Konrad Przygoda

A Novel Approach for Hardware Implementation of a Detuning Compensation Control System for SC Cavities

Superconducting (SC) resonant cavities seems to be an attractive option for various linear accelerators under construction. The nine-cell 1.3 GHz cavities have demonstrated gradients up to 38 MV/m. They are susceptible to small changes of dimension caused by mechanical vibrations, cooling systems, human activity (microphonics) and high gradients of RF field (Lorentz forces) as well. FPGA-based control system for cavity detuning compensation is presented.

Low-Latency Implementation of Coordinate Conversion in Virtex II pro FPGA

The paper presents a low-latency implementation of Cartesian-polar coordinate conversion in a Virtex II pro FPGA. The accuracy and resource consumption of the module is comparable to the one obtained with the Xilinx CORDIC IP Core, but the latency has been reduced to 65%. The application of the conversion module to the cavity detuning computation in low-level radio frequency control system for a FLASH accelerator has been also presented.

USB-based Background Mode Debugger For Freescale Processors

The application of an in-circuit emulator (ICE) also called on-circuit debugger (OCD) or background debug mode (BDM) can simplify arduous debugging process for embedded systems. Hardware debuggers allow to use an external computer for debugging, enable single stepping, breakpoints, and other resources provided by desktop computer programmers. The application of such circuits requisites high speed data rate transfers. Efficient communication protocols and USB-based devices can be easily implemented to improve advantages of using BDM interfaces. This paper presents an application of upgradable approach to implement USB-based debugger for Freescale microcomputers consisted of USB and field programmable gate array (FPGA) devices. Moreover the device can properly operate under most popular operating systems like UNIX or MS Windows

Real time control of RF fields using a MicroTCA.4 based LLRF system at FLASH

The Free Electron Laser in Hamburg (FLASH) is a large scale user facility, providing highly stable and brilliant laser pulses down to a wavelength of 4.1 nm. Essential for stable and reproducible photon beam is the precision control of the electron bunch parameters. The acceleration principle of the electron bunches at FLASH is based on superconducting RF technology (SRF), in which the RF fields are controlled by a digital low level RF (LLRF) system. This system has been recently upgraded to the Micro Telecommunication Computing Architecture (MicroTCA.4) to improve the performance of the field regulation. This paper presents the first measurements and operation experiences using this new electronic crate standard at a large scale research facility. RF field regulation is carried out by real time fast digital processing on several boards in a MicroTCA.4 crate, and slow automation routines running on a dual core i7 front-end CPU. Scalability and modularity of this system is one of the key parameters to meet the next steps, namely being the platform standard for the European X-ray free electron laser currently build at DESY.

Prototype Control System for Compensation of Superconducting Cavities Detuning Using Piezoelectric Actuators

Pulsed operation of high gradient superconducting radio frequency (SCRF) cavities results in dynamic Lorentz force detuning (LFD) approaching or exceeding the bandwidth of the cavity of order of a few hundreds of Hz. The resulting modulation of the resonance frequency of the cavity is leading to a perturbation of the amplitude and phase of the accelerating field, which can be controlled only at the expense of RF power. Presently, at various labs, a piezoelectric fast tuner based on an active compensation scheme for the resonance frequency control of the cavity is under study. The tests already performed in the Free Electron Laser in Hamburg (FLASH), proved the possibility of Lorentz force detuning compensation by the means of the piezo element excited with the single period of sine wave prior to the RF pulse. The X-Ray Free Electron Laser (X-FEL) accelerator, which is now under development in Deutsche Elektronen-Synchrotron (DESY), will consists of around 800 cavities with a fast tuner fixture including the actuator/sensor configuration. Therefore, it is necessary to design a distributed control system which would be able to supervise around 25 RF stations, each one comprised of 32 cavities. The Advanced Telecomunications Computing Architecture (ATCA) was chosen to design, develop, and build a Low Level Radio Frequency (LLRF) controller for X-FEL. The prototype control system for Lorentz force detuning compensation was designed and developed. The control applications applied in the system were fitted to the main framework of interfaces and communication protocols proposed for the ATCA-based LLRF control system. The paper presents the general view of a designed control system and shows the first experimental results from the tests carried out in FLASH facility. Moreover, the possibilities for integration of the piezo control system to the ATCA standards are discussed.

MicroTCA.4-Based RF and Laser Cavities Regulation Including Piezocontrols

In this paper, we present an universal solution for radio frequency (RF) and laser cavities regulation, including piezocontrols and drivers based on MicroTCA.4 electronics. The control electronics consists of an Analog to Digital Converter-advanced mezzanine card (AMC) with an analog rear transmission module (RTM) to downmix and measure the RF signals and a low cost AMC-based Field Programmable Gate Array mezzanine card carrier for fast data processing and digital feedback operation connected to an RTM piezodriver. For the RF cavity regulation, the piezodriver includes additional inputs to use piezoelements as active force sensors. The fine tuning of the laser is carried out using a cavity fiber stretcher. The coarse tuning of the supported optics is done using a piezomotor-driven linear stage. Both channels can be operated using digital feedback controllers. First, results from continuous wave operation of the RF field controller and the cavity active resonance control with the piezotuners are demonstrated. The laser lock application performance using both fine and coarse channel feedbacks is shown and briefly discussed.

Experience with Single Cavity and Piezo Controls for Short, Long Pulse and CW Operation

We present a compact RF control system for superconducting radio frequency (SCRF) single cavities based on MicroTCA.4 equipped with specialized advanced mezzanine cards (AMCs) and rear transition modules (RTMs). To sense the RF signals from the cavity and to drive the high power source, a DRTM-DWC8VM1 module is used equipped with 8 analog field detectors and one RF vector modulator. Fast cavity frequency tuning is achieved by piezo-actuators attached to the cavity and a RTM piezo-driver module (DRTMPZT4). Data processing of the RF signals and the real-time control algorithms are implemented on a Virtex-6 and a Spartan-6 FPGAs within two AMC’s (SIS8300-L2V2 and DAMC-FMC20). The compact single cavity control system was tested at Cryo Module Test Bench (CMTB) at DESY. Software and firmware were developed to support all possible modes, the short pulse (SP), the long pulse (LP) and CW operation mode with duty cycles ranging from 1% to 100%. The SP mode used a high power multi-beam klystron at high loaded quality factor (QL) of 3 · 106. For the LP mode (up to 50% duty cycle) and the CW mode a 120 kW IOT tube was used at QL up to 1.5 · 107. Within this paper we present the achieved performance and report on the operation experience on such system.

FPGA based RF and piezo controllers for SRF cavities in CW mode

Modern digital low level radio frequency (LLRF) control systems used to stabilize the accelerating field in facilities such as Free Electron Laser in Hamburg (FLASH) or European X-Ray Free Electron Laser (E-XFEL) are based on the Field Programmable Gate Array (FPGA) technology. Presently these accelerator facilities are operated with pulsed RF. In future, these facilities should be operated with continuous wave (CW) which requires significant modifications on the real-time feedbacks realized within the FPGA. For example, higher loaded quality factor of the cavities when operated in a CW mode requires sophisticated resonance control methods. However, iterative learning techniques widely used for machines operated in pulsed mode are not applicable for CW. In addition, the mechanical characteristic of the cavities have now a much more important impact on the choice of the feedback scheme. To overcome the limitations of classical PI-controllers novel realtime adaptive feed forward algorithm is implemented in the FPGA. Also, the high power RF amplifier which is an inductive output tube (IOT) for continuous wave operation instead of a klystron for the pulsed mode has major impact on the design and implementation of the firmware for regulation. In this paper, we report on our successful approach to control multi-cavities with ultra-high precision (dA/A<;0.01%, dphi<;0.02 deg) using a single IOT source and individual resonance control through piezo actuators. Performance measurements of the proposed solution were conducted at Cryo Module Test Bench (CMTB) facility.

MicroTCA.4 based Single Cavity Regulation including Piezo Controls

We want to summarize the single cavity regulation with MTCA.4 electronics. Presented solution is based on the one MTCA.4 crate integrating both RF field control and piezo tuner control systems. The RF field control electronics consists of RTM for cavity probes sensing and high voltage power source driving, AMC for fast data processing and digital feedback operation. The piezo control system has been setup with high voltage RTM piezo driver and low cost AMC based FMC carrier. The communication between both control systems is performed using low latency link over the AMC backplane with data throughput up to the 3.125 Gbps. First results from CW operation of the RF field controller and the cavity active resonance control with the piezo tuners are demonstrated and briefly discussed. INTRODUCTION The 1.3 GHz superconducting radio frequency (SCRF) cavities of modern linear accelerators like FLASH and European X-Ray Free Electron Linac (XFEL) are operated in short pulse (SP) mode with 1300 μs RF-pulse and repetition rate up to 10 Hz at high loaded quality factor (QL) above 3·10. During SP operation of the cavity, the 650 μs of RF-pulse can be efficiently used to accelerate up to 27.000 number of bunches per second (averaged over 10 successive RF pulses) with minimum bunch spacing of 222 ns and maximum charge per bunch of 1 nC. Since the bandwidth of the cavity resonator operated in SP mode is 433 Hz for FLASH and 283 Hz for XFEL (QL=4.6·10) with nominal operating gradient of 23.6 MV/m, the dominating effect of the RF field disturbance is Lorentz force detuning (LFD). As LFD is repetitive from pulse to pulse, adaptive feedforward methods for active compensation using piezo tuners can be applied [1]. For the continuous wave (CW) mode of operation of SCRF cavity at quality factor of more than 1.5·10 (5 times less bandwidth), the unpredictable microphonics becomes the main RF field disturbance source. In order to achieve stable acceleration of 100.000 number of bunches per second with nominal operating gradient of 7 MV/m (CW operation scenario for XFEL machine), the RF field stability requirements better than 0.01% for the amplitude and 0.01 degrees for the phase are the real challenge. Therefore, new control algorithms need to be developed and evaluated for the real environment conditions. Nowadays higher numbers of high energy research centers are switching from multi cavity (MC) to single cavity approach (SCA) operation. The SCA solution is giving a possibility of establishing in a short time a small facilities where the high current and low emittance (below 1 mm x mrad) CW electron beam at 2 ps rms bunch duration are the main goals for the experiment, i.e. Berlin Energy Recovery Linac Project bERLinPro at Helmholtz Zentrum Berlin (HZB). SUPERCONDUCTING RF CAVITY OPERATION IN CW MODE In order to operate SCRF cavity in CW mode, the several limitations need to be taken into account [2]. First of all the heat load at 2 K (1.8 K) shouldn’t exceed 20 W when considering single cryomodule (CM) consisted of 8 cavities. Heating of the higher order modes (HOM) couplers must not cause quenching of the cavity. Due to the fact all end-groups are cooled by means of heat conduction. The cryo plant capacity needs to be doubled due to increased dynamic heat load (max. 16 W for single CM). Finally, the CW high power RF sources need to be applied. The most promising solutions are Inductive Output Tubes (IOTs) with nominal output power of 120 kW or Solid State Power Amplifiers (max. output power of 3.8 kW per device). When considering all above constraints the following operating conditions for CW mode are defined (FLASH and XFEL):  Accelerating field gradient per cavity Eacc ~ 7 MV/m.  Nominal loaded Q of input coupler QL ~ 1.5·10  Maximum peak RF power per cavity ~ 3.8 kW  Maximum number of bunches per second ~ 100.000  Minimum spacing between bunches ~ 10 μs  Nominal/ Maximum charge per bunch ~ 0.1/ 0.5 nC  Nominal beam current ~0.010 mA. MICROPHONICS AND PIEZO TUNERS The cavities are detuned by external mechanical forces microphonics. The CW operated cavity with high loaded quality factor of order of 1.5·10 and narrow bandwidth of 87 Hz is very susceptible to disturbances of this kind. The first measurements carried out from XFEL CM installed in Cryo Module Test Bench (CMTB) facility at DESY show vacuum pumps as the main source of microphonics. As seen in Figure 1 the disturbance caused by vacuum pumps has dominant frequency of approx. 50 Hz with varying amplitude and phase. In addition slowly varying operating conditions such as helium pressure fluctuations can also cause detuning of the cavities. The peak-peak microphonics of more than 10 Hz can strongly modulate resonance frequency of 1.3 GHz of the cavity, especially when operated at gradient of 7 MV/m and ___________________________________________ † konrad.przygoda@desy.de THOAA03 Proceedings of IPAC2016, Busan, Korea ISBN 978-3-95450-147-2 3152 C op yr ig ht

First LLRF Tests of BERLinPro Gun Cavity Prototype

The goal of Berlin Energy Recovery Linac Project (BERLinPro) is the generation of a 50 MeV, 100-mA low emittance (below 1 mm mrad) CW electron beam at 2 ps rms bunch duration or below. Three different types of 1.3 GHz SRF modules will be employed: the electron gun, the booster and the main linac. Precise RF amplitude and phase control are needed due to the beam recovery process. In this paper we describe the first tests of the Low Level RF control of the first injector prototype at the HoBiCaT facility, implemented in the digital VME-based LLRF controller developed by Cornell University. Tuner movement control by an mTCA.4 system, together with further plans of using this technology will be also presented. INTRODUCTION The bERLinPro Energy Recovery Linac is a single pass, high average current and all superconducting CW driven ERL currently in construction by Helmholtz Zentrum Berlin (HZB). Its purpose is to serve as a prototype to demonstrate low normalized beam emittance of 1 mm·mrad at 100 mA and short pulses of about 2 ps [1]. bERLinPro will be formed by three 1.3 GHz modules with different characteristics and parameters [2]. The first module is a 1.4-cell gun cavity using a high quantum efficiency (QE) normal conducting multi-alkali cathode, which will deliver 2.3 MeV. The gun module is then followed by the booster module formed by three high power 2-cell booster cavities of Cornell type, where two of them deliver 2.1 MeV each and the third one is operated in zero crossing for bunch compression. The beam is merged into the main linac module consisting in three 7-cell cavities where it is accelerated to 50 MeV in a first pass and decelerated again to 6.5 MeV in a second pass. The beam is finally dumped in a 650 KW beam dump. The gun is one of the most critical components and in order to mitigate risk, it is being developed in several stages. The first one, the so-called Gun0, was a fully superconducting system with a super conducting lead deposited on the back. It allowed beam studies without a complex insert of a high QE normal conducting cathode in a SC environment, [3]. The prototype presented here, called Gun1.0, is a medium power version of the final high power structure and utilizes CW modified TTF-III couplers. It is a beam dynamic optimized design with high QE cathode insert system allowing the generation of a beam up to 4 mA, [4]. It will be used to study bERLinPro bunch parameters and the usage of high QE NC cathode within a SC environment. The last step in the gun development is the Gun2.0, which will feature two modified KEK c-ERL high power couplers [5] to allow 100 mA average current operation. Figure 1: Gun1.0 cavity’s cold mass with fundamental power couplers (left), blade tuner and cathode insert (right). GUN1.0 CAVITY After several vertical and horizontal tests at JLab and HZB where the Q0 specifications were met [2], cold mass assembly and first horizontal tests under module conditions in the horizontal bi-cavity testing facility (HoBICaT) at HZB have been carried out [6]. Table 1: Main Parameters of Gun1.0 Max E0 Max Pf QL 30 (MV/m) 20 KW 3·106 3·107 The cold mass consisting of the magnetic shielding, a blade tuner with a stepper motor and four piezo actuators, and the cathode insertion system, which includes a Petrov filter and a Helium gas cooler, was installed in HZB’s clean room together with the fundamental power couplers. Figure 1 depicts the gun cavity’s cold mass next to the HoBiCaT module. The installed coupler can stand an average input power up to 2 KW, but it is foreseen to equip later with modified warm part to allow 10 kW per coupler [7]. Unfortunately the penetration depth is lower than expected, which led to a higher QL and narrower bandwidth than expected. The last step in the cold mass assembly was to install the blade tuner including the motor and the piezo-actuators, whose pre-stress was adjusted by capacitance measurement. Table 1 shows the expected main parameters for the Gun1.0 cavity. The forward power will be delivered by two power couplers. ___________________________________________ * Work supported by German Bundesministerium für Bildung und Forschung, Land Berlin, and grants of Helmholtz Association † pablo.echevarria_fernandez@helmholtz-berlin.de Proceedings of IPAC2016, Busan, Korea TUPOW035 02 Photon Sources and Electron Accelerators A18 Energy Recovery Linacs (ERLs) ISBN 978-3-95450-147-2 1831 C op yr ig ht