Transcript Document

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 Facility
Ions: (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 system
Design values
Cooling section length
2m
Electron current
4A
Beam diameter
2.54 cm
Energy
100 keV
Normal operation
Energy
Current
Magnetic field
800 G
25 keV
100 – 250 mA
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)
Typical graphs at injection in COSY
Beam
shrinks and
decays
Initial losses
“Coherent”losses
The dependence on time
(a) neutrals generation rate and
(b) proton beam intensity (1.275·1010 protons/div).
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
Beam current signal, V
0.2
0.15
Ie = 0
Ie = 45 mA
Ie = 98 mA
Ie = 243 mA
0.1
0.05
0
0
25
50
Time, sec
0.14
0.12
8
Loss rate, sec^-1
Beam lifetime, sec
10
6
4
2
0.1
0.08
0.06
0.04
0.02
0
0
0
50
100
150
200
Electron current, mА
250
300
0
0.05
0.1
Ion beam current (relative units)
0.15
Equilibrium beam emittance
At small intensity
equilibrium between electron cooling and IBS leads to
  N0.6
At large intensity
Heating by high order resonances
H0 profiles
r  I-0.5

N
Q = const
Nonlinear field of the electron beam
CELSIUS:
Ion beam cross-section 70 x 58 mm
electron beam diameter 20 mm
COSY:
Ion beam cross-section 40 x 75 mm
electron 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
H0(t)
Initial losses
Coherent
oscillation start
(no losses!)
Oscillations
“jump”
(see next slide)
Ip(t)
Coherent instability development
1 (t = 0)
2 (t=8 s)
3 (t = 16 s)
1 injection (t = 0),
2 horizontal betatron oscillations start (t=8 s),
3 “jump” to vertical oscillations (t = 16 s), tjump< 0.5 s
Qx = 3.62
Qy =3.66
H. Stockhorst
5.Coherent instability
COSY: Sextupole correction
As result of
correction
accelerated
beam
increased in
two times
“Standard” setting
of sextupoles
Optimised setting
of sextupoles
Schottky Spectrum
Qx=3.609, Qy=3.694
x=−2.8, y=0.3
Qx=3.598, Qy=3.636
x=−2.4, y=−0.6
Instability suppression
Feedback system:
LEAR: (CERN) bandwidth 500 MHz - 81010 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)
4. Ion cloud in an electron cooling system
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
“Natural” neutralization
Potential at the electron
beam axis
I 
b
U  1  2 ln 
c 
a
Neutralization level
due to variation of the
vacuum chamber radius
 neutr
Neutralization measurements
Potential depression by space charge
45V/100mA (theo.)
30V/100mA (meas.)
Natural neutralization 34-37%
b2
2 ln
b1

b
1  2 ln 2
a
Vacuum chamber radius
At gun and collector
3.25 cm
At cooling section
7.5 cm
Control of the neutralization level
Trapped residual gas ions oscillate in the solenoid
magnetic field and electric field of the electron beam:
ZeB
2
2
  i 1   neutr    B / 4   B / 2  B 
Amp
2
Ze
ne
2
i 
2 Am p
Revolution frequency shift, Hz
“Shaker” – resonant excitation of the ion oscillations
0
-200 0
50
100
150
200
-400
-600
-800
-1000
-1200
-1400
Change of neutralization
leads to the shift
in proton revolution frequency
-1600
Shaker frequency, KHz
Ie = 250 mA
18 harmonics
Change of electron energy,
eV
Transverse shaking
20
28
H+
15
10
40
16 CO+
Xe+
5
0
0
Constant beam
50
revolution frequency
Change of electron energy, eV
A/Z of
residual gas
ions stored in
electron beam
N2 +
20
100
150
200
Shaker frequency, kHz
Longitudinal shaking
Ie = 170 mA
Revolution frequency
shift is compensated
by change of cathode
voltage
15
10
Ions traveling along cooler
5
0
0
50
100
Shaker frequency, kHz
150
200
Shaker is off
Resonance 100-120 kHz
Non resonance
excitation
Resonance 130-150 kHz
Conclusion
1. 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.