- Email: [email protected]
Absolute calibration of particle energy at VEPP-4M
Nuclear Instruments and Methods in Physics Research A 494 (2002) 81–85
Absolute calibration of particle energy at VEPP-4M$ V.E. Blinov, A.V. Bogomyagkov, S.E. Karnaev, V.A. Kiselev, B.V. Levichev, E.B. Levichev, O.I. Meshkov, S.I. Mishnev, A.I. Naumenkov, S.A. Nikitin*, I.B. Nikolaev, V.G. Popov, A.A. Polunin, I.Ya. Protopopov, D.N. Shatilov, E.A. Simonov, Yu.A. Tikhonov, G.M. Tumaikin BINP SB RAS, Acad. Lavrentiev Prospect 11, Novosibirsk 630090, Russia
Abstract We have started a new series of experiments on a precise measurement of J=C-, C0 - mesons and t-lepton masses at VEPP-4M collider with KEDR detector. Features of the method used for an absolute particle energy calibration based on the resonance depolarization are described. r 2002 Elsevier Science B.V. All rights reserved. PACS: 13.65; 29.20 Keywords: Electron–positron collider; Polarized beams; Resonance depolarization; Polarimeter; Intra-beam scattering
1. Introduction The most effective technique for a high-accuracy calibration of the particle energy in electron– positron storage rings is the method of resonance depolarization (RD) based on the measurement of the particle’s spin precession frequency in the guiding magnetic field. For the first time, RD was proposed and tested at VEPP-2 (Novosibirsk) in 1970 [1]. In 1975 the following development of RD was taken at the VEPP-2M collider [2,3]. Later it was successfully applied there for experiments on the high-precision measurements of the f-meson mass [4] and K 7 -meson mass [5]. In the beginning $ This work was supported in part by the Russian Fund of Basic Research N 01-02-17477. *Corresponding author. E-mail address: [email protected] (S.A. Nikitin).
of the 1980s a series of similar experiments were performed at VEPP-4 in respect of J=C-, C0 -, U-, U 0 - and U 00 - meson masses [6,7]. Recently, the detector KEDR [8] began its operation at the VEPP-4M collider, the modernized VEPP-4. We have started the experiment in which the accuracy of J=C and C0 mass determinations by the spin precession frequency will be significantly improved in comparison with the average world one. This experiment is an essential preliminary to the high-precision measurement of the tau lepton mass which we plan to perform using RD immediately in the vicinity of the tau production threshold ð1780 MeVÞ [9]. To use RD, one must have an opportunity to obtain the polarized beams, to observe their polarization in the storage ring as well as to produce a forced depolarization in an external spin resonance with the help of an oscillating e.m. field
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 1 4 4 9 - 3
82
V.E. Blinov et al. / Nuclear Instruments and Methods in Physics Research A 494 (2002) 81–85
(the TEM wave or the longitudinal magnetic field) whose frequency is scanned. At present we have more advanced systems to control the beam parameters, to observe the polarization and to produce the resonance depolarization as compared with the 1980s. All this permits us to obtain polarized beams in VEPP-4M with a high reliability and to calibrate the beam energy with an accuracy of one order better than that achieved in our early experiments at J=C and C0 peaks. In addition to indirect methods [10], we can directly study the long-term stability of VEPP-4M and partial terms of its elements to the energy variation.
2. Obtaining polarized beams At the energy range of J=C and C0 peaks, the VEPP-3 booster storage ring served formerly as a source of polarized particles for VEPP-4 and does it now for VEPP-4M (Fig. 1). Owing to the Sokolov– Ternov radiative mechanism the electron/positron beams become polarized in VEPP-3 with the design characteristic time tp E80 min at E ¼ 1550 MeV
ðJ=CÞ and tp E30 min at E ¼ 1840 MeV ðC0 Þ: A very large polarization time in VEPP-4M ðB102 hÞ does not allow one to obtain polarized beams immediately in the main ring. The operation time allotted for the polarization process is E2tp ; through this time, the special automatic control system holds the betatron tunes at VEPP-3 to be removed sufficiently from the nearest dangerous depolarizing spin resonance, determined by a combination of the spin and betatron frequencies. Because of a 3D spin evolution in the injection beam-line from VEPP-3 to VEPP-4M, the resulting vertical projection of the polarization vector differs from unity. As a consequence, the design maximal extent of polarization for injected electrons (P ) and positrons (Pþ ) does not exceed P ¼ 0:82; Pþ ¼ 0:85 at the injection energy E ¼ 1550 MeV and P ¼ 0:88; Pþ ¼ 0:54 at E ¼ 1840 MeV:
3. Touschek polarimeter The various methods for measurement of electron/positron beam polarization in storage
Scintillation Counters
DC Kicker
e
e Striplines
Polarized beams
VEPP-4M
45
DC
R=
BoosterPolarizer VEPP-3
.5 m
Pump
ITP ABS
8m R=
Separator Plates
Detector KEDR Fig. 1. Polarization devices at VEPP-4 complex.
V.E. Blinov et al. / Nuclear Instruments and Methods in Physics Research A 494 (2002) 81–85 Mon Feb 25 13:02:19 2002 Run 417
PNT6 1549.2
2 1 0 -1
0
200
400
E = (1547.649+-0.001)MeV
600
800
1000
0
1
1200
0
419341
419351
419361
419372
419381
419411
419421
419431
419442
419452
419461
419471
419481
419491
419501
419511
419520
419531
0
0
-5
419541
-4
419391
-3
419401
Chi2 / ndf = 23.84 / 23 Td = 613.4 ± 5.865 Delta = 3.723 ± 0.3096 Const = -3.334 ± 0.2247 Decline1 = 0.002479 ± 0.0006419 Decline2 = 0.0004397 ± 0.0005167
-2
0
rings were developed and used in BINP (Novosibirsk) during the last 30 years [3,11,13]. The most effective of them as applied to the low-energy range of VEPP-4M including J=C; C0 peaks as well as the tau production threshold is the observation of spin dependence in the intra-beam (Touschek) scattering. The cross-section of mutual scattering of electrons in the polarized beam is slightly less than in the unpolarized one. To observe the polarization of an extension P; one needs to depolarize the beam using an external spin resonance and determining the fact of depolarization by the jump pP2 in the counting rate of scattered beam particles [3]. The polarimeter device ‘‘Depolarizer-Counters’’ (DC) with a given title ‘‘Karakatitsa’’ is installed in the technical straight sections of VEPP-4M (Fig. 1). It is based on a cylindrical section of vacuum chamber of length 830 mm: There is a pair of scintillation counters at both ends of this section. Counters of each pair can be moved from opposite sides of the chamber inward the aperture in the horizontal plane and register the electrons scattered at the most part of the ring. Since the trajectories of Touschek pair electrons lie symmetrically on each side of the closed orbit, the twofold/four-fold coincidence circuits for the registration of pulses from counters are used. Simultaneously, a number of coincidences are measured for events in one of the pair counters delayed by the revolution period in reference to events in another. The result is subtracted from the total number of coincidences to decrease the uncorrelated background influence. To exclude the influence of changes in beam sizes, the closed orbit variations as well as the beam lifetime fluctuations, the method of ‘‘two bunches’’ is applied. The quantity 1 N2 =N1 is under observation where N1 and N2 are, respectively, the counting rates of the polarized bunch and the unpolarized one spaced at one-half turn. The numbers of particles in the bunches are equalized with an accuracy of a few percent by knocking surplus particles with the help of the inflector. Under positioning of counters at a distance of 1–1:5 cm from the beam, the counting rate in experiments makes up about 3–10 kHz at a beam current of 2–4 mA with a jump of 3–3:5% in
83
1400
Fd = (419458.6+-1.6)Hz
Fig. 2. Jump in 1 N2 =N1 (%) during the scan of the depolarizer frequency. The abscissa is the time in seconds.
good agreement with the calculation (at P ¼ 80%). In experiments at J=C energy range the typical jump is 10 times as a statistical error for 50 s (Fig. 2). We plan to use one more DC as well as a new polarimeter with the internal polarized target (ITP) installed in VEPP-3 for the experiment with t-lepton [9].
4. Depolarizer The two matched striplines of the VEPP-4M kicker (see Fig. 1) with a vertical gap of 80 mm are used to create the TEM wave moving towards the beam. The signal source is the frequency synthesizer controlled by a computer with a minimal bandwidth of Dfd of the order of a few Hz and the minimal rearrangement step of 1 Hz: For VEPP4M, 1 keV in the beam energy scale corresponds to 1:85 Hz in the depolarizer frequency fd : At J=C energy, the synthesizer frequency is scanned in the vicinity of a half revolution frequency f0 (about 400 kHz), i.e. a noninteger part of the spin precession frequency. A stability of f0 is B108 that ensures the energy stability better than 106 : The power wide-band amplifier can provide an amplitude of the voltage across striplines up to Ud E400 V: The rate t1 d of forced depolarization with the transverse field crucially depends on the absolute value of the spin response function jF n j [3]
V.E. Blinov et al. / Nuclear Instruments and Methods in Physics Research A 494 (2002) 81–85
at the place of the depolarizer location: 2 n2 t1 d pUd jF j =Dfd : At VEPP-4M, the design depolarization time td is about 2 s at E ¼ 1550 MeV with Ud E15 V; Dfd E4 Hz; jF n j2 ¼ 130: The typical depolarizer parameters in the new J=C experiment are as follows: the rearrangement step in the frequency ¼ 2 Hz; the bandwidth (due to modulation at the frequency fm ¼ 2 Hz) Dfd ¼ 4 Hz; the voltage amplitude ¼ 12 V; the average rate of the frequency rearrangement ¼ 0:2 Hz=s: The corresponding accuracy in the current energy determination is dEE72 keV: For comparison, the depolarizer device at VEPP-4 in the 1980s had a minimal band width of the order of 100 Hz (dEB750 keV). Near the t-lepton threshold energy the plates of the orbit electrostatic separator (Fig. 1), having at this energy range a crucially stronger factor jF n j2 compared with that of the kicker, as well as the longitudinal magnetic field are assumed to be used for RD [9].
2
1
dE /E * 10-4
84
0 -2.5
-1.5
-0.5
0.5
1.5
2.5
-1
-2
Corrector Current, A Fig. 3. Energy effect of one of the X -correctors in arcs (ZX ¼ 80 cm; the deflection efficiency C ¼ 0:55 mrad=A—the circles) and one in the straight section (ZX ¼ 31 cm; C ¼ 0:28 mrad=A—the squares).
6. Effect of orbit correctors 5. Field pulsations In the first RD experiments at VEPP-4M we faced an anomalously wide depolarization band D ¼ fdðupÞ fdðdownÞ E200 Hz (about 100 keV in energy units) where fdðupÞ and fdðdownÞ are respectively, the depolarization frequencies measured under scanning ‘‘upward’’ and ‘‘downward’’ of the depolarizer frequency. The spin precession line width due to the nonlinearity of the guiding magnetic field is of the order of 1 Hz and below, and the depolarizer line width is a few Hz. It has been found that a reason for the anomaly is the appearance of relatively large B50 Hz parasitic pulsations in main magnetic elements with an amplitude of 50 ppm in the mean field. This gave rise to a possibility to depolarize the beams at the side multiple lines due to the 50 Hz modulation of the spin frequency. We have applied the special circuit to suppress the pulsations in the feed system for the main field. As a result, the pulsations have dropped down to B2 ppm; the value of D has decreased to a level of 10–30 Hz (5–15 keV) and is determined by the energy drift during the time between RD runs.
Manipulating the horizontal closed orbit correctors may strongly affect the energy [10,12]. A single X -corrector creating the deflection angle y changes the energy by the relative value DE=E ¼ yZX =aL; where ZX is the radial dispersion function at the azimuth with this corrector, a is the momentum compaction and L is the perimeter. This effect has been demonstrated in the recent RD experiments at VEPP-4M. Fig. 3 shows the design (lines) and measured (squares and circles) energy deviation versus the current in coils of two X -correctors which differ in signs of the dispersion function. From the statistical model of Nc1 deflections, an estimate follows that DE=EB105 at the variation in rms orbit distortions of B100 mm for VEPP-4M [12].
7. Energy drift We have studied the long term stability of VEPP-4M keeping it without any controlled changes and calibrating the energy by RD at regular intervals of B1:5 h during 2 days. The observed energy drift (see Fig. 4) of a few tens of
Measured Beam Energy, MeV
V.E. Blinov et al. / Nuclear Instruments and Methods in Physics Research A 494 (2002) 81–85
85
tions of a monotonic behavior of the NMR data and variations in the rms orbit distortion o100 mm during a time interval between two energy measurements, the assured accuracy of the interpolated value of the particle energy is about 5–10 keV:
1845.97 1845.96 1845.95 1845.94 1845.93 1845.92 1845.91 9
21
33
45
57
Acknowledgements
Time, hrs
Fig. 4. Variations of the measured energy during the ‘‘longterm stability’’ run.
keV per day may be explained by the variations in temperature conditions at VEPP-4M depending on temperatures of the environment and the water cooling system. The measured drift makes up about 80 keV=1C change fixed by temperature sensors at the magnets. The variations in excess of 105 are correlated with a temperature drift of NMR data in the reference magnet.
8. Energy gap The tentative measurement of a difference between electrons and positrons in the energy has shown that the gap does not exceed a value of 10 keV which is needed to be refined. Contribution of the orbit separation bump in the technical section to the energy gap is negligible.
9. Resume Accuracy of the current energy determination of about 72 keV has been achieved. Under condi-
We thank E. Shubin, V. Cherepanov and A. Karpov for technical support in using the depolarizer device.
References [1] Yu. Shatunov, G. Tumaikin, private communication. [2] A.D. Bukin et al., Proceedings of the Fifth International Symposium on High Energy and Elementary Particle Physics, Warsaw, 1975, p. 138. [3] Ya.S. Derbenev, et al., Part. Accel. 8 (1978) 115. [4] A.D. Bukin, et al., Yadernaya Fiz. 27 (1978) 976. [5] L.M. Barkov, et al., Nucl. Instr. and Meth. B 148 (1979) 53. [6] A.A. Zholents, et al., Phys. Lett. B 96 (1980) 214. [7] A.S. Artamonov, et al., Phys. Lett. B 137 (1984) 272. [8] V.V. Anashin, et al., Nucl. Instr. and Meth. A 478 (2002) 420. [9] V.E. Blinov et al., Proceedings of PAC2001, Chicago, 2001, Vol. 5 (5), p. 3317. [10] M. Sullivan et al., Proceedings of PAC2001, Chicago, 2001, Vol. 5 (5), p. 3570. [11] A.E. Bondar et al., Proceedings of the 12th International Conference on High Energy Accelerators FNAL, Batavia, IL, 1983, p. 240. [12] V.E. Blinov, A.V. Bogomyagkov, V.A. Kiselev, S.A. Nikitin, K.Yu. Todyshev, G.M. Tumaikin, Nucl. Instr. and Meth. A 492 (2002) 68. [13] S.I. Mishnev et al., in: SPIN 2000, Proceedings of the 14th International Spin Physics Symposium AIP, New York, Vol. 570, 2001, p. 892.
Absolute calibration of particle energy at VEPP-4M$ V.E. Blinov, A.V. Bogomyagkov, S.E. Karnaev, V.A. Kiselev, B.V. Levichev, E.B. Levichev, O.I. Meshkov, S.I. Mishnev, A.I. Naumenkov, S.A. Nikitin*, I.B. Nikolaev, V.G. Popov, A.A. Polunin, I.Ya. Protopopov, D.N. Shatilov, E.A. Simonov, Yu.A. Tikhonov, G.M. Tumaikin BINP SB RAS, Acad. Lavrentiev Prospect 11, Novosibirsk 630090, Russia
Abstract We have started a new series of experiments on a precise measurement of J=C-, C0 - mesons and t-lepton masses at VEPP-4M collider with KEDR detector. Features of the method used for an absolute particle energy calibration based on the resonance depolarization are described. r 2002 Elsevier Science B.V. All rights reserved. PACS: 13.65; 29.20 Keywords: Electron–positron collider; Polarized beams; Resonance depolarization; Polarimeter; Intra-beam scattering
1. Introduction The most effective technique for a high-accuracy calibration of the particle energy in electron– positron storage rings is the method of resonance depolarization (RD) based on the measurement of the particle’s spin precession frequency in the guiding magnetic field. For the first time, RD was proposed and tested at VEPP-2 (Novosibirsk) in 1970 [1]. In 1975 the following development of RD was taken at the VEPP-2M collider [2,3]. Later it was successfully applied there for experiments on the high-precision measurements of the f-meson mass [4] and K 7 -meson mass [5]. In the beginning $ This work was supported in part by the Russian Fund of Basic Research N 01-02-17477. *Corresponding author. E-mail address: [email protected] (S.A. Nikitin).
of the 1980s a series of similar experiments were performed at VEPP-4 in respect of J=C-, C0 -, U-, U 0 - and U 00 - meson masses [6,7]. Recently, the detector KEDR [8] began its operation at the VEPP-4M collider, the modernized VEPP-4. We have started the experiment in which the accuracy of J=C and C0 mass determinations by the spin precession frequency will be significantly improved in comparison with the average world one. This experiment is an essential preliminary to the high-precision measurement of the tau lepton mass which we plan to perform using RD immediately in the vicinity of the tau production threshold ð1780 MeVÞ [9]. To use RD, one must have an opportunity to obtain the polarized beams, to observe their polarization in the storage ring as well as to produce a forced depolarization in an external spin resonance with the help of an oscillating e.m. field
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 1 4 4 9 - 3
82
V.E. Blinov et al. / Nuclear Instruments and Methods in Physics Research A 494 (2002) 81–85
(the TEM wave or the longitudinal magnetic field) whose frequency is scanned. At present we have more advanced systems to control the beam parameters, to observe the polarization and to produce the resonance depolarization as compared with the 1980s. All this permits us to obtain polarized beams in VEPP-4M with a high reliability and to calibrate the beam energy with an accuracy of one order better than that achieved in our early experiments at J=C and C0 peaks. In addition to indirect methods [10], we can directly study the long-term stability of VEPP-4M and partial terms of its elements to the energy variation.
2. Obtaining polarized beams At the energy range of J=C and C0 peaks, the VEPP-3 booster storage ring served formerly as a source of polarized particles for VEPP-4 and does it now for VEPP-4M (Fig. 1). Owing to the Sokolov– Ternov radiative mechanism the electron/positron beams become polarized in VEPP-3 with the design characteristic time tp E80 min at E ¼ 1550 MeV
ðJ=CÞ and tp E30 min at E ¼ 1840 MeV ðC0 Þ: A very large polarization time in VEPP-4M ðB102 hÞ does not allow one to obtain polarized beams immediately in the main ring. The operation time allotted for the polarization process is E2tp ; through this time, the special automatic control system holds the betatron tunes at VEPP-3 to be removed sufficiently from the nearest dangerous depolarizing spin resonance, determined by a combination of the spin and betatron frequencies. Because of a 3D spin evolution in the injection beam-line from VEPP-3 to VEPP-4M, the resulting vertical projection of the polarization vector differs from unity. As a consequence, the design maximal extent of polarization for injected electrons (P ) and positrons (Pþ ) does not exceed P ¼ 0:82; Pþ ¼ 0:85 at the injection energy E ¼ 1550 MeV and P ¼ 0:88; Pþ ¼ 0:54 at E ¼ 1840 MeV:
3. Touschek polarimeter The various methods for measurement of electron/positron beam polarization in storage
Scintillation Counters
DC Kicker
e
e Striplines
Polarized beams
VEPP-4M
45
DC
R=
BoosterPolarizer VEPP-3
.5 m
Pump
ITP ABS
8m R=
Separator Plates
Detector KEDR Fig. 1. Polarization devices at VEPP-4 complex.
V.E. Blinov et al. / Nuclear Instruments and Methods in Physics Research A 494 (2002) 81–85 Mon Feb 25 13:02:19 2002 Run 417
PNT6 1549.2
2 1 0 -1
0
200
400
E = (1547.649+-0.001)MeV
600
800
1000
0
1
1200
0
419341
419351
419361
419372
419381
419411
419421
419431
419442
419452
419461
419471
419481
419491
419501
419511
419520
419531
0
0
-5
419541
-4
419391
-3
419401
Chi2 / ndf = 23.84 / 23 Td = 613.4 ± 5.865 Delta = 3.723 ± 0.3096 Const = -3.334 ± 0.2247 Decline1 = 0.002479 ± 0.0006419 Decline2 = 0.0004397 ± 0.0005167
-2
0
rings were developed and used in BINP (Novosibirsk) during the last 30 years [3,11,13]. The most effective of them as applied to the low-energy range of VEPP-4M including J=C; C0 peaks as well as the tau production threshold is the observation of spin dependence in the intra-beam (Touschek) scattering. The cross-section of mutual scattering of electrons in the polarized beam is slightly less than in the unpolarized one. To observe the polarization of an extension P; one needs to depolarize the beam using an external spin resonance and determining the fact of depolarization by the jump pP2 in the counting rate of scattered beam particles [3]. The polarimeter device ‘‘Depolarizer-Counters’’ (DC) with a given title ‘‘Karakatitsa’’ is installed in the technical straight sections of VEPP-4M (Fig. 1). It is based on a cylindrical section of vacuum chamber of length 830 mm: There is a pair of scintillation counters at both ends of this section. Counters of each pair can be moved from opposite sides of the chamber inward the aperture in the horizontal plane and register the electrons scattered at the most part of the ring. Since the trajectories of Touschek pair electrons lie symmetrically on each side of the closed orbit, the twofold/four-fold coincidence circuits for the registration of pulses from counters are used. Simultaneously, a number of coincidences are measured for events in one of the pair counters delayed by the revolution period in reference to events in another. The result is subtracted from the total number of coincidences to decrease the uncorrelated background influence. To exclude the influence of changes in beam sizes, the closed orbit variations as well as the beam lifetime fluctuations, the method of ‘‘two bunches’’ is applied. The quantity 1 N2 =N1 is under observation where N1 and N2 are, respectively, the counting rates of the polarized bunch and the unpolarized one spaced at one-half turn. The numbers of particles in the bunches are equalized with an accuracy of a few percent by knocking surplus particles with the help of the inflector. Under positioning of counters at a distance of 1–1:5 cm from the beam, the counting rate in experiments makes up about 3–10 kHz at a beam current of 2–4 mA with a jump of 3–3:5% in
83
1400
Fd = (419458.6+-1.6)Hz
Fig. 2. Jump in 1 N2 =N1 (%) during the scan of the depolarizer frequency. The abscissa is the time in seconds.
good agreement with the calculation (at P ¼ 80%). In experiments at J=C energy range the typical jump is 10 times as a statistical error for 50 s (Fig. 2). We plan to use one more DC as well as a new polarimeter with the internal polarized target (ITP) installed in VEPP-3 for the experiment with t-lepton [9].
4. Depolarizer The two matched striplines of the VEPP-4M kicker (see Fig. 1) with a vertical gap of 80 mm are used to create the TEM wave moving towards the beam. The signal source is the frequency synthesizer controlled by a computer with a minimal bandwidth of Dfd of the order of a few Hz and the minimal rearrangement step of 1 Hz: For VEPP4M, 1 keV in the beam energy scale corresponds to 1:85 Hz in the depolarizer frequency fd : At J=C energy, the synthesizer frequency is scanned in the vicinity of a half revolution frequency f0 (about 400 kHz), i.e. a noninteger part of the spin precession frequency. A stability of f0 is B108 that ensures the energy stability better than 106 : The power wide-band amplifier can provide an amplitude of the voltage across striplines up to Ud E400 V: The rate t1 d of forced depolarization with the transverse field crucially depends on the absolute value of the spin response function jF n j [3]
V.E. Blinov et al. / Nuclear Instruments and Methods in Physics Research A 494 (2002) 81–85
at the place of the depolarizer location: 2 n2 t1 d pUd jF j =Dfd : At VEPP-4M, the design depolarization time td is about 2 s at E ¼ 1550 MeV with Ud E15 V; Dfd E4 Hz; jF n j2 ¼ 130: The typical depolarizer parameters in the new J=C experiment are as follows: the rearrangement step in the frequency ¼ 2 Hz; the bandwidth (due to modulation at the frequency fm ¼ 2 Hz) Dfd ¼ 4 Hz; the voltage amplitude ¼ 12 V; the average rate of the frequency rearrangement ¼ 0:2 Hz=s: The corresponding accuracy in the current energy determination is dEE72 keV: For comparison, the depolarizer device at VEPP-4 in the 1980s had a minimal band width of the order of 100 Hz (dEB750 keV). Near the t-lepton threshold energy the plates of the orbit electrostatic separator (Fig. 1), having at this energy range a crucially stronger factor jF n j2 compared with that of the kicker, as well as the longitudinal magnetic field are assumed to be used for RD [9].
2
1
dE /E * 10-4
84
0 -2.5
-1.5
-0.5
0.5
1.5
2.5
-1
-2
Corrector Current, A Fig. 3. Energy effect of one of the X -correctors in arcs (ZX ¼ 80 cm; the deflection efficiency C ¼ 0:55 mrad=A—the circles) and one in the straight section (ZX ¼ 31 cm; C ¼ 0:28 mrad=A—the squares).
6. Effect of orbit correctors 5. Field pulsations In the first RD experiments at VEPP-4M we faced an anomalously wide depolarization band D ¼ fdðupÞ fdðdownÞ E200 Hz (about 100 keV in energy units) where fdðupÞ and fdðdownÞ are respectively, the depolarization frequencies measured under scanning ‘‘upward’’ and ‘‘downward’’ of the depolarizer frequency. The spin precession line width due to the nonlinearity of the guiding magnetic field is of the order of 1 Hz and below, and the depolarizer line width is a few Hz. It has been found that a reason for the anomaly is the appearance of relatively large B50 Hz parasitic pulsations in main magnetic elements with an amplitude of 50 ppm in the mean field. This gave rise to a possibility to depolarize the beams at the side multiple lines due to the 50 Hz modulation of the spin frequency. We have applied the special circuit to suppress the pulsations in the feed system for the main field. As a result, the pulsations have dropped down to B2 ppm; the value of D has decreased to a level of 10–30 Hz (5–15 keV) and is determined by the energy drift during the time between RD runs.
Manipulating the horizontal closed orbit correctors may strongly affect the energy [10,12]. A single X -corrector creating the deflection angle y changes the energy by the relative value DE=E ¼ yZX =aL; where ZX is the radial dispersion function at the azimuth with this corrector, a is the momentum compaction and L is the perimeter. This effect has been demonstrated in the recent RD experiments at VEPP-4M. Fig. 3 shows the design (lines) and measured (squares and circles) energy deviation versus the current in coils of two X -correctors which differ in signs of the dispersion function. From the statistical model of Nc1 deflections, an estimate follows that DE=EB105 at the variation in rms orbit distortions of B100 mm for VEPP-4M [12].
7. Energy drift We have studied the long term stability of VEPP-4M keeping it without any controlled changes and calibrating the energy by RD at regular intervals of B1:5 h during 2 days. The observed energy drift (see Fig. 4) of a few tens of
Measured Beam Energy, MeV
V.E. Blinov et al. / Nuclear Instruments and Methods in Physics Research A 494 (2002) 81–85
85
tions of a monotonic behavior of the NMR data and variations in the rms orbit distortion o100 mm during a time interval between two energy measurements, the assured accuracy of the interpolated value of the particle energy is about 5–10 keV:
1845.97 1845.96 1845.95 1845.94 1845.93 1845.92 1845.91 9
21
33
45
57
Acknowledgements
Time, hrs
Fig. 4. Variations of the measured energy during the ‘‘longterm stability’’ run.
keV per day may be explained by the variations in temperature conditions at VEPP-4M depending on temperatures of the environment and the water cooling system. The measured drift makes up about 80 keV=1C change fixed by temperature sensors at the magnets. The variations in excess of 105 are correlated with a temperature drift of NMR data in the reference magnet.
8. Energy gap The tentative measurement of a difference between electrons and positrons in the energy has shown that the gap does not exceed a value of 10 keV which is needed to be refined. Contribution of the orbit separation bump in the technical section to the energy gap is negligible.
9. Resume Accuracy of the current energy determination of about 72 keV has been achieved. Under condi-
We thank E. Shubin, V. Cherepanov and A. Karpov for technical support in using the depolarizer device.
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