Physics issues of storage ring 

Gang XU

IHEP, Apr. 26, 2006

contents

• Beam-beam effects and luminosity

• Impedance and instabilities

• Optics issues

• commissioning softwares

• Summary

Tune survey with 11mrad crossing angle by Cai’s Code

Tune survey with 11mrad crossing angle(by Zhang’s Code)

• Although there is some difference between Cai’s code and Zhang’s code, the higher luminosity means closer to the half integer

• Even for Qx=0.51, L is just about 60% of L0 according to Cai’s code.

)(

)(m)()1(1017.2)s(cm0

*311-2-

cm

AIkGeVERL

y

bby

According to the formula of L0, decreasing β*y and increasing kb can increase the L0

β*y is by decreasing αp from 0.0235 to 0.0188, σz=1.08cm, β*y=1.2cmThe microwave instability threshold will decrease, HOM heating increase

Kb increase from 93 to 120 even 130bunch spacing 6ns, e-cloud acceptable, parasitic beam-beam effect acceptable, RF power increase 30%, HOM heating increase

So L0 will increase by factor (1.5/1.2)*(120/93)=1.6Lmax=1.6*0.6*L0~L0---1×1033 could be reached

Impedance and instabilities• impedance of pumping screen in IR

Width 4mm ， depth 2mm ， step 6mm ，number 27

Loss factor 2.25×10-4V/pC(1.78V/pC).Z0||=2.4×10-4Ω(0.23Ω)

• Fast valve near SC-cavity

Model in mafia

Loss factor 0.175V/pC.It can not be used in BEPCII

• Impedance In-vacuum wiggler 4W2

the shielding chamber with 1mm gap

Inducing part has been shieldedBut the HOM heating is serious The structure of shielding chamber must be modified

The simulation of the electron cloud in the magnetic fields

Because of the short circumference of the BEPC storage ring, most of the arc regions are occupied by magnets. So it is necessary to study the distribution and the motion of electron cloud in different magnetic fields. A two- dimensional code evoluted from the ECIC was used to simulate the motion, formation, distribution of the electron cloud in drift, dipole, quadrupole, sextupole and solenoid field regions. In dipole magnetic field region without considering the fringe field, the magnetic field is only in vertical direction.

B=By the electrons in the cloud are confined to move in tight vertical helices whose radius is typically a few

microns, and whose cyclotron frequency is f=eB/2m, B=8000Gs, f=22.3GHz. The main consequence of the

cyclotron motion of the electrons is the severe suppression of the horizontal component of the velocity of

the electrons in the cloud.

For the quadrupole magnetic field, B can be expressed by

,1

,

,

1

1

1

x

B

Bk

xkB

ykB

y

y

x

For the sextupole magnetic field, B can be expressed by

2

2 22

2

2 2

,

1 ( ),2

1.

x

y

y

B k xy

B k x y

Bk

B x

In a uniform solenoid field, the magnetic field is only in longitudinal direction, i.e., B=Bz.

Distribution of electron cloud in various kinds of magnetic field (left: antechamber chamber; right: elliptic chamber)

(a: field free region; b: dipole field; c: quadrupole field; d: sextupole field; e: solenoid field with Bz=10Gs )

Electron cloud density in elliptic chamber and antechamber (left: antechamber; right: elliptic chamber)

The uniform solenoid field is the most effective way to confine the photoelectrons. All of the photoelectrons are confined to the vicinity of the vacuum chamber wall. So in the design of the BEPCII, solenoids will be wound on the vacuum chamber of the straight sections with the magnetic field of 30Gs, which is enough to clear the electron cloud in the central region.

Comparing the length of solenoid region in KEKB and BEPCII

The drift length in BEPCII is much shorter than that in KEKB.The occupation of all the drift region in BEPCII will profit to increase the luminosity.

KEKB LER

BEPCII BEPCII

Circumference C （ m ） 3016.26 237.6 237.6

total drift length LD （ m ） 2331.4 49117.87

(free magnetic field region)

solenoid length LS （ m ） 230010.35

(including bellows)

10.35(including bellows)

LD/C 77.29% 20.62% 49.62%

LS/C 76.25% 4.35% 4.35%

LS/LD 98.65% 21.12% 8.78%

• Sorting for bending magnets

before sorting 3~5mm COD

after sorting 0.3~0.5mm COD

• Sorting Quadrupole --- select quadrupoles with better quality for positron ring

• 6.51/5.58 working point

• αp=0.0188

• Spare IR scheme design

• Related issues of Vacuum chamber deformation

0

5

10

15

20

25

0 5 10 15 20 25

x / s x

y/s

y

0

5

10

15

20

25

0 5 10 15 20 25

x / s xy

/sy

Dynamic aperture with/without aliagnment errors for 6.51/5.58

The dynamic aperture is less than 6.53/5.58

-1.5 -1 -0.5 0.5 1 1.5

-0.02

-0.01

0.01

0.02

Using two bending magnets replace the SC-dipolesChanging the polarity of the two magnets to connect SR ring, e+/e- ring

Orbit of the backup IR scheme

dot-dash: SR mode solid: e- ring dotline: e+ ring

parameters Unit SR mode e+/e- ring

束流能量 E GeV 2.50 1.89

注入能量 Einj GeV 1.89 1.89

同步环周长 C m 241.13 237.53

同步环回旋频率 f0 MHz 1.2433 1.2621

谐波数 h 402 396

高频频率 frf MHz 499.8 499.8

高频腔压 Vrf MV 3.0 1.5

每圈同步辐射能量损失 U0 keV 334 121

同步辐射阻尼时间 x/y/e ms 12/12/6 25/25/12.5

束流强度 I mA 250 -

同步辐射功率 P0 (不包括插入件 ) kW 84 -

束流自然能散度 s e0 6.66104 5.21104

动量压缩因子 p 0.0152 0.0224

束流自然均方根束长 s z0 mm 11 -

束流自然发射度 x0 nmrad 123 154

工作点 x/y/s 7.28/5.18/0.034 6.57/5.61/0.0334

横向自然色品 x0 /y0 9.0/8.3 -8.2/-8.5

backup IR scheme design

Related issues of Vacuum chamber deformation

• Physical aperture ok• Impedance ok• E-cloud acceptable• Heating from SR light serious

Acceptable mini-gap : 11mm(15mm designed)

Potential issues: damage by heating, need more attentions

High level application(softwares)

• Optics(done) calculation (beta, tune, dispersion, emittance, bunch length, RF buc

ket-height) match (beta, tune, dispersion) chromaticity correction dynamic aperture and Poincare phase-plot main magnets (main bending, (de)focusing quadrupoles) setting optics record• Collision adjusting(done) IR orbit feedback(in preparation)• Orbit correction(nearly done)• Beam response-matrix based on windows-epics• Others in preparation (transport line optics calculation and match,or

bit adjusting at injection point, injection adjusting(repetition,kicker/suptum setting, filling pattern), orbit bump in ring)

RF phase adjustment panel for longitudinal separation during Injection and collision conditions tuning in the horizontal plane

Collision conditions tuning in the vertical plane and some simulation results during collision tuning in both horizontal and vertical plane

Detailed information during collision tuning in horizontal plane for the elimination of vertical crossing angle at the IP

Collision condition tuning monitored by 8-pole BPM in both horizontal and vertical plane

Control panel for Waist-y* scan

Control panel for the local coupling compensation in the IR. (Anti-solenoid system)

Control panel for the global coupling compensation. (Skew quadrupole system)

Orbit correction

• Measure orbit of ring with good BPMs or with all “not good” BPMs.• Display the measured, calculated, golden, reference, statistical COD,

and the differences between measured and calculated, measured and reference, and statistical and reference COD at each BPM along the ring.

• Display the above CODs or differences of COD’s in different regions (IR/ARCs/RF/INJ) around the ring and any assigned BPM nearby regions.

• Display the max. and rms values of COD around the ring. • Display all the Twiss parameters at every element around the ring.• Calculate the COD with different methods (SVD, MICADO), and diff

erent beam conditions. • Compare the calculated COD with the measured one.• Set the strengths of correctors according to the calculated COD. • Continuous COD correction (CCC) has the orbit corrected every 20

seconds during routine operation.• Save the measured orbit, reference orbit, golden orbit, and the differ

ence between measured and reference orbits.• List the IR related info.

Beam response-matrix(windows-epics)

• Fit quadruple gradient changes that best correct both beta and dispersion functions

• Fit BPM gains and coupling• Fit horizontal and vertical corrector magnet kicks and

coupling• Fit energy change at correctors• Fit skew gradients • Save the results of every iteration to the designated file • The number of iteration and from which to start can be

selected

Functions

Summary

• By decreasing β*y and increasing bunch number, the luminosity could reach 1×1033

• The lattice with β*y=1.2cm, αp=0.0188 and Qx=6.51 still need research

• The 70% physics part of commissioning software has been finished