VEPP-4M OPERATION IN THE LOW-ENERGY RANGE
E.Levichev for the VEPP-4M team
Budker Institute of Nuclear Physics, Novosibirsk 630090, Russia.
Abstract
The possibility of effective operation of themodified e+e− collider VEPP-4M with the new detector
KEDR in the low-energy range of 1-1.8 GeV isdiscussed. This energy range is unusual for the facility
but it is of interest for measuring the total cross sectionof the e+e− annihilation into hadrons and studying theJ/ψ physics. We plan to perform in the nearest future anexperiment on the high precision measurement of the τlepton mass in the vicinity of the threshold of its
production (1.78 GeV), using the method of theresonance depolarization. Preliminary experimental
data on the luminosity optimization at low energies aregiven and analyzed. The most important for VEPP-4M
problems related with luminosity, such as the singlebeam effect of vertical oscillations self-excitation
depending on the beam current and length, non-linear
beam dynamics, application of two dipole wiggler
magnets of a 1.8 T field for raising the peak luminosity
due to the beam "blow-up" and increase in radiation
damping decrements are under consideration.

1 INTRODUCTION
VEPP-4M [1] is a modernized VEPP-4 collider
that was in successful operation in 1980-1985. Main
goals of the modernization beside installation andcommissioning of the new KEDR detector [2], include:•••

Development of the tagging system for scatteredelectrons and positrons that based on the magnetic
spectrometer and provides precise determination ofthe γγ -invariant mass.
Modification of the final focus quadrupoles toreach β y* = 5 cm.
Design, production and commissioning of twoinsertions in the middle of the collider arcs withadditional set of vertical separators for 2×2 bunches operation.

2 LOW ENERGY LUMINOSITY
2.1 Collective effects
Due to the low radiation damping rates (1/τ ~ 10 s-1)
different collective effects even in the single bunch
operation mode are pronounced. A dominantlongitudinal instability is caused by a rather large
longitudinal impedance (Re(Z||) ≈ 4 kOhm) of vacuum
chamber and parasitic HOM of the five old-type
accelerator cavities (f0 = 181 MHz). Since at the low
energy we need 2 cavities only, the rest 3 were tuned
down for ∆f = 12 kHz (that is equal approximately tothe synchrotron frequency) to introduce artificial
damping of the dipole oscillation mode with the ratethat exceeds the radiation one by a factor of 100.
For the 2×2-bunches mode we are planning toinstall two active third-harmonic cavities to control thesynchrotron oscillation frequency for each bunch
independently.
Vertical Instability
1- BPM reading
2 - PMT readingV
0.500

10.000-0.5002-1.000-1.500
1.90

1.80

1.70

1.60

1.50

1.40

1.30

1.201.10

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00

The maximum designed energy of VEPP-4M is of6 GeV and its circumference is equal to 366 m.
Basically, this facility was intended to study physics ofϒ - meson and two-photon processes. However,
because of the strong interest in J/ψ and ψ′ physics inrecent years, it was proposed firstly to concentrate thefacility efforts in the low energy range of E = 1.5 – 1.8GeV

The low energy region is not typical for VEPP-4Mand special beam studies are required to investigate thebeam behavior and obtain maximum available
luminosity. Just that purpose was emphasized during
two machine runs in 1996-1998 and 2000-till today. A
shutdown in between was mainly due to the KEDR
assembling completion and VEPP-4M latticemodification. Several runs of the collider with thedetector for scanning the J/ψ -energy range werededicated to system testing both at KEDR and VEPP4M.
The results obtained overview of problems anddiscussion of the ways of their curing are presented inthe paper.

SECFig.2.1.1 Observed signal of the vertical instability.

In a transverse plane, a notable beam instability
associate with a low frequency vertical oscillation at abunch current about 2 mA, which provides a significant
vertical beam size increasing and reduces theluminosity. This effect (Fig.2.1.1) looks like the socalled “saw-tooth” instability and acts equally to bothe+e− beams. Despite the origin of this instability is still
unclear we can cure it by increasing twice the radiation
damping with the help of dipole wigglers. Further
investigation of this effect is under way.

2.1 Beam-beam performance
Previously the VEPP-4 peak luminosity was
about 5 × 1030 cm-2s-1 at E =5 GeV. Now for the sameenergy of VEPP-4M we hope to reach 2×1031 cm-2s-1
with the existing optics and 1032 cm-2s-1 with two
superconducting wigglers and β* reduced down to 2.5cm.
At low energy the luminosity reduces
drastically (∝E4) and different problems arise due tothe low damping rates. To improve the luminosity
performance we use a pair of normal-conducting
wigglers with the peak field H = 1.8 T, which allows
one to increase the horizontal emittance by a factor of 4
at 1.5 GeV. A numerical simulation of the beam-beaminteraction with the LIFETRACK code [3] shows thatwith the help of the wigglers we can reach theluminosity L ≈ 1030 cm-2s-1 at the 1×1 mode for 1.5
GeV. Fig.2.2.1 demonstrates the luminosity
improvement due to the wigglers excitation.

The experimental data with the switching-on
wigglers correspond to ξy = 0.046 and the beam
lifetime τ = 1.3 hour. During this run we have foundthat the maximum possible emittance increasing (4times) by the wigglers is not the optimal one from theviewpoint of highest luminosity and optimization of thewiggler parameters will be continued. The reason as it
seems now is the dynamic aperture limitation.
Table 2.2.2: VEPP-4M luminosity (experiment)
Wigglers
Current
Lmax(mA)(cm-2s-1)
Off
1.5
1.6×1029On
3.2
0.7×1030

2.3 VEPP-4M non-linear beam dynamics
The horizontal beam size and energy spreadincreased by the wigglers can cause the current loss if
the dynamic aperture (DA) is rather small. A single
beam non-linear motion for VEPP-4M is driven bystrong chromatic sextupoles. It is known that there are
two main mechanisms of the short-term DA limitation
for the sextupole-driven electron machines. If the tune
point is chosen in the vicinity of the strong sextupole
resonance, the DA is determined by the resonance fixedpoints. Otherwise, small high-order resonances provide
particle diffusion through the overlapped stochastic
layers. The latter is true for the VEPP-4M tune pointthat situates slightly above the half-integer resonance(both for horizontal and vertical motion). The numeric
simulation results (Fig.2.3.1) show high orderresonance islands and the stochastic particle motion.

Fig.2.2.1 VEPP-4M luminosity with- andwithout the dipole wigglers.

Fig.2.3.1 VEPP-4M phase-space plot.

Table 2.2.1: VEPP-4M luminosity (LIFETRACK)(ξx = 0.015, ξy = 0.03)
Wigglers
Current
Lmax(mA)(cm-2s-1)
Off1
1.8×1029On5
1.1×1030

Non-linear beam dynamics experiments havebeen performed at VEPP-4M to measure phase space
trajectories, dynamic aperture and non-linear tune shift.
Experimental set up comprises the single-turn BPM,
horizontal and vertical kickers and movable scrapers.
The measurement of the detuning coefficientsdefined asTable 2.2.1 lists the simulation results while
the experimental ones are shown in Table 2.2.2.

∆Q x = C xx J x + C xy J y ,∆Q y = C xy J x + C yy J y ,where Jij is the action variable yields Cxx = 1.4 mm-1,
Cxy = -1.0 mm-1 and Cyy = -5.2 mm-1. These values areapproximately twice as large as those predicted bymodel simulation, and further investigation of theadditional non-linearity sources is necessary. A
comparison of the measured and estimated dynamic
aperture also shows a discrepancy with a factor of two.
However, insertion the measured values of the detuning
coefficients in the model estimation significantly
reduces the difference between measurement andprediction (Fig.2.3.2).

inside the vacuum chamber. According to estimation,
the relative error of the energy will be about 3×10-5
with the polarization level of ~80%. All elements of thesystem are designed and its manufacturing is inprogress. First experiments are expected in June-July,2001.
Additionally we consider a different possible
scheme for the accurate on-line beam energy
measuring. The method, previously demonstrated at
BESSY-I [5], based on the CO2 laser photons (ε = 0.12eV) back scattered against the VEPP-4M e+e− beams.
To detect the back scattered γ-rays we are planning touse the 500 mm3 HPGe detector that can be calibrated
precisely in the ~1-3 keV region by a radioactive
isotope. Calculation shows that to have the scatteredphotons in the energy range of 1-3 keV, which isconvenient for registration, we need to modify theBESSY-I scheme and insert a laser cavity into thestorage ring vacuum chamber at the angle of ~80° to thebeam axis.

Fig.2.3.2 VEPP-4M dynamic aperture. Line – the initialmodel, dots – the measurement, crosses – the modelwith the measured detuning coefficients.
Two ways of the DA improving were checked
experimentally:••

Re-distribution of the chromatic sextupoles inorder to reduce the main resonances driving terms
allows increasing the DA by a factor of 1.5.
Controlling the detuning coefficients with the helpof octupole magnets. In spite of good simulation
results this method fails in the DA opening andfurther experiments are being planned.

3 PRECISE ENERGY MEASUREMENT
To measure the J/ψ resonance and τ lepton
mass, a precise calibration of the beam energy is highly
desired. We are planning to do that using the electrons
prepolarized in VEPP-3 (E = 2 GeV, τP ≈ 30 min).
Depolarization of the beam in VEPP-4M with the lowamplitude RF-kicker allows one to measure the spin
precession frequency (through the intrabeam scatteringcount rate reading) and hence the beam energy [4]. TheVEPP-4M depolarization time is rather large (≈100hours), so we expect an effective suppresion of the spin
diffusion due to the quantum fluctuation, magnet field
ripple, etc. The registration system includes two pairsof scintillator counters, which can be moved precisely

Fig.3.1 Precise energy measurement with theCompton back-scattering.
Fig.3.1 shows the simulation results for the100 W CO2 laser and 1000 s measuring time.
According to the simulation, the relative energy error
can be as small as 5÷6×10-5.

4 KEDR STATUS
KEDR is a general-purpose detector with a 1 T
solenoid magnetic field for experiments at VEPP-4M inthe energy range of 2E = 2 ÷ 12 GeV. Now all thesystems of KEDR (including the vertex detector, driftchamber, aerogel Cherenkov counters, LKr calorimeter,
superconducting coils, etc.) are assembled andcommissioned. In a test run the magnetic field reached
a 0.7 T level that is quite enough to get the optimalresolution at the VEPP-4M low energy mode. During
this run, the influence of the longitudinal field on theVEPP-4M beam behaviour was studied and different
schemes for the betatron coupling compensation wereconsidered.
To check the KEDR acquisition systems a test
scanning of the J/Ψ meson was performed.
Detector KEDR is equipped with the tagging
system, which provides registration of the scatteredelectrons and positrons in the γγ experiments. In situ
calibration of the tagging system is available with thelaser photon backward Compton scattering. Thetagging range for γ-quanta is 2÷61 % of the e+e− beamenergy with the energy resolution about 1÷3 % of theγ-quantum energy in the whole tagging range.

5 PROBLEMS AND FUTURE PLANS
At present one of the main problems of theVEPP-4M facility is the low efficiency of the positron
injection in the cascade injection scheme: linac (e−, 50MeV) → converter (e+, 8 MeV) → synchrotron (350MeV) → VEPP-3 (up to 2 GeV) → VEPP-4M. To
overcome this disadvantage a new injection facility atBINP now is underway. It will consist of two linacs inseries (300 MeV for e− and 510 MeV for e+e−) and a510 MeV damping ring. A planning efficiency of thisfacility is around 2×1010 e+e−/second.
To inject the beam in the VEPP-4M vacuum
chamber, a 2.5 GeV booster synchrotron is now underconsideration. Main parameters of the synchrotron are
given in Table 5.1.
Table 5.1 Main parameters of the VEPP-4M booster
synchrotron.
Injection energy
Extraction energy
Beam current
Circumference
Repetition rate
Horizontal emittance
Dipoles NoQuads No
Sextupoles No

0.51 GeV
2.5 GeV
200 mA132 m
1 Hz78 nm284016The booster synchrotron is of the usual
racetrack type with 12 FODO cells per arc and two 3m-longdispersion-freestraight
sections
toaccommodate two RF cavities and injection/extraction
equipment. It is planned now to accelerate a 200 mA
maximum current in 50 bunches.

5 ACKNOWLEDGMENT
This article is a brief review of the work doneby many dedicated people at the BINP VEPP-4M

facility. Many special thanks to V.Kiselev, S.Nikitin,
N.Muchnoi, V.Blinov.

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[2] Yu. Tikhonov, NIM A453 (2000) 54.
[3] D.Shatilov, Part. Acc. 52, 65 (1996).
[4] Ya.S.Derbenev et al, Part.Acc. v.9, No.4, pp.247265 (1979).
[5] R.Klein et al, J.Synchrotron Rad.(1998), 5, 392394.