The HERA-B experiment utilizes an internal wire target in the halo of the stored proton beam in order to study CP violation. As operational experience shows, interaction rates tend to be extremely sensitive to tiny orbit jitter amplitudes. In order to stabilize these interaction rates, various methods have been studied to increase diffusion in the transverse proton beam tails without affecting the luminosity at the electron-proton collider experiments ZEUS and H1. Tune modulation was found to be a promising method for this task. The present status of these experiments willbe reported

Ehret, K

Funcke, M

Issever, S

Montag, C

Montag, C.

Ehret, K.

Funcke, M.

Issever, S.

DESY

Transverse Beam Tail Shaping in HERA-p by Means of Tune Modulation

DESY Publication Database

CERN Document Server

TRANSVERSE BEAM TAIL SHAPING IN HERA-P BY MEANS OF TUNEMODULATIONC. Montag∗† , DESY, Hamburg, GermanyK. Ehret, M. Funcke, S. Issever, University of Dortmund, GermanyAbstractThe HERA-B experiment utilizes an internal wire tar-get in the halo of the stored proton beam in order to studyCP violation. As operational experience shows, interactionrates tend to be extremely sensitive to tiny orbit jitter am-plitudes. In order to stabilize these interaction rates, vari-ous methods have been studied to increase diffusion in thetransverse proton beam tails without affecting the luminos-ity at the electron-proton collider experiments ZEUS andH1. Tune modulation was found to be a promising methodfor this task. The present status of these experiments willbe reported.1 INTRODUCTIONTo study CP violation, the HERA-B detector has beeninstalled in the straight section West of the HERA protonring. Using an internal wire target in the halo of the storedproton beam, the B-meson decay B 0 → J/ψK0s is beinginvestigated.To achieve the required interaction rate of N˙interaction =40MHz, each bunch crossing has to provide about four in-teractions. The primary vertices of these interactions mustexhibit a sufficient spatial distribution in order to allow aclear reconstruction. Therefore eight target wires of 50µmdiameter are arranged around the beam. To ensure equalcontribution of all four wires to the overall interaction rate,each wire is equipped with a stepping motor that allows ac-curate target movements in nominal steps of 50 nm. Theinteraction rate is kept constant using a 10Hz wire positionfeedback, thus allowing to counteract slow beam orbit vari-ations by target position adjustment.As it was observed, this slow wire position feed-back is not sufficient to ensure the required in-teraction rate stability. Especially at high inter-action rates above some 20MHz large fluctuationswere observed, with relative rms deviations σrel =√〈(N˙interaction − 〈N˙interaction〉)2〉/〈N˙interaction〉 of morethan 50%. This can be explained by lack of diffusion intothe transverse beam tails, thus requiring the target wires tobe positioned extremely close to the beam center, at typ-ically 3 . . . 4 transverse rms beam sizes. This tight targetaperture in turn leads to sharp “edges” of the initially gaus-sian distribution, resulting in a high sensitivity of the inter-action rate to beam orbit vibrations.To overcome this effect, a re-population of the transversebeam tails is required. Since HERA is not operated at any∗ christoph.montag@desy.de† Now at BNL, Upton, NY 11973dedicated run mode for HERA-B only, but simultaneouslyhas to provide high luminosity for the colliding-beam ex-periments ZEUS and H1, this “tail shaping” has to be donein such a way that the detrimental effects at H1 and ZEUS,like luminosity degradation or increased background, are assmall as possible. For this reason a simple slow transverseblow-up of the beam is not tolerable, and a more sophisti-cated method has to be chosen.It is well known that tune modulation in connection witha strong nonlinearity, in particular the beam-beam inter-action, results in an increased diffusion in the transversebeam tails, while the beam core, where the linear part ofthe beam-beam force dominates, is practically not affected[1, 2, 3]. These are just the properties required for “tailshaping”.In the presence of tune modulation, additional resonancesidebands are created. When the tune coincides with oneof these sidebands, this leads to increased diffusion. Thecorresponding resonance condition can be written ask ·Q+ l · fmodfrev= m, k, l,m integer, (1)where Q is the betatron tune and fmod, frev are the modu-lation frequency and the revolution frequency, respectively.2 EXPERIMENTAL SET-UPIn the 920 GeV superconducting HERA proton storagering the main quadrupoles are connected in series with themain dipole magnets. For a fine-adjustment and control ofthe tunes two families of superconducting quadrupoles areinstalled in each quadrant of HERA. In the quadrant Westthe chopper power supplies of these magnets are modifiedto provide an extra control input to allow for tune modula-tion with frequencies up to some 1.5 kHz.Due to the large inductance of the superconductingquadrupole magnets, high frequency modulation signalsare effectively suppressed, while low frequencies of a fewHertz are counteracted by internal feedback loops of thepower supplies. Figure 1 shows the measured magnet cur-rent (peak value) as a function of the modulation frequencyfor 1 Volt modulation amplitude of the power supply inputsignal and DC currents of 30 A and 60 A, respectively. Inboth planes a magnet current of 1 A corresponds to a tunechange ∆Q of about 5 · 10−3.The tune modulation input signal is provided by a PC withDATEL PC-420 [5] wave generator board, remotely con-trollable from the HERA control room. This board is capa-ble of generating an arbitrary, continuous, periodic signal.0-7803-7191-7/01/$10.00 ©2001 IEEE. 1699Proceedings of the 2001 Particle Accelerator Conference, Chicago00.10.20.30.40.50.60.70.80.90 10 20 30 40 50 60 70 80 90 100magnet current/Afrequency/Hzmagnet current/AFigure 1: Measured AC magnet current as function of fre-quency for an input voltage of 1 V and two DC magnet cur-rents of 30 A (lower) and 60 A (upper curve), respectively[4]. The lines have been introduced to guide the eye.3 EXPERIMENTSDuring the dedicated tune modulation experiments aswell as during regular luminosity operation the tunes wereset to Qx = .297 horizontally and Qy = .292 verti-cally. Therefore the closest resonances of even order areQx,res = 0.3 = 3/10 and Qy,res = 0.286 = 4/14. Tak-ing into account a total beam-beam tune shift of roughly∆Qx = 1 · 10−3, ∆Qy = 2.5 · 10−3 (the negativesign occurs due to positron-proton operation) and a revo-lution frequency frev = 47.3 kHz, the resonance condi-tion is therefore fulfilled for a tune modulation frequencyof fmod ≈ 100Hz horizontally, and fmod ≈ 280Hz ver-tically. Here we have taken into account the betatron fre-quencies of particles in the transverse beam tails rather thanthe beam core.The effect of tune modulation on the beam halo was studiedin two steps. First a single target wire (Inner I) was movedinto the beam halo until an interaction rate of 10 MHz wasreached. After retracting the wire by 100µm the target po-sition control was switched off, leaving the wire positionconstant. After some decay time a lower stable interactionrate of roughly 0.3 MHz was reached.When the tune modulation was switched on, the new stableinteraction rate was recorded. Figure 2 shows the achievedinteraction rates for different tune modulation parameters,withU(t) =∑iUi · sin(2πfit) (2)being the power supply AC input voltage that translates intoAC magnet current according to Figure 1. For all measure-ments, the amplitude of all frequency components U i ofthe input signal was equal, but due to the frequency de-pendency of the output current this was not the case in theresulting AC magnet signal.As Figure 2 shows, the maximum interaction rate wasachieved by simultaneous application of two modulationfrequencies, namely 190 Hz and 10, 20, or 40 Hz. At anAC magnet current of 1.37A, an interaction rate of 23resp. 30MHz could be achieved. When the 190Hz tunemodulation component was switched off, the interactionrate significantly reduced to 0.35 resp. 0.6MHz, though therelative reduction in the total AC peak current to 1.25Awas much smaller. When the 190Hz component wasturned on again at a reduced amplitude for both compo-nents, the resulting interaction rate of 0.9MHz was signif-icantly larger than with 10Hz only even for a lower to-tal AC magnet current of 1.05A. Furthermore Figure 2shows that the relation between total tune modulation am-plitude and resulting HERA-B interaction rate is even bet-ter when the 10Hz component is replaced by a 20Hz or40Hz signal, while keeping the 190Hz component. Butsince these frequencies are already strongly suppressed dueto the large magnet inductance the maximum achievable in-teraction rate is still much smaller than in the case with the10Hz component. In a later dedicated HERA run in August10-11100 0.5 1Figure 2: Interaction rates with fixed wire for different tunemodulation parameters. For each setting, the input signalvoltages for each frequency component are identical, buttheir sum might be varied. Due to the strong frequencydependency of the magnet power supply output current, theresulting total tune modulation amplitude is different fordifferent frequency parameter sets.2000 the effect of tune modulation on HERA-B interactionrate stability was studied in detail. For this purpose thewire position feedback was switched on and a single targetwire was moved towards the beam until a stable interactionrate of 20MHz was reached. To avoid artificial effects dueto the identical 10Hz frequencies used for interaction ratedata sampling as well as tune modulation, a slightly dif-ferent tune modulation frequency of 8Hz resp. 12Hz wasused.In a first step a tune modulation signal of 12Hz frequencyand 0.5V resp. 1.0V amplitude was applied to the beam.A second sine wave of equal voltage amplitude was added,the frequency being varied from 100Hz to 500Hz in stepsof 100Hz. Figure 3 depicts the resulting relative rms inter-action rate width σrel as a function of the frequency of this1700Proceedings of the 2001 Particle Accelerator Conference, Chicagosecond input signal for different tune modulation depths.In contrast to the measurements with a fixed target wire00.20.40.60.810 100 200 300 400 500rel. rms width of HERA-B interaction ratefrequency component f2/Hzrel. rms width of HERA-B interaction raterel. rms width of HERA-B interaction rateFigure 3: Interaction rate stability for different tune mod-ulation frequencies and three different amplitudes (+ :0.5V, × : 1.0V, ∗ : 2.0V).position, the interaction rate stability does not exhibit anysignificant dependency on the second frequency f 2. Therate stability depends strongly on the amplitude of the in-put signal, and thus on the resulting tune modulation depth.This was also observed when the 12Hz was replaced by a8Hz signal.When the tune modulation is switched on, the filling patternof the electron beam is clearly reflected in the HERA-B in-teraction rates pe bunch, Figure 4. This is a clear indicationA]µelectron bunch current [0 20 40 60 80 100 120 140 160interaction rate per bunch [kHz]020406080100120140160180200220240Tune Modulation OFFfilled proton bunchespilot punchesnot filled proton bunchesA]µelectron bunch current [0 20 40 60 80 100 120 140 160interaction rate per bunch [kHz]020406080100120140160180200220240Tune Modulation ON filled proton bunchespilot punchesnot filled proton bunchesFigure 4: HERA-B interaction rate per bunch vs. bunchcurrent of the corresponding electron bunch. Note that thelast two proton bunches (“pilot bunches”) in each 60 bunchtrain have no counterpart in the electron beam.that the observed effect is indeed dominated by the beam-beam interaction in combination with tune modulation, andnot just an orbit effect due to dipole fields caused by orbitdistortions in the modulated quadrupoles.In parallel to these experiments the specific luminosity atthe two colliding beam experiments ZEUS and H1 wasrecorded. As Figure 5 shows, these tune modulation stud-ies did not result in any decrease of specific luminosity.Furthermore, no significant increase of experimental back-10-111056722 24 26 28Figure 5: HERA-B interaction rate (top) and specific lumi-nosity during a dedicated run for tune modulation studies(bottom). The luminosity tends to decrease due to slow or-bit drifts, but recovers to the initial value after tuning.ground was observed during several weeks of operation ofthe tune modulation during regular HERA luminosity runs.4 CONCLUSIONIt has been successfully demonstrated that the interactionrate at the internal halo target of the HERA-B experimentcan be significantly stabilized by means of tune modula-tion. Since this method does not result in any detrimentaleffects such as increased background rates or degradationof specific luminosity at the colliding beam experimentsZEUS and H1, it has become a standard procedure in regu-lar HERA runs.5 REFERENCES[1] S. Peggs, Nonlinear Dynamics Experiments, in: A. W. Chao,M. Tigner (eds.), Handbook of Accelerator Physics and Engi-neering, World Scientific, Singapore, 1999[2] L. Evans, CERN 84-15[3] O. S. Bru¨ning, DESY 94-085[4] I. Ludwig, private communication[5] DATEL PC-420 User Manual1701Proceedings of the 2001 Particle Accelerator Conference, Chicago

Transverse beam tail shaping in HERA-p by means of tune modulation

Transverse beam tail shaping in HERA-p by means of tune modulation

Abstract

Similar works

Full text

Available Versions

DESY

DESY Publication Database

CERN Document Server