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.