A background on the Europan Spallation Source is given, and our work on three control problems important for the design of its linear accelerator are presented

Bernhardsson, Bo

Johansson, Anders J

Johansson, Rolf

Olofsson, Björn

Troeng, Olof

English

Lund University Publications

LUND UNIVERSITYPO Box 117221 00 Lund+46 46-222 00 00Control Problems at the European Spallation SourceTroeng, Olof; Olofsson, Björn; Bernhardsson, Bo; Johansson, Anders J; Johansson, RolfPublished in:Abstracts of Reglermöte 20162016Link to publicationCitation for published version (APA):Troeng, O., Olofsson, B., Bernhardsson, B., Johansson, A. J., & Johansson, R. (2016). Control Problems at theEuropean Spallation Source. In Abstracts of Reglermöte 2016Total number of authors:5General rightsUnless other specific re-use rights are stated the following general rights apply:Copyright and moral rights for the publications made accessible in the public portal are retained by the authorsand/or other copyright owners and it is a condition of accessing publications that users recognise and abide by thelegal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private studyor research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portalRead more about Creative commons licenses: https://creativecommons.org/licenses/Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will removeaccess to the work immediately and investigate your claim.Control Problems at the EuropeanSpallation SourceOlof Troeng ∗ Björn Olofsson ∗ Bo Bernhardsson ∗Anders J Johansson ∗∗ Rolf Johansson ∗∗ Department of Automatic Control, Lund UniversityE-mail: {oloft, bjorno, bob, Rolf.Johansson}@control.lth.se∗∗ Department of Electrical and Information Tech., Lund UniversityE-mail: anders_j.johansson@eit.lth.se1. THE EUROPEAN SPALLATION SOURCEThe European Spallation Source (ESS), currently underconstruction in Lund, will be the world’s brightest neutronsource, allowing researchers to see and understand previ-ously invisible structures in for example superconductingmaterials, pharmaceuticals, solar cells, fuel cells, catalysts,and engineering materials. The neutrons will be generatedby smashing protons, traveling at 96 % the speed of light,into a tungsten target. For illustrations see Figures 1-3.Fig. 1. Visualization of ESS’ architectural design. In thedistance the world’s most brilliant synchrotron lightsource, MAX IV, is seen. Photo credit: ESS.Fig. 2. Single-pulse brightness for the world’s leadingneutron sources at 5Å wavelength, Peggs et al. (2013).(a) X-Rays (b) NeutronsFig. 3. Different details appear in the two images due tothat neutrons and X-rays are attenuated differently.Photo credit: PSI.The protons will be accelerated by electromagnetic fieldsconfined in 155 radio-frequency cavities along the 500 me-ter long, most powerful linear accelerator ever built(Figure 4). The first nine cavities (one radio frequencyquadrupole, three bunchers, five drift tube linacs) aremade of copper and operated at around room temperature.The succeeding cavities are made of niobium and cooled byliquid helium to 2 K, making them superconducting andallowing stronger electric fields and lower electrical losses.protonsource 1 RFQ 3 bunchers 5 DTLs26 spokecavities120 ellipticalcavitiestargetFig. 4. Block diagram of the ESS accelerator.Radioactivation due to escaped protons will be significantaround the accelerator. For this reason the acceleratoris placed in an underground tunnel while all electronicequipment is kept in buildings at the surface. The highcharge density of the accelerated proton bunches putsunprecedented requirements on amplitude and phase sta-bility of the cavity fields to keep activation within tolerablelevels.At the Department of Automatic Control at Lund Univer-sity we are working with control design for the amplitudeand phase regulation, and also temperature stabilizationfor the distribution of the phase reference. We are alsoinvolved in the control design for damping the mechanicalresonances in the superconducting cavities that are excitedby the photon pressure of the pulsed electric fields. Thesethree control problems are briefly introduced below. Thespace does not allow us to go into details but more back-ground and experimental results will be presented at themeeting.2. CAVITY FIELD CONTROLThe block diagram of a single RF station is shown inFigure 5. The dynamics for the electric modes in the cavityare given byëk(t) + 2αkė(t) + ω2kek(t) = κgk i̇g(t) + κbk i̇b(t), (1)where ωk is the resonance frequency and αk the half band-width of mode k. The parameters κgk and κbk quantifyhow the power supplied by the power amplifier, modeledas a current ig, and how the accelerated particle bunches,modeled as a current ib, couple to the cavity field. Thecontrol system is able to modulate ig, which acts as thecontrol input.The objective is to control the accelerating mode, whichhas a frequency matched to the proton bunch frequency.Analysis and controller implementation is done in thebase-band (s 7→ s − iωRF) around the nominal RF fre-quency ωRF. Considering the accelerating mode and theclosest parasitic mode, a linear model is given byP (s) = PPA(s)e−iωRFτe−τs×[c0κg0/2s+ α0ω0 + i∆ω0+c1κg1/2s+ α1ω1 + i∆ω1], (2)where ∆ωk = ωRF − ωk, ck describe the coupling to themeasurement probe, τ is the system time delay and PPA(s)is the linearized amplifier dynamics. Note that the processtransfer function (2) contains multiple complex coefficients(colored blue). We have realized that complex transferfunctions offer improved insight compared to considering(2) as a real two-input two-output system which is donein the cavity field control literature.The controller will be implemented in an FPGA, allowingsampling speeds of 10 MHz. The bandwidth of the fieldcontrol loops will be around 100 kHz for the warm cavitiesand 30 kHz for the cold cavities. It is interesting to notethat most of the limitations on the bandwidth and controlperformance are due to the 1 µs time delay τ .CavityiiiiPAAC/DC converterCVPSiguyφrefibFig. 5. Block diagram of RF station driving one cavity.Variations in the voltage VPS supplied to the poweramplifier and the accelerated current ib affect thesystem as load disturbances on the cavity input. Tem-perature regulation of the line between the referenceoscillator and the controller is discussed in Section 3.3. TEMPERATURE CONTROL OF PHASEREFERENCE LINEThe phase setpoints of all cavities are given relative thephase of an ultra-stable reference clock. The phase refer-ence is distributed along the accelerator as an electromag-netic wave in a rigid coaxial line made of copper. Varia-tions in the air temperature of the tunnel induce lengthvariations of the coaxial line and consequently changesthe phase of the wave at the tap points. These variationswill be counter-acted by insulating the reference line andmounting temperature sensors and heating elements insidethe insulation. The heating power is controlled by PIcontrollers, with one controller for every 10—20 m.The requirement on phase stability of the reference signalstranslates to a requirement on temperature stability of±0.1 ◦C. Experimental results from a prototype setuphave demonstrated that the system achieves the requiredstability when subject to sinusoidal variations in the airtemperature with a period of 1 h and an amplitude of 5 ◦C.4. TUNING CONTROLThe superconducting cavities, made of thin (4 mm) nio-bium, deform due to the photon pressure of the electricfields. This changes the resonance frequency of the cavities,increasing the required power to maintain the fields atthe nominal frequency. To counter-act time-varying de-formations the cavities are equipped with stepper motorsfor slow, coarse tuning and fast piezo-tuners for precisiontuning. Both feedback and feedforward will be used in thetuning control algorithms.Fig. 6. Expected Lorenz force detuning of ESS spokecavity, in millimeters, Peggs et al. (2013).ACKNOWLEDGEMENTSWe would like to acknowledge helpful discussions withpeople at ESS, Lund University and accelerator facilitiesthat we have visited.REFERENCESPeggs, S. et al. (2013). ESS technical design report.Technical Report ESS-doc-274.

Control Problems at the European Spallation Source

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Control Problems at the European Spallation Source

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