) is mainly used for the performance evaluation of STF(Superconducting RF Test Facility) LLRF. Installing the custom-made FPGA board equipped with ten ADCs and two DACs is considering for up-grade of the rf driver and rf monitoring system in the injector linac. Development of the high-speed data acquisition system that combines commercial FPGA board ML555 and FastADC(ADS5474 14bit, 400MS/s) is carried out. Result of those data acquisition systems will be summarized.

THP107 Performance of Digital Low-Level RF Control System with Four Intermediate Frequencies 1045
  • T. Matsumoto, S. Fukuda, H. Katagiri, S. Michizono, T. Miura, Y. Yano
    KEK, Ibaraki

In a superconducting accelerator, an FPGA/DSP-based low-level rf (LLRF) system with feedback control is adopted to satisfy the requirement of stability in the accelerating field. An rf probe signal picked up from cavity is down-converted to an intermediate frequency and sampled by an analog-to-digital converter (ADC) in the digital LLRF control system. In order to decrease the number of the ADCs required for vector sum feedback operation, a digital LLRF control system using different intermediate frequencies has been developed. At STF (Superconducting RF Test Facility) in KEK, the digital LLRF system with four intermediate frequencies was operated and the rf field stability under the feedback operation was estimated using a superconducting cavity. The result of the performance will be reported.

THP108 Performance of Digital LLRF System for STF in KEK 1048
  • S. Michizono, S. Fukuda, H. Katagiri, T. Matsumoto, T. Miura, Y. Yano
    KEK, Ibaraki

RF operation has started at the STF (Superconducting RF Test Facility) in KEK. The digital feedback system, which consists of one FPGA, ten 16-bit ADCs and two 14-bit DACs, was installed in order to satisfy the rf-field regulation requirements of 0.3% rms and 0.3 deg.rms in phase. The rf field stability under various feedback parameters are presented. Various studies were also carried out such as cavity detuning measurements (microphonics, quench detection, etc.). These results will also be summarized.

THP109 Measurements of Feedback-Instability Due to 8/9π and 7/9π Modes at KEK-STF 1051
  • T. Miura, S. Fukuda, H. Katagiri, T. Matsumoto, S. Michizono, Y. Yano
    KEK, Ibaraki

In the superconducting rf test facility (STF) at KEK, high power tests of the nine-cell superconducting cavity for the international linear collider (ILC) have been performed. Although the cavity was operated in π-mode, the feedback instability due to 8/9π and 7/9π modes was observed in the STF. The intensities of 8/9π and 7/9π modes were measured by changing the feedback loop-delay and stable/unstable region appeared periodically as expected.

THP110 Pulse-by-Pulse Switching of Beam Loading Compensation in J-PARC Linac RF Control 1054
  • T. Kobayashi
    JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
  • S. Anami, Z. Fang, S. Michizono, S. Yamaguchi
    KEK, Ibaraki
  • E. Chishiro, H. Suzuki
    JAEA, Ibaraki-ken

For the J-PARC linac low level rf system, in order to compensate beam-loading change by pulses in the operation of 25 Hz repetition, a function that switches the feed-forward control parameters in every pulse were installed into the digital accelerating-field control system. The linac provides a 50 mA peak current proton beam to a 3 GeV rapid-cycling synchrotron (RCS). Then the RCS distributes the 3-GeV beam into a following 50 GeV synchrotron (main ring, MR) and the Materials and Life Science Facility (MLF), which is one of the experimental facilities in the J-PARC. The 500-us long macro pulses from the ion source of the linac should be chopped into medium pulses for injection into the RCS. The duty (width or repetition) of the medium pulse depends on which facility the RCS provides the beam to the MR or MLF. Therefore the beam loading compensation needs to be corrected for the change of the medium pulse duty in the 25 Hz operation.

THP111 LLRF Control System Using a Commercial Board 1057
  • H.S. Kim, Y.-S. Cho, H.-J. Kwon, K.T. Seol
    KAERI, Daejon

The requirements for the field amplitude and phase stability of the PEFP linac are 1% and 1 degree, respectively. To achieve the requirements, a digital LLRF control system has been developed using a commercial digital board for general purpose(FPGA). The feedback with PI control and feedforward are implemented in the FPGA. The LLRF control systems are currently used for the linac test. In this paper, test results and discussion on the advantage and disadvantage of the LLRF system based on a commercial board are presented.

THP112 Numerical Simulation of the INR DTL A/P Control System 1060
  • A.I. Kvasha
    RAS/INR, Moscow

Stabilization of amplitude and phase in linear accelerator cavities can be realized by means of control systems, operating both in polar (A/P) and rectangular (I/Q) coordinates. In analyzing of linear control systems, as a rule, transfer functions are used, which, in turn, are the symbolic representation of the linear differential equation, connecting the input and output variables. It's well known that generally in A/P coordinate it is impossible to get two separate linear differential equations for amplitude and phase of rf voltage in a cavity except for estimating of the control system stability in the small near steady state values of variables. Nevertheless, there is a possibility of numerical simulation of nonlinear A/P control system using up-to-date programs. Some results of the simulation are presented.

THP113 Optimal Coupler and Power Settings for Superconductive Linear Accelerators 1063
  • J. Branlard, B. Chase, S. Nagaitsev, O.A. Nezhevenko, J. Reid
    Fermilab, Batavia

Funding: FRA
In this paper we present a model for the rf power distribution to multiple super-conductive cavities from a single klystron. The goal of this model is to find a distribution scheme in which the cavities are operated as close to their quench limit as possible. The approach presented in this work consists of setting all cavities to the same QL value by adjusting the power coupler, and optimizing the power (Pk) distribution individually to each cavity to maximize the vector sum voltage. The proposed approach yields an operating gradient very close to the theoretical limit and offers a great operational benefit as the gradient stability is conserved for any beam current.

C. Nantista, K.L.F. Bane, C. Adolphsen, RF Distribution Optimization in
the Main Linacs of the ILC. Proceedings of PAC07, Albuquerque,
New Mexico, USA.

THP114 New LLRF System for Fermilab 201.25 MHz Linac 1066
  • T.A. Butler, L.J. Allen, J. Branlard, B. Chase, E. Cullerton, P.W. Joireman, M.J. Kucera, V. Tupikov, P. Varghese
    Fermilab, Batavia

The Fermilab Proton Plan, tasked to increase the intensity and reliability of the Proton Source, has identified the Low Level RF (LLRF) system as the critical component to be upgraded in the Linac. The current 201.25 MHz Drift Tube Linac LLRF system was designed and built over 35 years ago and does not meet the higher beam quality requirements under the new Proton Plan. A new VXI based LLRF system has been designed to improve cavity vector regulation and reduce beam losses. The upgrade includes an adaptive feedforward system for beam loading compensation, a new phase feedback system, and a digital phase comparator for cavity tuning. The new LLRF system is phase locked to a temperature stabilized 805 MHz reference line, currently used as frequency standard in the higher energy accelerating section of the Linac. This paper will address the current status of the project, present the advancements in both amplitude and phase stability over the old LLRF system, and discuss commissioning plans.

THP115 Optimizing Cavity Gradients in Pulsed Linacs Using the Cavity Transient Response 1069
  • G.I. Cancelo, A. Vignoni
    Fermilab, Batavia

Pulsed Linac accelerators are being designed powering a string of cavities from one klystron. A typical low level rf control loop controls the amplitude and the phase of the klystron's rf power; however, the loop cannot dynamically control individual cavity amplitude and phases. The problem is further complicated by the need to obtain the maximum possible acceleration from the rf unit. Proton Linacs (HINS, ProjectX) add extra complexity. A rf unit may need cavities operating at different synchronous phases. Particles travel cavities at increasing velocities, which implies different beam loading conditions. For pulsed proton Linacs amplitude and phase stability are crucial for beam stability. The usual steady state approach determines optimality conditions for minimum generator power as a function of rf parameters. This approach does not provide constant amplitude and phases when the beam is on. In this paper we propose a novel theory using the cavity transient response. The transient response allows setting flat cavity gradients (A and phi) for each cavity in the unit. The optimized rf parameters for the transient response are the cavity coupling parameter and cavity tuning angle.

THP116 Real Time RF Simulator (RTS) and Control 1072
  • G.I. Cancelo, K.R. Treptow, A. Vignoni, T.J. Zmuda
    Fermilab, Batavia
  • C. Armiento
    University of Pisa and INFN, Pisa

A multi cavity real time rf simulator and PID control has been implemented on a Xilinx Virtex-4 FPGA. The rf simulator simulates an entire rf unit with up to 4 cavities connected to a single simulated klystron. Each cavity is allowed to have its own set of parameters, set point gradients, synchronous phases, and beam loadings. The simulator is built based on an interdependent electrical and mechanical model of a cavity. The electrical model is a 1st order differential equation in the complex phase space. The mechanical model is a 2nd order differential equation of the Lorentz force detuning on the cavities. Other spurious effects as microphonics and noises can be added using an external source or a memory table. The simulator has been optimized for size and utilizes only one Xilinx DSP block per cavity. A typical Virtex-4 has of the order of 100 DSP blocks. The simulator bandwidth is 1MHz which is plenty for niobium type superconducting cavities which have a loaded Q of about 3 million and a half bandwidth of about 250 Hz. The Real Time simulator is currently running on hardware comprised by an ESECON LLRF controller* and a Linux based VME processor.

*ESECON, 14 channel LLRF controller, Low Level Radio Frequency Workshop (LLRF07), Knoxville, Tennessee, October 22-25, 2007, presentation 031.

THP117 Design and Evaluation of the Low-Level RF Electronics for the ILC Main LINAC 1075
  • U. Mavric, B. Barnes, J. Branlard, B. Chase, D.W. Klepec, V. Tupikov
    Fermilab, Batavia

Funding: Work supported by Fermi Research Alliance LLC. Under DE-AC02- 07CH11359 with the U.S. DOE
The proposed 30 km long ILC electron/positron collider is pushing the limits not only in basic physics research but also in engineering. For the two main Linacs, the pulsed rf power that is feeding the high number of SCRF cavities (~17,000) must to be regulated to app. 0.1% for amplitude and 0.2 deg for phase. The regulation of phase and amplitude is carried out by the analog/digital electronics also denoted as the low-level rf control system. Besides meeting the regulation specifications, the low-level rf must be reliable, robust and low cost. In the paper we present a possible hardware solution that addresses these issues. We also reveal the main design strategies that allowed us meeting the conflicting demands of the system. The system is evaluated on a cavity emulator implemented on the FPGA, which shows that system performance is within the specifications. Finally, we discuss the obtained results and give some suggestions for future work.

THP118 A Femtosecond-Level Fiber-Optics Timing Distribution System Using Frequency-Offset Interferometry 1078
  • J.W. Staples, J.M. Byrd, L.R. Doolittle, G. Huang, R.B. Wilcox
    LBNL, Berkeley, California

Funding: This work was supported by the Office of Science, U. S. Department of Energy, under Contract No. DE-AC02-05CH11231.
A fiber-based frequency and timing distribution system based on the principle of heterodyne interferometry has been in development at LBNL for several years. The temporal fiber drift corrector has evolved from an rf-based to an optical-base system, from mechanical correctors (piezo and optical trombone) to fully electronic, and the electronics from analog to fully digital, all using inexpensive commodity fiber components. Short-term optical phase jitter and long-term phase drift are both in the femtosecond range over distribution paths of 2 km and more. The temperature dependence of group and phase velocity correction is measured and applied. We will discuss the results of field tests, integration into various client subsystems and further applications.


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