COULOMB ’05

Experiments with Cooled Beams at COSY

A.Lehrach, H.J.Stein, J. Dietrich, H.Stockhorst, R.Maier, D.Prasuhn, V.Kamerdjiev, COSY, Juelich,

I.Meshkov, Yu.Korotaev, A.Sidorin, A.Smirnov, JINR, Dubna

Contents

1. Introduction: Electron cooling at COSY

2.“Electron heating”

3. Coherent instability 4. Ion cloud in an electron cooling system

COSY Accelerator FacilityIons: (pol. & unpol.) p and d

Momentum: 300/600 to 3700 MeV/c for p/d, respectively

Circumference of the ring: 184 m

Injection: 45 MeV H-, D- stripping injection Intensity 8 mA: 1011 protons coasting beam

Electron Cooling at injection

Stochastic Cooling above 1.5 GeV/c

4 internal and 3 external experimental areas

COSY Electron Cooling systemDesign valuesCooling section length 2 mElectron current 4 ABeam diameter 2.54 cmEnergy 100 keV

Normal operationEnergy 25 keVCurrent 100 – 250 mAMagnetic field 800 G

Applications

1. On-turn extraction using diagnostics kicker (JESSICA)

2. Increase of the beam quality for slow extraction (TOF)

3. Increase of polarized beam intensity (cooling-stacking)

Beam shrinks and decays

Typical graphs at injection in COSY

The dependence on time

(a) neutrals generation rate and

(b) proton beam intensity (1.275·1010 protons/div).

Initial losses

“Coherent”losses

2. «Electron heating»

«Measurements of electron cooling and «electron heating» at CELSIUS» D.Reistad et al.

Workshop on Beam Cooling, Montreux, 1993

In presence of the electron beam the ion beam lifetime is much shorter:

50 - 100 sec without electron beam

0.5 - 1 sec at electron current of 100 mA

COSY, detuned electron beam

0

0.05

0.1

0.15

0.2

0 25 50

Time, sec

Be

am

cu

rre

nt

sig

na

l, V

0

2

4

6

8

10

0 50 100 150 200 250 300

Electron current, mА

Bea

m li

fetim

e, s

ec

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.05 0.1 0.15

Ion beam current (relative units)

Lo

ss

rat

e, s

ec^

-1

Ie = 0Ie = 45 mAIe = 98 mA Ie = 243 mA

r I-0.5

N

Q = const

Equilibrium beam emittanceAt small intensity

equilibrium between electron cooling and IBS leads to

N0.6

H0 profiles

At large intensityHeating by high order resonances

Nonlinear field of the electron beam

CELSIUS:Ion beam cross-section 70 x 58 mmelectron beam diameter 20 mm

COSY:Ion beam cross-section 40 x 75 mmelectron beam diameter 25.4 mm

Two-beam instability

V.Parkhomchuk, D.Pestrikov, Coherent instabilities at electron cooling, Workshop on Beam Cooling, Montreux, 1993

3.Coherent instability at COSY

Single injection in COSY

Ip(t)

H0(t)

Initial losses

Coherent oscillation start(no losses!)

Oscillations “jump” (see next slide)

Coherent instability development

1 injection (t = 0),

2 horizontal betatron oscillations start (t=8 s),

3 “jump” to vertical oscillations (t = 16 s), tjump< 0.5 s

1 (t = 0)

2 (t=8 s)

3 (t = 16 s)Qx = 3.62Qy =3.66

H. Stockhorst5.Coherent instability

COSY: Sextupole correction

“Standard” setting

of sextupoles

Optimised setting of sextupoles

As result of correction accelerated beam increased in two times

Qx=3.609, Qy=3.694 x=−2.8, y=0.3

Qx=3.598, Qy=3.636 x=−2.4, y=−0.6

Schottky Spectrum

Instability suppression

Feedback system:

LEAR: (CERN) bandwidth 500 MHz - 81010 protons

COSY: bandwidth 70 MHz - 1011 stored protons

Variation of electron beam energy, CELSIUS:

Most effective square-wave modulation

50 V amplitude at 115 keV electron beam energy

“Hollow beam”,

Measuring a hollow electron beam profile,  A. V. Bubley,

V. M. Panasyuk, V. V. Parkhomchuk and V. B. Reva,

NIM A 532 (October 2004)

P. Zenkevich, A. Dolinskii and I. Hofmann Dipole instability of a circulating beam

due to the ion cloud in an electron cooling system, NIM A 532 (October 2004)

E.Syresin, K.Noda, T.Uesugi, I.Meshkov, S.Shibuya,Ion lifetime at cooling stacking injection in HIMAC,

HIMAC-087, May 2004

4. Ion cloud in an electron cooling system

“Natural” neutralization

a

b

c

IU ln21

Potential at the electron beam axis

Neutralization level due to variation of the vacuum chamber radius

a

bb

b

neutr2

1

2

ln21

ln2

Neutralization measurements

Vacuum chamber radiusAt gun and collector 3.25 cmAt cooling section 7.5 cm

Natural neutralization 34-37%

Potential depression by space charge45V/100mA (theo.)30V/100mA (meas.)

Control of the neutralization level

“Shaker” – resonant excitation of the ion oscillations

Trapped residual gas ions oscillate in the solenoid magnetic field and electric field of the electron beam:

2/4/1 22BBneutri

pB Am

ZeB

p

ei Am

nZe

2

22

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

0 50 100 150 200

Shaker frequency, KHz

Re

volu

tio

n f

req

ue

ncy

sh

ift,

Hz

Change of neutralizationleads to the shift

in proton revolution frequency

Ie = 250 mA18 harmonics

0

5

10

15

20

0 50 100 150 200

Shaker frequency, kHz

Ch

an

ge

of

ele

ctro

n e

ne

rgy,

e

V

A/Z of residual gas ions stored in electron beam

0

5

10

15

20

0 50 100 150 200

Shaker frequency, kHz

Ch

an

ge

of

ele

ctro

n e

ne

rgy,

eV

Transverse shaking

Longitudinal shaking

16

40

28

Ions traveling along cooler

Ie = 170 mARevolution frequency shift is compensatedby change of cathode

voltage

CO+

N2+

Xe+

H+

Constant beam

revolution frequency

Non resonance excitation

Shaker is off

Resonance 100-120 kHz Resonance 130-150 kHz

Conclusion1. Electron cooling permits to form ion beams

at high phase space density, however the problems of beam stability specific for electron cooler rings appear.2. First problem relates to interaction of an ion

circulating in the ring with nonlinear field of cooling electron beam.3. Second problem is connected with development of coherent instability in cooled ion beam.4. The threshold of this instability can be reduced when “secondary” ions of residual gas are being stored in the cooling electron beam.

5. The threshold of this instability can be increased when feedback system and control of “the natural neutralization” (with a shaker, for instance) are applied.