The RFQD is designed to decelerate antiprotons of momentum 100 MeV/c (kinetic
energy 5.33MeV)down to a kinetic energy variable between ~10 keV and 120 keV.
Inside the RFQ body, at ground potential, the rf structure of the four-rod type
is mounted on insulating supports. It can be biased between plus/minus 60 kV dc
to achieve the continuous adjustment of output energy required by the ASACUSA
experiment at the CERN Antiproton Decelerator AD. The different parts of the
system are described and the present status reported

Bylinsky, Y.

Lombardi, A. M.

Pirkl, W.

English

arXiv

Bylinsky, Y

Lombardi, AM

Pirkl, W

CERN Document Server

RFQD - A decelerating radio frequency quadrupole for the CERN antiproton facility

The RFQD is designed to decelerate antiprotons of momentum 100 MeV/c (kinetic energy 5.33 MeV) down to a kinetic energy variable between ~10 keV and 120 keV. Inside the RFQ body, at ground potential, the rf structure of the four-rod type is mounted on insulating supports. It can be biased between ±60 kV dc to achieve the continuous adjustment of the output energy required by the ASACUSA experiment at the CERN Antiproton Decelerator AD. The different parts of the system are described and the present status reported.The RFQD is designed to decelerate antiprotons of momentum 100 MeV/c (kinetic energy 5.33MeV)down to a kinetic energy variable between ~10 keV and 120 keV. Inside the RFQ body, at ground potential, the rf structure of the four-rod type is mounted on insulating supports. It can be biased between plus/minus 60 kV dc to achieve the continuous adjustment of output energy required by the ASACUSA experiment at the CERN Antiproton Decelerator AD. The different parts of the system are described and the present status reported

Lombardi, A.M.

EUROPEAN ORGANISATION FOR NUCLEAR RESEARCHCERN - PS DIVISIONCERN/PS 2000-061 (PP)RFQD -  A  DECELERATING  RADIO FREQUENCY QUADRUPOLE FORTHE CERN ANTIPROTON FACILITYY. Bylinsky*, A.M. Lombardi, W. PirklAbstractThe RFQD is designed to decelerate antiprotons of momentum 100 MeV/c (kineticenergy 5.33 MeV) down to a kinetic energy variable between ~10 keV and 120 keV.Inside the RFQ body, at ground potential, the rf structure of the four-rod type ismounted on insulating supports. It can be biased between ±60 kV dc to achieve thecontinuous adjustment of the output energy required by the ASACUSA experiment atthe CERN Antiproton Decelerator AD. The different parts of the system are describedand the present status reported.Presented at the XXth International Linac Conference,21-25 August 2000, Monterey, California, USAGeneva, Switzerland                                                          * Visitor from INR, Moscow, Russia18 August 2000RFQD -  A  DECELERATING  RADIO FREQUENCY QUADRUPOLE FORTHE CERN ANTIPROTON FACILITYY. Bylinsky*, A.M. Lombardi, W. Pirkl, CERN, Geneva, Switzerland                                                          * Visitor from INR, Moscow, RussiaAbstractThe RFQD is designed to decelerate antiprotons ofmomentum 100 MeV/c (kinetic energy 5.33 MeV) downto a kinetic energy variable between ~10 keV and120 keV. Inside the RFQ body, at ground potential, the rfstructure of the four-rod type is mounted on insulatingsupports. It can be biased between ±60 kV dc to achievethe continuous adjustment of the output energy requiredby the ASACUSA experiment at the CERN AntiprotonDecelerator AD. The different parts of the system aredescribed and the present status reported.1  DESCRIPTION OF THE SYSTEMThe RFQ key parameters are presented in Table 1. Table 1: RFQ key parametersOperating frequency 202.5 MHzShunt impedance 13.8 kΩVane voltage 167 kVMaximum electric field 33 MV/mDissipated power 1.1 MWMinimum aperture 0.4 cmVane modulation factor 2.9Input energy 5.33 MeVOutput energy with internaldc post-deceleration63 keV (RFQ )± 60 keV (dc)Transverse acceptance 15pi mm mradEnergy acceptance ± 0.9×10-3Decelerating efficiency 45% The main girder serves as a common support for themedium-energy beam transport (MEBT), the RFQ tankand the low-energy beam transport (LEBT), see Fig.1.The MEBT starts with two diagnostic boxes joined by abeam transformer. They contain retractable scintillatorscreens, a silicon strip detector and a Faraday cup. Asmall rf cavity attached to the RFQ tank allows the inputbeam energy to be varied. A second cavity of the sametype is mounted about 6 m upstream (not shown) to bunchthe beam.The RFQ tank contains the rf structure which consistsof a floating "ladder" with 34 rf cells, where the 35 rungscarry the four modulated RFQ electrodes. HV shields aremounted on top and bottom of the ladder which is held inplace by 5 ceramic insulators.The LEBT contains a first solenoid; a first steerer; adiagnostic box with large pumping port and retractableFaraday cup; a second steerer; a SEM wire chamber 95%transparent to the beam that was contributed byASACUSA; a second solenoid. A measurement linedescribed in paragraph 3 is initially attached instead of thephysics experimental apparatus, to assess the RFQperformance.Three rf amplifier chains power the buncher, energycorrector cavity and RFQ, respectively. Each chaincomprises 3 servo loops to stabilise amplitude, phase andresonator tuning. The sheer length of the RFQ (3.55 m)made the usual concept of movable tuners impractical, sothe system frequency is determined by the resonancefrequency of the RFQ. The error signal generated by thestandard tuning criteria is fed to a "motor emulatorcircuit" which in turn acts on the frequency of the masteroscillator. The two low-power resonators use motorizedtuners that have to follow the RFQ frequency and absorbalso the tuning variations of their respective cavity. Thepower rating of the RFQ amplifier chain was raised frominitially 0.7 MW initially to 2 MW by the addition of adriver stage, to cope with the increased powerrequirement due to Q-factor lower than expected.Figure 1: The girder with the MEBT (left), RFQ tank (centre) and LEBT (right)2   MECHANICS AND VACUUMMechanical layout: The RFQ tank is a cylinder3555 mm long, made of 15 mm thick sheet of Cr.Ni.18-10 (Inox 304L). The ladder is milled out of two OFEcopper blocks; the electrodes also made of Cu-OFE andare screwed on the ladder together with spiral rfcontacts. A dedicated carriage running on demountablerails allows to slide the rf structure into the tank. Nospecial cooling is necessary since the average powerdissipation is less than 100 W.Vacuum: Three turbo pumps (250/1400/250 l/s) andthree ion pumps (400/800/125 l/s) are mounted in thethree girder areas. The LEBT acts as a buffer betweenthe RFQ and the physics apparatus, where there ismuch higher pressure for the first phase of ASACUSA,but UHV later on. Differential pumping is facilitated bythe small output aperture of the RFQ. A pressure of afew times 10-8 mbar was attained in the RFQ afterconditioning with the LEBT end closed. An additionalsublimation pump of 2000 l/s is foreseen and bake-outup to 150°C can be applied later if needed.3  BEAM DYNAMICS ANDMEASUREMENTSElectrode design: The procedure for the design ofthe RFQ decelerator has been reported in [2]. Theincoming antiproton beam at 5.33 MeV is bunched by acoaxial TEM double-gap resonator, delivering aneffective voltage of 47 keV. A corrector cavity,identical to the buncher, is placed at the RFQ entranceto counteract the dc bias of the RFQ ladder, thuspreserving the full energy acceptance. The distancebetween the centre of the buncher and the RFQdetermines the longitudinal matching into thedecelerating bucket. Magnetic quadrupoles in theupstream MEBT and a conventional radial matchingsection in the RFQ perform the transverse matching tothe time-varying FODO system of the RFQ. Themodulated part of the electrodes (3.15 m) deceleratesthe beam to 63 keV. The modulation factor ismaximum at the beginning and decreases as the beam isdecelerated in order to keep the maximum field on thevane tip below 1.8 times the Kilpatrick limit, and inorder to keep the longitudinal radius of curvature above5 mm for ease of machining. The beam sizes in the twotransverse planes of the decelerated particles areequalised by a carefully designed output matcher. Thebeam can be further accelerated/decelerated as it exitsthe RFQ by the dc bias (±60 kV) of the rf structure. Inthe LEBT a first solenoid focuses the highly divergentbeam and makes it almost parallel. A second solenoidat the end focuses the beam into the experimentalapparatus. Simulation results: The beam dynamics simulationshave been performed with the codes PARMULT,PARMILA and PATH. The expected beam phase space"portraits" at 63 keV are shown in Fig. 2.  Figure 2: The input (upper) and output (lower) phasespace "portraits"  Measurement line: In order to assess the RFQperformance, an additional measurement line has beenarranged downstream of the LEBT. It comprises a 20°spectrometer magnet, a pair of steerers, a quadrupoletriplet, a Faraday cup and a multi-wire profile monitor(see Fig. 3). This set-up allows one to evaluate thebeam transmission, output energy, energy dispersionand beam transverse emittance. The whole set-up hasbeen tested with a proton beam of 30 – 80 keV atCERN. Calibration factors for the magnetic fieldstrength were obtained and diagnostic devices weretested at low beam intensity (beam pulse of 50 nA and5 µs). For transverse emittance evaluation we use awell-known technique based on the profilemeasurements for different triplet settings. The testresults were in good agreement with directmeasurements performed by means of a slit – collectoremittance measuring device in transport line of theduoplasmatron source.  Figure 3: Layout of the measurement line. 4  RF ISSUESParasitic modes: The floating ladder permits thepropagation of unwanted parasitic modes that maybuild up dangerous voltages between the rf structureand the grounded cylinder. These are the TEM(n) andthe TE(1,1,n) families that have to be damped ordetuned. Low-level tests on a 1/3 scale model startedearlier to devise a suitable strategy. Terminating theunwanted modes in capacitively-coupled absorbers ledto unreasonably high power losses; therefore (lossless)detuning by 2 pairs of "horns" has been implemented.They consist of open-ended coaxial stubs, close to λ/4long, mounted on top and bottom at the two ends of theladder. A single stub length was found that moves bothfamilies sufficiently away from the operating frequencywithout the need to shift each family separately.Power feeding: Two horizontal coupling loops feedthe RFQ at its centre. They are located at a distance of25 mm from the electrodes to avoid sparking. Theircoupling falls short by a factor of about two, hence atransformation circuit is built into the coaxial feederlines inside the tank. It consists of a low-Z sectionfollowed by a folded high-Z section to achieve correctmatching. The two power feeder lines are composed of2 x 4 cables RG218 for high flexibility. A braidedshield has been added to the outside of each cable toreduce rf emissions. A 4/1 splitter is provided at eachRFQ terminal and an 8/1 splitter at the amplifier.Field flattening: The initially measured resonancefrequency, and also the field pattern, differedconsiderably from the predictions obtained from theusual design programs. This can in part be explained bythe substantial length of this RFQ: it becomes highlysensitive to slight local tuning variations due to thelarge leverage arm of 2.3λ, while the practical limit onthe number of mesh points reduces at the same time theprecision of a full-length simulation. Movable tuningplates were provisionally mounted in each of the 34 rfcells for the experimental determination of the correctcell heights. The results were then made permanent byslight re-machining of the ladder. Additionally, themounting of small "fingers" allowed flexible fine-tuning of the assembly. A field flatness of about ±1%was achieved.Improvement of the Q-factor: The initiallymeasured Q-factor was only 3800, about 36% of thetheoretical value. Experimental studies on a single full-size cell allowed one to identify some of the lossfactors. In particular it was found that the losses in thestainless-steel cylinder were much higher thanpredicted by simulation. The whole inner cylinder ofthe RFQ was therefore copper plated, the ladder and theelectrodes carefully polished, and all contactsimproved. By these measurements a Q-factor of 5100was finally obtained. In parallel an additional driverstage was included into the rf chain to cover all powerrequirements with comfortable reserve.High power tests: The RFQ accepted full field aftertwo days of conditioning; the dc bias of 60 kV on theladder is also reliably sustained. The pulse fault rate isvirtually zero; in early tests the equipment ranuninterrupted and unattended over one prolongedweekend.Radio Frequency Interference: The RFQ will haveto work in an environment that is very susceptible toelectromagnetic perturbations. Emitted rf power fromthe system has been reduced to about 1 W, thanks toheroic efforts of the RF Group. It was neverthelessdecided to enclose the two large amplifiers (and lateralso the pre-driver) in a Faraday cage to minimise theperturbation to the physics experiments.5  NEXT STEPSThe system has been transported to the "Institut forFysik" in Aarhus/Denmark where the performance ofthe RFQ will be assessed using a high-quality protonbeam from the Tandem generator. After return toCERN, the RFQ will provide a decelerated antiprotonbeam for physics from November 2000.ACKNOWLEDGEMENTSThe construction of this RFQ would not have beenpossible without the participation of the PS RF Groupand the EST Division. Our thanks go to them and thenumerous other persons who actively supported thisproject.REFERENCES[1] T. Azuma et al., “Atomic Spectroscopy andCollisions Using Slow Antiprotons,” ASACUSACollaboration, CERN/SPSC 97-19, CERN/SPSCP-307 (1997).[2] A.M. Lombardi., “Accelerating and DeceleratingRFQs,” Proc. 1998 Linear Accelerator Conf.,Chicago, 377-381, 1998

RFQD: a decelerating radio frequency quadrupole for the CERN antiproton facility

RFQD - a Decelerating Radio Frequency Quadrupole for the CERN Antiproton
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RFQD - a Decelerating Radio Frequency Quadrupole for the CERN Antiproton Facility

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