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IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4, August 1983 BEAM DIAGNOSTIC INSTRUMENTATION AT CESR* D. Rice, G. Aharonian, K. Adams, M. Billing, G. Decker, C. Dunnam, M. Giannella, G. Jacksgn, R. Littauer, B. McDaniel, D. Morse, S. Peck, L. Sakazaki, J. Seeman, R. Siemann, and R. Talman Cornell University, Ithaca, New York, 14853 Summary --_ _-
We d scribe various beam diagnostic devices in use at CESRP , an 8 GeV electron-positron storage ring operating primarily in the 4.7 to 5.5 GeV bealn energy range. Getting tne last 20% of performance depends to 5 orne extent tuning and on empirical appropriate presentation of various parameters is very important. Several devices are most useful in machjne studies and we describe their operation. The individually regulated quadrupoles' in CESR provide unique opportunities for lattice measurements and calibration of beam position monitors. Performance ---
Optimization ____---_
During routine high energy physics running the CESK operator has available displays of the usual parameters, i.e., betatron and synchrotron tunes, beam current and lifetime, some indication of background rates in the experimental detectors, and beam profile. Betatron tunes are derived from differentially connected pairs of beam position monitor (BPM) buttons (see fig. 1) and displayed on swept frequency type spectrum analyzers. These units (one for vertical, one for horizontal) were selected for their ease of use and digitally controlled receiver which provides the accuracy necessary to properly set the betatron tunes in preparation for colliding beams (to.1 kHz or oQ=i0.00025). Having a separate analyzer for each plane allows the operator to place the target betatron frequency for each in the center, providing a convenient reference point. Several analyzers also permits close monitoring of vertical normal mode (beam-beam) tune split which is very useful for maximizing luminosity. The synchrotron frequency is observed from the horizontal betatron channel. Since the RF field is not changed often and there are no hysteresis effects to worry about, this signal is not watched regularly. VAR. PULSE BEAM ATTEN. , STRETCHER PICKUP BUTTONS LOW PASS FILTER -* QD-----L L-3
Y
+
7-Q
DIFFERENTIAL AMPLIFIER
d& 7
1: AGC
Figure
1
Betatron
Oscillation
Signal
Processor
Beam currents are rneasured using a ferrite transformer placed around a metalized ceramic vacuum chamber. This signal is filtered and digitized with a 16 bit resolution integrating AFEntri;o insure We monatonicity). One of the system mini-computers does the lifetime calculation. The present system can measure a 300 minute lifetime to 5% * Supported by the + Present address:
U.S. National SLAC, Stanford,
Science Foundation CA, 94305
We plan on replacing the ferrite in 30 seconds. pickup with a tape-wound iron core transformer to reduce position sensitivity and to use a dedicated averaging and lifetime micro-computer to do The primary current reference is a calculations. CESR transductor (similar to those used in the quadrupole circuits') placed around tne beam pipe with a DC current calibration wire threading the cores. Since this signal is somewhat noisy it is continually monitored during runs and compared with the ferrite providing very long integrating transformer signal, times. from from signals calculated Luminosity is conventional luminosity monitors at each of the two This information is presented to experimental areas. the operator both as a digital reading and on analog From luminosity, beam current, and knowledge meters. of the IR B functions the linear beam-beam tune shift This value is then displayed paralneter is calculated. both digitally and on a computer generated time Since CESR is generally operating history (figure 2). in the saturated tune shift reyime , the linear tune shift is an excellant performance yardstick, invariant with respect to most accelerator operating conditions. The operator can easily determine when more tuning would likely improve the luminosity. accurate luminosity Unfortunately, obtaining an minutes requires several usually measurement Thus, when possible, the splitting of integration. modes ("X" and "71") from the normal the vertical beam-beam interaction is observed on the spectrum Betatron tunes and skew quads are adjusted analyzer. for maximum tune split. Even the tune split observation requires several and under of averaging to get a clean signal seconds visible. The mode is not some conditions the "I? vertical profile signal from an oscillating mirror radiation scanner looking at visible synchrotron instantaneous feedback to tuning almost provides Since the vertical beam sizes are determined changes. by the beam-beam interaction, and not primarily directly by changes in magnetic elements, the fact that the synchrotron light port is not at the IR does utility. The its significantly compromise not electron and positron scanners are swept by a common signal and the outputs displayed in an X-Y format. While highest luminosity tuning conditions are not directly indicated by this display alone, operators conditions with particular correlate good soon patterns. Figure 2 (next page) shows a single video display available to the operator both current values making of parameters in digital form (right side of display) history of singles rates, tune shift and an 8 hour and electron and positron beam lifetimes. parameter, -Beam Uiagnostic ----~ --Beam Position _---
Hardware ---
Monitors ----
Beam position monitor pickups are installed next to from 4 buttons 2 each quadrupole in CESR. The signals cm in diameter are carried to a 6 pole, single circuit relay by short pieces of RI;-55 cable. A 5/8 coaxial
0018-9499/83/0800-2190$01.OOO1983 IEEE
2191 coaxial inch diameter foam dielectric cable carries the signal up to' 60 meters to one of 6 sets of processing electronics. Here the amplitude is settable adjlrsted by remotely attenuators and amplifiers pulse is digitized and made and the available to the control system computer. The electronics also have an optional RF gate to separate bunches at least 5 nanoseconds apart. The dynamic range of the processor without changing amplifiers or attenuators is approximately 2.5 to 1. This is just adequate to take care of variation in attenuation from the nearest to farthest monitor and also cover the range between amplifier/attenuator steps. Figure 3 below snows the basic system.
appropriate betatron frequency. Its phase locked oscillator is interfaced to the control computer and a around the ring varying program steps torage change in individual quads and measuring t fl e resulting betatron frequency. An alternative method is to stimulate the beam with a few microsecond long kicker and measure the beam position on successive turns after the excitation. The envelope of the measured positions follows the square root of e. We have tried using a dual channel FFT analyzer to measure betatron phase advance between buttons and derive the b function from the phase measurements. However, we found that the BPM processing electronics dependent shift which has an amplitude phase introduces excesgive noise. This technique has been two used to measure BH since measurement by the other methods above is difficult. Measurement of n is accomplished by subtracting two orbits taken at different RF frequencies. Vertical n's in the arcs of less than 10 cm can be detected. Transfer -_-
Figure
2
uning
Paralneter
Display
PULSE STRETCHER
7,
I
I
RF GATE SWITCHABLE ATTENUATORS/ AMPLIFIERS
EPM
Figure
3
Beam Position
AND
Monitor
Signal
HOLD
Path
Function -----
Measurements -------
spectrum A dual channel fast fourier transform been used analyzer has for transfer function measurements. (Since the revolution frequency in CESR is 390 kHz, a heterodyne unit must be used to transfer the frequencies of interest to the D-100kHz input range of the analyzer.) Figure 4 shows several presentations of the same input signal. 4a is the rms power spectrum from the vertical betatron signal. X and n modes are clearly visible in this example, but other peaks are present, potentially causing data to be misinterpreted. 4b and 4c show the transfer function phase and amplitude. The phase shows 180 confirming degree steps coincident with both peaks, that we chose the right ones. The coherence spectrum in 4d is highest when there is a good correlation between the excitation (fed into the reference channel The low coherence of the analyzer) and the response. area nearest the higher frequency (7) mode corresponds to the horizontal betatron frequency.
Since the current in each quadrupole in CESR can be individually varied, the BPM adjacent to each can be calibratea witn respect to the magnetic center of that quad. The current is changed and the movement of the beam l/4 wavelength away is recorded. .The position of the beam in that quad is then moved using a local bump until a cnange in quad current no longer deflects the beam. The BPM next to the quad is now read out and that reading defined to be zero. The result is reproducable to tU.1 mm over a several day period. After the first calibration was done, the residual ripple after a global orbit flattening proceedure was reduced from l-2 mm rms to about 0.5 mm rms.
(0)
(b)
75 ohm resistive terminations which are an integral part of each beam pickup button not only reduce checking reflections but provide a simple means of proper operation of the RF relays in the system. The processing electronics has a resistance measurement capability which perrnits continuity checks and tests for sticky contacts.
(cl (d)
Measurements .-Y -and r- ~---_--__ Since the quads may be adJusted independently, measurement of the r function is straight forward. A tune tracker is available to lock on to the
Figure
4
FFT Analyzer
Outputs
2192 The transfer function capabilities have also been used coherent frequency shifts. longitudinal to measure information is very helpful in Again, the phase response frequencies when the pinning down exact amplitude signal is confused by noise or multiple modes (figure 5).
spectrum analyzer the FFT Used as a single channel advantages over the swept has definite approach Transient changes in betatron tunes, frequency type. such as might be caused by a bad power supply, are much more likely to be seen since it samples all frequencies simultaneously. If averaging is not used, be displayed, approximately 5 spectra per second can giving almost real-time response. Beam Profile
Measurements
In addition to the synchrotron light scanner mentioned other profile measurement techniques above, several Scrapers are available to have been used at CESK. of the transverse distributions. explore the tails Once beam lifetime as a function of position has been beam distribution may be actual measured the Vertical scrapers have been used calculated3. understand some aspects of the effectively to vertical of the beam beam-beam induced enlaryement A more sophisticated variation is to use dimensions '. a low Z material such as beryllinm as a probe and photon flux with a down measure the br mstrahlJng apparatus allows deeper This stream counter. 5 into the beam before the lifetime is penetration affected, as well as a simpler interpretation of the This apparatus and results are described raw data. elsewhere in this conference . A precision measurelnent of vertical profile is being undertaken in the CHESS synchrotron light facility at CESK.6 IJsing monochromated photons, a moving slit, and slices through betatron phase analyzer, final energy While results are preliminery at space may be made. approach is quite useful for detailed this time, this measurement of phase space in the 0 to 5 sigma range. and synchronous The addition of a beam chopper detector is expectea to extend the useful range to 7 making the apparatus very useful for sigma or so, exploring the interesting tails of the beam. ~Tjne
Plane -
Scanning ------
A computerized scan of the tune plane is available to explore potential operating points in a limited area, The program SCAN first steps rapidly through an area looking for and tagginy poor lifetime regions. This avoiding the poor is followed by a slower pass, lifetime regions, where luminosity, singles rates, and video lifetime are measured and presented on a display.
References 1. 2. 3. 4.
B. McDaniel, IEEE NS-28, 1984 (1981) Hartill and Rice, IEEE NS-26, 4078 (1979) iM. Billing, CBN 82-43, Cornell Univ. this conference, paper L-10 Seeman et al.,
