Bellato, M.

Bortolato, D.

Gelain, F.

Isocrate, R.

Marcato, D.

Munaron, Enrico

Pavinato, S.

Pedretti, D.

English

D. Bortolato

S. Pavinato

M. Bellato

F. Gelain

R. Isocrate

D. Marcato

MUNARON, ENRICO

D. Pedretti

Archivio istituzionale della ricerca - Università di Padova

UPGRADE OF THE LLRF CONTROL SYSTEM AT LNLD. Bortolato∗, S. Pavinato, INFN, Laboratori Nazionali di Legnaro, Legnaro,ItalyM. Bellato2, F. Gelain1, R. Isocrate2, D. Marcato1, E. Munaron1, D. Pedretti11INFN, Laboratori Nazionali di Legnaro, Legnaro,Italy2INFN, Sezione di Padova, Padova, ItalyAbstractFor the SPES project at Legnaro National Laboratories(LNL), a Low-Level Radio Frequency (LLRF) has been de-signed to have ﬂexibility, reusability and an high precision.It is an FPGA-based digital feedback control systemusingRF ADCs for the direct undersampling and it can controlat the same time eight diﬀerent cavities. The LLRF systemwas tested on the ﬁeld with an accelerated beam. In thelast year some improvements on the ﬁrmware, software andhardware of the control system have been done. In this pa-per the results carried out in the more recent tests, the futureworks and the upgrades of the system will be detailed.INTRODUCTIONSPES (Selective Production of Exotic Species) [1] is anew facility under construction at LNL. The main goal isthe production of neutron-rich exotic beams to perform re-search in nuclear structure and in interdisciplinary ﬁeldslike medical and biological. Then the Radioactive IonBeam (RIB) from SPES can be boost through the linear ac-celerator ALPI [2] and sent to the three experimental halls.Some upgradeswhichwould make ALPI suitable as an RIBaccelerator are in commissioning [3]. ALPI is a supercon-ducting linear formed by 96 QWR cavities. The ﬁrst groupof cavities resonate at 80MHz, while that of a second groupresonate at 160MHz. In order to boost also the RIB com-ing from SPES, ALPI must be modernized. Besides theresonators themselves, all their ancillary components needto be modernized: RF controllers, RF control system, RFpower ampliﬁers, and couplers, pickup and tuners. The RFcontrol system upgrade are here detailed. In particular thispaper is focused on the digitalization of the signals, howthis process is related to the control performance and on theﬁrmware/software modiﬁcations done during the last year.RF CONTROLLEREach RF controller [4], [5] controls up to eight cavi-ties. The RF signals picked-up from the cavities are under-sampled by RF ADCs. The digitized signals are elaboratedby a ﬁeld programmable gate array (FPGA) which imple-ments a proportional-integral controller. The signals pro-cessed by the FPGA are up-converted by DACs, hence theharmonic of interest is ﬁltered-out and sent to power ampli-ﬁers and then to the cavities.A block diagram of the LLRF controller for the cavitiesis shown in Fig. 1. It is based essentially on three boards:∗ bortolato@lnl.infn.it.........USBSPI...............MASTEROSCILLATORDRS1 DRS2DRS3DRS4DRS4DRS3DRS2DRS1USBDRS 4EVALUATIONBOARD...POWER MONITORPOWERAMPLIFIERSRF FRONTEND......FPGAXILINXXC7K355TGDR/SELFIELDCONTROL100 MHzVCXO100 MHZQZ OSC.......RF CONTROLLER(RF IOC)I2CHelicalResonator Filters8  RF QWR  CAVITIESFORWARD POWERREFLECTED POWERFORWARD POWERFORWARD POWERREFLECTED POWERREFLECTED POWERCHIPSCOPEXILINXLABTOOLSEPICSIOC PCADCADCPLLADCDACDACSPIIPBUSCOREFigure 1: Block diagram of the LLRF systemthe RF input-output controller (RF IOC), the RF front end(RFFE) and the power monitor (PM) [4]. The RF IOC im-plement the control law. The RFFE board adapts the ampli-tude level of the RF signals from the cavities to the ADCs ofthe RF IOC and from the DACs of the RF IOC to the powerampliﬁers. The PM board measures the reﬂected powersand the forward powers of the eight controlled cavities.One of the most critical point next analyzed is the digital-ization of the signals.ADCIn order to simplify the hardware design the direct downconversion(DDC)was adopted. The signals picked up fromcavities have frequencies that lie in the second or thirdNyquist zone. DDC preserves the signal information if thesampling frequency is at least double the bandwidth of theRF signals [6]. This condition is complied. Furthermorethe RF signals are sampling in quadrature in order to ex-tract the in-phase and quadrature components. From thesecomponents it is direct get the amplitude and phase of theﬁeld resonating in cavities. The components are physicalmeaningful and can be controlled independently.ADCs have to guarantee an high resolution and an highsampling rate. The sampling rate is important since theDDC was chosen, while the high resolution is related tothe stability of phase and amplitude in cavity. For a mod-16th Int. Conf. on Accelerator and Large Experimental Control Systems ICALEPCS2017, Barcelona, Spain JACoW PublishingISBN: 978-3-95450-193-9 doi:10.18429/JACoW-ICALEPCS2017-TUPHA117TUPHA117678ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2017).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Feedback Control and Process Tuningern heavy ion linac like ALPI, it is required a phase stabil-ity and a amplitude stability of at least 0.5° and 0.5% rms,respectively. In other words the phase and gradient stabil-ity can be studied from the phase error and amplitude error.From these values we can extract the ADC speciﬁcation forsignal-to-noise ratio (SNR) [7]. According with the ampli-tude error the SNR is:SNRMag = −20 ∗ log(ErrMAGNITUDE) (1)while according with the phase error the SNR is:SNRPh = −20 ∗ log(sin(45° + ErrPHASE) − cos(45°))(2)To calculate the SNR due to the phase error, we need tosolve the amplitude diﬀerence between the in-phase I andquadrature components Q. The worst case is when the mag-nitude of I is equal to the magnitude of Q, in example at45° [7]. The SNR requirement has to be kept in the wholegradient dynamic range |EMIN | → |EMAX |, that in ALPItypical conﬁgurations is |EMIN | = 3.5 MV/m in a lowbeta cavity and |EMAX | = 5MV/m in a medium beta cav-ity. Hence the ADC must have a SNRADC greater than:SNRADC ≥ max(SNRMag, SNRPh)+20∗log( |EMAX ||EMIN |)(3)where 20 ∗ log(|EMAX/|EMIN |) ≈ 3dB.A gradient stability of 0.5% rms means a SNR =−20 log(0.005) = 46dB. Whereas a phase requirementof 0.5° implicates a SNR = −20 ∗ log(sin(45 + 0.5) −cos(45)) = 44.2dB. The SNR of an ideal ADC is given bythe following equation:SNRADC = 6.02 ∗ N + 1.76 (4)This means that the ADCs have to have at least an equiv-alent number of bit equals to ENOB = (SNRADC −1.76)/6.02 = 7.9 → 8, with SNRADC = 46 + 3 = 49dB.SLOW CONTROLThe slow control system refers to the diﬀerent com-munication protocols and associated ﬁrmware or software,needed to monitor and control the electronic programmabledevices present in the RF controller. In this project theFPGA, housed in the RF IOC, communicates with a remoteor local host using standard hardware such as USB or Ether-net. The communications are used for monitoring or com-municationwith the accelerator control systems. A commu-nication controller in the FPGA and a driver in the host ex-change data with the USB or Ethernet communication link.In the RF IOC there are two diﬀerent slow control systemcommunication link.• The ﬁrst one is based on a USB communication link.The serial link in the board is formed by the chain usb-connector, CYUSB3014 and some GPIO in the FPGA,that implements inside it a usb core block. The coreacts as a USB device that transfer a byte stream inboth direction over the bus. CYUSB3014 is a super-speed peripheral controller, providing integrated andﬂexible features. It has a fully conﬁgurable, parallel,general programmable interface, which can connect toan FPGA. Basically it is a programmable statemachinethat enables a ﬂexible interface that may function ei-ther as a master.• Instead of the other one the communication link is aGigabit Ethernet. The communication is based on IP-bus protocol [8]. On the board there is a speciﬁc bidi-rectional high-speed serial link in the QSFP connector,linked to a GTX transceiver in the FPGA. Since thecommunication link is a Ethernet cable, a 1000BASE-T COPPER SFP Optical Transceiver is used with theQSFP connector.IPbus allows the communication between the RF con-troller and the accelerator control system, without placinga computer, where an EPICS IOC can run, close to the con-troller. The software IOC is executed on a virtual machine.In this way you can exploit the advantages given by a virtualmachines, constantly backed-up and easy to maintain, andreduce the wiring required. Therefore the communicationvia IPBUS was preferred on USB one. In spite of the USBcommunication has been used for the ﬁrst test, IPBUS pro-tocol is going to be used in the stable version of the controlsystem.IPBUS COREIPbusCORETRANSCEIVERTRANSCEIVERTRANSCEIVERregistersI2CSPIInterfaceControl loopparametersDigital SignalProcessing (PI)ADCDACEPICSFigure 2: Block diagram of the ﬁrmware implemented.The IPbus protocol allows to control hardware through avirtual bus. This bus has 32 bits address and 32 bits data.It uses standard IP based on gigabit ethernet connection. It16th Int. Conf. on Accelerator and Large Experimental Control Systems ICALEPCS2017, Barcelona, Spain JACoW PublishingISBN: 978-3-95450-193-9 doi:10.18429/JACoW-ICALEPCS2017-TUPHA117Feedback Control and Process TuningTUPHA117679ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2017).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.is the interface between the RF IOC and the IOC EPICSbased, that allows to set and get registers implemented inthe FPGA ﬁrmware. The IPbus protocol belong to the ap-plication layer in the OSI model. Each IPbus host device (inthis project the RF IOC board) has an IP address and a portnumber where it accepts IPbus control packets. The IPbusﬁrmware suite was downloaded from the CERN repositoryhttps://svnweb.cern.ch/trac/cactus.The registers are the so called "Conﬁguration registers","control loop registers" and "transceiver registers" as shownin Fig. 2.RegistersThe FPGA ﬁrmware [9] implements a set of control andstatus registers, through which the accelerator control sys-tem, via IPbus protocol, can set the amplitude and phaseloop parameters and the conﬁguration parameters for theICs on the RF IOC, RFFE and PM boards. These regis-ters in Fig. 2 are labeled as "Conﬁguration registers" and"control loop registers". The latter is devoted for the con-ﬁguration parameters and amplitude/phase loop parametersrespectively. Actually it is formed by eight blocks of regis-ters: one block for each cavity controlled. These registers,in the IPbus philosophy, are the slave blocks.The "Conﬁguration registers" are used to conﬁgure theICs on the RF IOC, RFFE and PM board. Furthermore theyread the digitized values of RF power from the PM board.These ICs implement an SPI or IIC interface in order to ac-cess their registers. From the perspective of the acceleratorcontrol system the SPI or IIC interface is totally transparentand the access to the ICs internal registers is done via reador write IPbus commands. Both protocols have a master-slave architecture with a single master. The master deviceoriginates the frame for reading and writing. In this casethe master device is the FPGA, while the slaves are the ICslisted before. The lowest-level read and write functions tothe memory-mapped registers, implemented in the FPGA,are based on the read and write function respectively, pro-vided by the μHAL [8]. This is a C++ library that is a com-ponent of the IPbus suite, too.MEASUREMENTAfter having developed the FPGA ﬁrmware and the IOCEPICS, it was possible conﬁgure the boards. When theEPICS architecture to control the RF controller was com-pleted rf control system validation started. It consisted inthe installation of the new RF controller with its control sys-tem EPICS based in the ALPI facility. The cavities wereSEL locked and a beam was accelerated. The stability ofthe LLRF controller was assessed. Suitable indicators forvalidate the stability performance are the rms value of theresidual errors, in phase and magnitude, of the RF IOCboard. A good visualization of the diﬀerent perturbationfrequency is achieved calculating the integrated rms detun-10 2 10 3 10 4Frequency [Hz]00.050.10.15Δφrms [deg]Figure 3: Cumulative spectrum of the phase error for a lowbeta cavity SEL driven.10 2 10 3 10 4Frequency [Hz]01234567Δ M/Mrms×10 -4Figure 4: Cumulative spectrum of the amplitude error for alow beta cavity SEL driven.ing spectrum [10], [11]:Δd(fn)rms =√∑n1 |F [Δd(t)]i|2√2(5)where F [Δd(fn)rms] represents the spectrum of thetime series collected. It is gotten accumulating up to theperturbation frequency fn, the absolute value squared of theFourier components F [Δd(t)]i.Figure 3 shows the integrated phase error for a low betacavity. This graph shows the eﬀect of the disturbances atmicrophonic frequencies.Instead of in the graph of the integrated ﬁeld error Fig. 4,there is a source of noticeable amplitude error around the10 KHz. At the moment it is unknown the source of thisdisturbance.From Fig. 3 and 4, it appears that the total value of thephase ﬂuctuation is 0.145° rms, while for the ﬁeld ﬂuctua-tion is 6.9 · 10−4.Substituting in equation 1 ErrMAGNITUDE = 6.9 · 10−4and in equation 2 ErrPHASE = 0.145 and ﬁnally applying16th Int. Conf. on Accelerator and Large Experimental Control Systems ICALEPCS2017, Barcelona, Spain JACoW PublishingISBN: 978-3-95450-193-9 doi:10.18429/JACoW-ICALEPCS2017-TUPHA117TUPHA117680ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2017).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Feedback Control and Process Tuningthe equation 3 the SNR estimated is:SNRADC = max(63.22, 55.0) + 20 ∗ log( 53.5) ≈ 66dB(6)Making the assumption of chapter the ENOB is calculatedas:ENOB = SNRADC − 1.766.02 = 10.7 (7)This value is better than the ENOB requirements.CONCLUSIONThe measurements show that the new RF system guar-antees a phase and an amplitude stability margins consis-tent with those required for a heavy ion linear acceleratoras ALPI.The measurements done to qualify RF control systemhave been come to an end. The hardware developed is nowready for the ﬁrst production in order to substitute the oldanalog controllers.REFERENCES[1] G. Prete et al., Status of the SPES Facility for Acceleration ofHigh Intensity Protons and Production of Exotic Beams, EPJWeb of Conferences 66, 11003 (2014).[2] A. Dainelli et al., Commissioning of the ALPI post-accelerator, Nucl. Instr. and Meth A 382 (1996) 100-106.[3] G. Bisoﬃ et al., ALPI Setup as the SPES Accelerator of ExoticBeams, EPJ Web of Conferences 66 , 11003 (2014).[4] D. Bortolato et al., New LLRF control system at LNL, Proceed-ings of Real Time Conference (RT), 2016 IEEE-NPSS.[5] S. Pavinato et al., Development of a digital LLRF control sys-tem at LNL, Proceedings of LINAC Conference, 2016.[6] Walt Kester, Editor with the technical staﬀ of Analog Devices,Data Conversion Handbook.[7] C. Hovater, et al. Direct Digital Down/Up Conversion for RFControl of Accelerating Cavities[8] Tom Williams for the CMS Collaboration, IPbus A ﬂexibleEthernet-based control system for xTCA hardware, CMS Con-ference Report -2014/334.[9] Notes on Firmware Implementation of an IPbus SoC Bus,V1.0 23/5/2012, DMN.[10] A. Neumann et al., Analysis and active compensation of mi-crophonics in continuous wave narrow-bandwidth supercon-ducting cavities, Physical Review Special Topics - Accelera-tors and Beams 13, 082001 (2010).[11] M. Konrad, ADevelopment and commissioning of a digital rfcontrol system for the S-DALINAC and migration of the accel-erator control system to an EPICS-based system, PhD Thesis,2013.16th Int. Conf. on Accelerator and Large Experimental Control Systems ICALEPCS2017, Barcelona, Spain JACoW PublishingISBN: 978-3-95450-193-9 doi:10.18429/JACoW-ICALEPCS2017-TUPHA117Feedback Control and Process TuningTUPHA117681ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2017).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.

UPGRADE OF THE LLRF CONTROL SYSTEM AT LNL

Padua Research Archive

https://www.research.unipd.it/bitstream/11577/3255747/1/tupha117.pdf

UPGRADE OF THE LLRF CONTROL SYSTEM AT LNL

Abstract

Similar works

Full text

Available Versions

Archivio istituzionale della ricerca - Università di Padova

Padua Research Archive