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1、1,High Sensitivity Tune Measurement using Direct Diode Detection,Acknowledgments to the Sponsor and the BIW Committee,2,High Sensitivity Tune Measurement using Direct Diode Detection,Bergoz Instrumentation is celebrating 30 years of service !,3,High Sensitivity Tune Measurement using Direct Diode De
2、tection,Let the money prize be used for important things ,The money prize will be entirely forwarded for supporting therapy of a child,Szymon Janowski, 3 years, suffers from DiGeorge syndrome (DGS),2012 Beam Instrumentation Workshop, April 15-19 2012, Newport News, VA,4,High Sensitivity Tune Measure
3、ment using Direct Diode Detection 2012 Faraday Cup Talk Marek Gasior Beam Instrumentation Group, CERN,Outline: Basics of tune measurement Challenges of the LHC tune measurement History of the development Principles of the direct diode detection (3D) Measurement examples Other development triggered b
4、y the diode detection project,5,High Sensitivity Tune Measurement using Direct Diode Detection,Betatron motion, tune, and resonance,Q betatron tune, betatron wave number q fractional tune, operation point All particles undergo betatron motion, forced by the machine quadrupoles. Betatron oscillations
5、 must not superimpose in-phase on themselves after few revolution periods. In-phase superposition of betatron oscillations leads to a resonant amplitude build-up. Eventually betatron oscillations may not fit into available aperture and beam can be lost. Real life is more complex than that: two motio
6、n planes, horizontal and vertical; coupling between the planes; oscillation amplitude in each plane changes from one location to another according to a function (so called -function); betatron motion with larger amplitudes is nonlinear.,6,High Sensitivity Tune Measurement using Direct Diode Detectio
7、n,Importance of tunes in real life a 2008 LHC startup example,A recipe to start (a circular) accelerator: First you use the BPMs to steer the beam to make first turn You measure the tunes to set them in good values to make the beam circulating. Once beam circulates you can play with machine paramete
8、rs to increase the beam lifetime, including the tunes.,10:20 almost one turn.,10:41 first turn.,20:25 a few turns, large oscillations, maybe a (1/3, 1/2) resonance ?,21:37 Ralph measures the tunes to be indeed on a resonance and corrects them. Then beam circulates.,6,10/09/2008 From the LHC logbook:
9、 “The World watches LHC”,7,High Sensitivity Tune Measurement using Direct Diode Detection,Tune diagram,Resonance condition:,8,High Sensitivity Tune Measurement using Direct Diode Detection,LHC tune diagram,LHC physics tunes: H: 0.31, V: 0.32 The blue dot comes from a real measurement during collisio
10、ns at 4 TeV. LHC injection tunes: H: 0.28, V: 0.31 LHC integer tunes: H: 59, V: 64 LHC tunes must be kept on the design values within 0.001. LHC has a tune feed-back system, i.e. during the ramp the LHC tunes are measured continuously and the readings are used to calculate the necessary corrections
11、applied to the quadrupoles to keep the tunes on the design values.,9,High Sensitivity Tune Measurement using Direct Diode Detection,Tune measurement the principle,Beam betatron oscillations are observed on a position pick-up Oscillations of individual particles are incoherent an excitation needed fo
12、r “synchronization” The machine tune is the frequency of betatron oscillations related to the revolution frequency. Betatron oscillations are usually observed in the frequency domain. With one pick-up only betatron phase advance form one turn to the next is known only fractional tune can be measured
13、.,10,High Sensitivity Tune Measurement using Direct Diode Detection,Excitation price and limits, emittance blow-up,Beam size is defined by the incoherent betatron motion of all particles. Smaller size = more luminosity. Particles have some momentum spread, leading to the spread in their deflection b
14、y the quadrupoles and finally to the spread in the frequency of the betatron oscillations. Protons do not forget: once hit they oscillate (practically) forever. LHC colliding beam size is in the order of 100 m, so the excitation must be kept in the 1 m range. LHC has a sophisticated collimation syst
15、em, with some 100 collimators. Any oscillations significant w.r.t. the beam size are converted by the collimators into beam loss,11,High Sensitivity Tune Measurement using Direct Diode Detection,Tune measurement challenges the time domain view,Linear processing of position pick-up signals Dynamic ra
16、nge problems: Signals related to betatron oscillations are small with respect to the beam offset signals. Even if the beam is centred, the subtraction of the signals from the pick-up opposing electrodes is not perfect. The leakage is in the order of 1-10 % for ns beam pulses. Options to decrease bea
17、m offset signals: centre the beam; centre the pick-up; equalise the signals by attenuating the larger one (electronic beam centring).,12,High Sensitivity Tune Measurement using Direct Diode Detection,Tune measurement challenges the freqnecy domain view,Spectrum envelope is defined by the bunch shape
18、. For a gaussian bunch the spectrum envelope is gaussian as well. The harmonic structure of the beam spectrum is defined by the time beam time structure. Short single bunches give large spectra, with many lines; most often this is the largest challenge for the tune meas. system. The LHC bunch length
19、 (4) is about 1 ns and the corresponding bunch power spectrum cut-off is about 500 MHz. With just one bunch in the machine the revolution spectral lines are spaced by 11 kHz, so by 500 MHz there are some 50 000 of them and some 100 000 betatron lines. In the classical tune measurement method only on
20、e betatron line is observed, so in the LHC case it is only some 10-5 (-100 dB) of the total spectral content. This results in very small signals, requiring low noise amplifiers and mixers, which have small dynamic ranges; they can be saturated even with relatively small beam offset signals. The “ord
21、er of magnitude” estimates for pick-up signals, assuming electrode distance 100 mm and 100 V electrode signals: 1 mm beam offset signal: 1 mm / 100 mm * 100 V 1 V 1 m beam oscillation signal: 1 m / 100 mm * 100 V 1 mV 1 m beam oscillation signal observed at a single frequency: 1 mV * 10-5 10 nV,13,H
22、igh Sensitivity Tune Measurement using Direct Diode Detection,What a young engineer read in the LHC Design Report in 2004,13.7.1 General tune Measurement System This system will allow the measurement of tune via standard excitation sources (single kick, chirp, slow swept frequency, and noise). It sh
23、ould operate with all filling patterns and bunch intensities and be commissioned early after the LHC start-up. Even with oscillation amplitudes down to 50 m, a certain amount of emittance increase will result, limiting the frequency at which measurements can be made. It will therefore probably be un
24、suitable for generating measurements for an online tune feedback system. Dedicated stripline couplers will be mounted on 2-3 m long motorised supports that can be displaced horizontally or vertically with a resolution of 1 or 2 microns. There will be two such supports near Q6 and Q5 left of IR4 and
25、another two at Q5 and Q6 right of IR4. They will measure in one plane only to profit from the high h or v near each quadrupole (typically 400 m). Also mounted on each support will be the resonant pick-up (see Sec. 13.7.3) and a standard warm button BPM dedicated to providing positions for a slow fee
26、dback loop keeping the BPM centred about the beam. This will allow the electrical aperture of the tune coupler to be reduced to measure small position deviations about the closed orbit. Also, taking into account the higher signal level from the coupler design, the dedicated tune pick-ups will have a
27、 much higher sensitivity than the normal closed orbit BPMs for transverse oscillation measurements. If necessary, electronic common mode rejection could also be included in the processing chain. Measurements of individual bunch positions have been requested for this system. When the oscillation ampl
28、itude is sufficiently large, the orbit BPMs can also be used for tune measurement. It will be possible to measure the betatron function and phase advance all around the ring with them. () 13.7.3 High Sensitivity Tune Measurement System The beam is excited by applying a signal of low amplitude and hi
29、gh frequency, fex, to a stripline kicker. fex is close to half the bunch spacing frequency, fb, (for the nominal 25 ns bunch spacing fb = 40 MHz). The equivalent oscillation amplitude should be a few micrometers or less at a -function of about 200 m. A notch filter in the transverse feedback loop su
30、ppresses the loop gain at this frequency, where instabilities are not expected to be a problem. If the excitation frequency divided by the revolution frequency corresponds to an integer plus the fractional part of the tune then coherent betatron oscillations of each bunch build up turn by turn (reso
31、nant excitation). A batch structure with a bunch every 25 ns “carries” the frequency fex as sidebands of the bunch spacing harmonics (i.e. at (N 40 MHz) fex). A beam position pick-up is tuned to resonate at one of these frequencies. By linking the generation of the excitation signal and the processi
32、ng of the pick-up signal in a phase-locked loop (PLL) feedback circuit, the excitation can be kept resonant and the tune can be determined continuously. Emittance growth is controlled by maintaining the excitation level as small as possible, compatible with the required measurement precision and rat
33、e. Since the first derivative of the phase as a function of frequency goes through a maximum at the central value of the tune, this method gives the highest precision for a given oscillation amplitude. The tune values produced could be used as input to a tune feedback loop. It must be emphasized tho
34、ugh that the present design of the system is optimised for luminosity runs with batched beams with 25 ns bunch spacing. The handling of other bunch spacings at multiples of 25 ns should be possible, but the magnitude of the pick-up signal diminishes with increasing bunch spacing. The development of
35、a PLL tune measurement system for the LHC is being done in collaboration with Brookhaven National Laboratory, where a similar system has been installed in RHIC. It is clear that the system will not be operational during the early stages of commissioning the LHC. Indeed, the beams planned for initial
36、 commissioning (single bunch, 43 equally spaced bunches, etc.) are incompatible with this method.,14,High Sensitivity Tune Measurement using Direct Diode Detection,LHC tune measurement according to the LHC Design Report,Two tune measurement systems: general: standard excitation sources (single kick,
37、 chirp, slow swept frequency, and noise); high sensitivity: resonant PLL excitation, optimised for 25 ns bunch spacing and full machine. 3 sets of pick-ups: 2-3 m long stiplines for the general system; resonant pick-ups for the high sensitivity PLL system; button pick-ups dedicated to providing posi
38、tions for a slow feedback loop keeping the tune pick-up centred about the beam. Beam oscillations made with the “standard excitation sources “ with amplitudes in the order of 50 m are considered small. Tune feed-back considered as probably not possible with the general tune measurement system (too l
39、arge excitation necessary). Dealing with the revolution frequency background: striplines on motorised supports which can be displaced in 1 or 2 micron steps; electronic common mode rejection. “The development of a PLL tune measurement system for the LHC is being done in collaboration with Brookhaven
40、 National Laboratory, where a similar system has been installed in RHIC”. “It is clear that the (PLL) system will not be operational during the early stages of commissioning the LHC”.,13.7.1 General tune Measurement System This system will allow the measurement of tune via standard excitation source
41、s (single kick, chirp, slow swept frequency, and noise). It should operate with all filling patterns and bunch intensities and be commissioned early after the LHC start-up. Even with oscillation amplitudes down to 50 m, a certain amount of emittance increase will result, limiting the frequency at wh
42、ich measurements can be made. It will therefore probably be unsuitable for generating measurements for an online tune feedback system. Dedicated stripline couplers will be mounted on 2-3 m long motorised supports that can be displaced horizontally or vertically with a resolution of 1 or 2 microns. T
43、here will be two such supports near Q6 and Q5 left of IR4 and another two at Q5 and Q6 right of IR4. They will measure in one plane only to profit from the high h or v near each quadrupole (typically 400 m). Also mounted on each support will be the resonant pick-up (see Sec. 13.7.3) and a standard w
44、arm button BPM dedicated to providing positions for a slow feedback loop keeping the BPM centred about the beam. This will allow the electrical aperture of the tune coupler to be reduced to measure small position deviations about the closed orbit. Also, taking into account the higher signal level fr
45、om the coupler design, the dedicated tune pick-ups will have a much higher sensitivity than the normal closed orbit BPMs for transverse oscillation measurements. If necessary, electronic common mode rejection could also be included in the processing chain. Measurements of individual bunch positions
46、have been requested for this system. When the oscillation amplitude is sufficiently large, the orbit BPMs can also be used for tune measurement. It will be possible to measure the betatron function and phase advance all around the ring with them. () 13.7.3 High Sensitivity Tune Measurement System Th
47、e beam is excited by applying a signal of low amplitude and high frequency, fex, to a stripline kicker. fex is close to half the bunch spacing frequency, fb, (for the nominal 25 ns bunch spacing fb = 40 MHz). The equivalent oscillation amplitude should be a few micrometers or less at a -function of
48、about 200 m. A notch filter in the transverse feedback loop suppresses the loop gain at this frequency, where instabilities are not expected to be a problem. If the excitation frequency divided by the revolution frequency corresponds to an integer plus the fractional part of the tune then coherent b
49、etatron oscillations of each bunch build up turn by turn (resonant excitation). A batch structure with a bunch every 25 ns “carries” the frequency fex as sidebands of the bunch spacing harmonics (i.e. at (N 40 MHz) fex). A beam position pick-up is tuned to resonate at one of these frequencies. By linking the generation of the excitation signal and the processing of the pick-up signal in a phase-locked loop (PLL) feedback circuit, the excitation can be kept resonant and the tune can be determined continuously. E