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Beam energy measurement and stabilization at the storage ring VEPP-2M
Nuclear Instruments
and Methods in Physics Research A 359 (1995) 419-421
NUCLEAR INSTRUMENTS
&METNoDs IN PNVSCS RESEARCH Section A
ELSEVIER
Beam energy measurement and stabilization at the storage ring VEPP-2M A. Lysenko, I. Koop, A. Polunin, E. Pozdeev
*,
V. Ptitsin, Yu. Shatunov
Budker Institute of Nuclear Physics, Novosibirsk, 630090, Russian Federation
Abstract Adequate use of storage rings as synchrotron radiation sources requires exact knowledge of parameters like beam energy. This article discusses the long term experience of beam energy measurements at the storage ring VEPP-2M.
1. Introduction One of the most important properties of synchrotron radiation is the dependence of the spectrum on the particle energy. About twenty years ago the method of absolute calibration of particle energy in a storage ring was developed in Institute of Nuclear Physics in Novosibirsk. This method is based on the measurement of the spin precession frequency Ll of the polarized electrons, given by:
where w,, is the revolution frequency, C; and /_Lare the anomalous and normal parts of the magnetic moment, respectively, and y is the gamma factor. It may be measured with a resonant depolarization of the beam by an external oscillating electromagnetic field. The depolarization is observed by alternation of some process with a probability depending on the initial electron spin state (for example, intra-beam scattering).
2. Beam polarization in BEP
The strongest resonances are: v = n and v = N - v~,~, N is the periodicity number (N,,, = 12, NvEPP_aM= 4). Figs. 1 and 2 show the results of the numerical calculation of the depolarization time for BEP and VEPP-2M (in order to estimate the power of the depolarizing resonances a reasonable strength skew-quadrupole was placed as an imperfection in the unperturbed storage ring lattice). According to calculations, the strength of resonant harmonics ok of resonances v = 4 - v, z and the normal rate of the BEP energy change satisfy, the so-called fast resonance crossing condition [2,3]:
where E is a detuning from resonance. However, it is more difficult to pass the imperfection resonances v= n. To arrange fast crossing of resonances Y = 1 the scheme which shunts the BEP coil is used (BEP bending magnets and lenses are fed consequently and betatron tunes are not changed). It has a decay time of 35 ms and a 40 MeV energy jump. It corresponds to an increase of the crossing rate to ten times. In this way, the region of the VEPP-2M energies measured with the resonant depolarization method was extended from 200 MeV to 650 MeV.
The scheme of the resonant depolarization method applied to VEPP-2M has some special features. To avoid depolarization by spin resonances at VEPP-2M, the beam is polarized because of the Sokolov-Ternov polarizing effect [l] in the damping ring BEP. The polarization time rp for the maximum energy of BEP is less than one hour. When the BEP energy is changed to the VEPP-2M injection energy, a polarized positron beam passes through some spin depolarizing resonances, defined by the general resonant condition: v=n+mu,+kv,+1v,. 02
* Corresponding
author.
Elsevier Science B.V. S.SDf 0168-9002(94)01671-2
03
Fig. 1. Depolarization
04 Energy
(C-v)
05
06
07
time at BEP versus energy.
X. SR FOR METROLOGY
420
A. Lysenko et al. /Nucl.
Instr. and Meth. in Phys. Res. A 359 (1995) 419-421
3. Energy measurement at VEPP3M After the polarization in BEP a positron beam is transferred into VEPP-2M with 1% loss of the polarization degree. Unlike at BEP it is important to choose betatron tunes at VEPP-2M before the energy measurement in order to move away far from resonances u = 4 - v,,,. In VEPP2M the beam is depolarized by the longitudinal oscillation magnetic field. The frequency of the longitudinal magnetic field is slowly changed in the region close to:
37 289
I
370991 400
‘\
j 420
fl-kw,
fd=-g+
Fig. 3. Coefficient
k is an integer and it is equal to 0, 1 or 2 in the real condition. The method of resonant depolarization allows the energy in VEPP-2M to be measured with the highest rational accuracy (lo-‘).
4. Energy stabilization at VEPP-2M There are two main causes for a reduction of the accuracy of the energy calibration and stability at VEPP2M. Firstly, due to the ripple of the magnetic field in the bending magnets the spin precession frequency varies from one moment to another. It reduces the accuracy of the energy calibration. Secondly, the instability of the environment temperature causes optical alignment distortions and energy drift. In order to improve the stability of the magnetic field in the bending magnets, the feeding system of VEPP-2M was altered [4]. An electrical scheme reducing the ripple of the supply current was constructed. Its filter operates in the frequency region from 1 Hz to dozens kHz. Such a system allows ripple to be suppressed to order 10m6. Also it is important to avoid the slow drift of the magnetic field. The applied compensation scheme includes a magnetometer using NMR phenomena; when turned on it computes feedback. It provides the stability of the magnetic field level with an accuracy of lo-‘.
440
Energy
k = E/B
460
480 (WJ)
‘I
500
520
versus VEPP-2M energy.
Also a position monitoring system measuring misalignments of the storage ring elements was produced [4]. This includes invar pivots connecting magnets and lenses with position monitors. It controls misalignments in the range f 0.4 mm with an accuracy of 0.01 mm. The results of the measurements are analyzed by a computer. Thus, it is possible to compensate the beam energy instability caused by the optical alignment distortions. Some results of energy measurements with a fixed magnetic field and various environment temperatures were compared with results of the position monitoring system. Such a comparison allows the thermal coefficient of the energy instability to be determined: 1 AE z E = (1 f 0.1) x 1o-4
2
This is enough; it will stabilize the energy in VEPP-2M during a long time period with a rational accuracy of 10e5.
5. Energy reproducibility at VEPP3M Sometimes it is not necessary to calibrate the energy exactly when an experiment is carried out. If ratio E/B is known, the energy at the storage ring may be reproduced with sufficient accuracy. In order to obtain the ratio E/B many results of the energy calibration with the resonant depolarization method were compared in a wide range with the magnetic field value. Fig. 3 shows the ratio E/B versus the VEPP-2M energy. The observed behavior is explained by the effective shortening of the bending magnets due to the iron saturation when the magnetic field rises. In order to reproduce the energy with the known dependence k(E) = E/B, the magnetic field must be equal to: B = E/k(E).
Fig. 2. Depolarization
time at VEPP-2M versus energy.
The reproducibility of the energy calibration is not worse than 10e4.
at VEPP-2M
without
A. Lysenko et al. / Nucl. Instr. and Meth. in Phys. Res. A 359 (1995) 419-421
References [l] A. Sokolov and 1. Ternov, Dokl. Akad. Nauk SSSR 153 (1963) 1052. [2] Ya. Derbenev, A. Kondratenko and A. Skrinsky, J. Exp. Teor. Phys. 60 (1971) 1216.
421
[3] Ya. Derbenev and A. Kondratenko, J. Exp. Teor. Phys. 62 (1972) 430. [4] Yu. Shatunov et al., Proc. 7th All-Union Particle Accelerator Conf., Dubna, 1981, V.l, p.338.
X. SR FOR METROLOGY
and Methods in Physics Research A 359 (1995) 419-421
NUCLEAR INSTRUMENTS
&METNoDs IN PNVSCS RESEARCH Section A
ELSEVIER
Beam energy measurement and stabilization at the storage ring VEPP-2M A. Lysenko, I. Koop, A. Polunin, E. Pozdeev
*,
V. Ptitsin, Yu. Shatunov
Budker Institute of Nuclear Physics, Novosibirsk, 630090, Russian Federation
Abstract Adequate use of storage rings as synchrotron radiation sources requires exact knowledge of parameters like beam energy. This article discusses the long term experience of beam energy measurements at the storage ring VEPP-2M.
1. Introduction One of the most important properties of synchrotron radiation is the dependence of the spectrum on the particle energy. About twenty years ago the method of absolute calibration of particle energy in a storage ring was developed in Institute of Nuclear Physics in Novosibirsk. This method is based on the measurement of the spin precession frequency Ll of the polarized electrons, given by:
where w,, is the revolution frequency, C; and /_Lare the anomalous and normal parts of the magnetic moment, respectively, and y is the gamma factor. It may be measured with a resonant depolarization of the beam by an external oscillating electromagnetic field. The depolarization is observed by alternation of some process with a probability depending on the initial electron spin state (for example, intra-beam scattering).
2. Beam polarization in BEP
The strongest resonances are: v = n and v = N - v~,~, N is the periodicity number (N,,, = 12, NvEPP_aM= 4). Figs. 1 and 2 show the results of the numerical calculation of the depolarization time for BEP and VEPP-2M (in order to estimate the power of the depolarizing resonances a reasonable strength skew-quadrupole was placed as an imperfection in the unperturbed storage ring lattice). According to calculations, the strength of resonant harmonics ok of resonances v = 4 - v, z and the normal rate of the BEP energy change satisfy, the so-called fast resonance crossing condition [2,3]:
where E is a detuning from resonance. However, it is more difficult to pass the imperfection resonances v= n. To arrange fast crossing of resonances Y = 1 the scheme which shunts the BEP coil is used (BEP bending magnets and lenses are fed consequently and betatron tunes are not changed). It has a decay time of 35 ms and a 40 MeV energy jump. It corresponds to an increase of the crossing rate to ten times. In this way, the region of the VEPP-2M energies measured with the resonant depolarization method was extended from 200 MeV to 650 MeV.
The scheme of the resonant depolarization method applied to VEPP-2M has some special features. To avoid depolarization by spin resonances at VEPP-2M, the beam is polarized because of the Sokolov-Ternov polarizing effect [l] in the damping ring BEP. The polarization time rp for the maximum energy of BEP is less than one hour. When the BEP energy is changed to the VEPP-2M injection energy, a polarized positron beam passes through some spin depolarizing resonances, defined by the general resonant condition: v=n+mu,+kv,+1v,. 02
* Corresponding
author.
Elsevier Science B.V. S.SDf 0168-9002(94)01671-2
03
Fig. 1. Depolarization
04 Energy
(C-v)
05
06
07
time at BEP versus energy.
X. SR FOR METROLOGY
420
A. Lysenko et al. /Nucl.
Instr. and Meth. in Phys. Res. A 359 (1995) 419-421
3. Energy measurement at VEPP3M After the polarization in BEP a positron beam is transferred into VEPP-2M with 1% loss of the polarization degree. Unlike at BEP it is important to choose betatron tunes at VEPP-2M before the energy measurement in order to move away far from resonances u = 4 - v,,,. In VEPP2M the beam is depolarized by the longitudinal oscillation magnetic field. The frequency of the longitudinal magnetic field is slowly changed in the region close to:
37 289
I
370991 400
‘\
j 420
fl-kw,
fd=-g+
Fig. 3. Coefficient
k is an integer and it is equal to 0, 1 or 2 in the real condition. The method of resonant depolarization allows the energy in VEPP-2M to be measured with the highest rational accuracy (lo-‘).
4. Energy stabilization at VEPP-2M There are two main causes for a reduction of the accuracy of the energy calibration and stability at VEPP2M. Firstly, due to the ripple of the magnetic field in the bending magnets the spin precession frequency varies from one moment to another. It reduces the accuracy of the energy calibration. Secondly, the instability of the environment temperature causes optical alignment distortions and energy drift. In order to improve the stability of the magnetic field in the bending magnets, the feeding system of VEPP-2M was altered [4]. An electrical scheme reducing the ripple of the supply current was constructed. Its filter operates in the frequency region from 1 Hz to dozens kHz. Such a system allows ripple to be suppressed to order 10m6. Also it is important to avoid the slow drift of the magnetic field. The applied compensation scheme includes a magnetometer using NMR phenomena; when turned on it computes feedback. It provides the stability of the magnetic field level with an accuracy of lo-‘.
440
Energy
k = E/B
460
480 (WJ)
‘I
500
520
versus VEPP-2M energy.
Also a position monitoring system measuring misalignments of the storage ring elements was produced [4]. This includes invar pivots connecting magnets and lenses with position monitors. It controls misalignments in the range f 0.4 mm with an accuracy of 0.01 mm. The results of the measurements are analyzed by a computer. Thus, it is possible to compensate the beam energy instability caused by the optical alignment distortions. Some results of energy measurements with a fixed magnetic field and various environment temperatures were compared with results of the position monitoring system. Such a comparison allows the thermal coefficient of the energy instability to be determined: 1 AE z E = (1 f 0.1) x 1o-4
2
This is enough; it will stabilize the energy in VEPP-2M during a long time period with a rational accuracy of 10e5.
5. Energy reproducibility at VEPP3M Sometimes it is not necessary to calibrate the energy exactly when an experiment is carried out. If ratio E/B is known, the energy at the storage ring may be reproduced with sufficient accuracy. In order to obtain the ratio E/B many results of the energy calibration with the resonant depolarization method were compared in a wide range with the magnetic field value. Fig. 3 shows the ratio E/B versus the VEPP-2M energy. The observed behavior is explained by the effective shortening of the bending magnets due to the iron saturation when the magnetic field rises. In order to reproduce the energy with the known dependence k(E) = E/B, the magnetic field must be equal to: B = E/k(E).
Fig. 2. Depolarization
time at VEPP-2M versus energy.
The reproducibility of the energy calibration is not worse than 10e4.
at VEPP-2M
without
A. Lysenko et al. / Nucl. Instr. and Meth. in Phys. Res. A 359 (1995) 419-421
References [l] A. Sokolov and 1. Ternov, Dokl. Akad. Nauk SSSR 153 (1963) 1052. [2] Ya. Derbenev, A. Kondratenko and A. Skrinsky, J. Exp. Teor. Phys. 60 (1971) 1216.
421
[3] Ya. Derbenev and A. Kondratenko, J. Exp. Teor. Phys. 62 (1972) 430. [4] Yu. Shatunov et al., Proc. 7th All-Union Particle Accelerator Conf., Dubna, 1981, V.l, p.338.
X. SR FOR METROLOGY