Diagnostics for intense e-cooled ion

beamsby Vsevolod Kamerdzhiev

Forschungszentrum Jülich, IKP, COSY

ICFA-HB2004, Bensheim, October 19, 2004

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Content

• Objects of diagnostics

• What is an electron-cooled ion beam from

the diagnostics point of view?

• Parameters to be measured and

corresponding diagnostic methods.

• Diagnostics for electron-cooled beams,

difficulties and advantages.

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Objects of diagnostics

• Electron beam– High beam power– Beam pipe is inside the solenoid

• Electron-cooled ion beam– Intensities differ in orders of magnitude– High beam density– Small transverse dimensions– Small momentum spread

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• E-beam position• Space charge field

of the e-beam• Current• Temperature• Neutralization

Measured parameters (e-beam)

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Measured parameters (ions)

• Beam current • Position along the orbit• Momentum• Momentum spread• Profile• Emittance• Tune• BTF

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Interceptive methods or not?

• Interceptive methods– Not suitable for a circulating beam (operation)– Any probe will melt when inserted in the dc

electron beam

• Not interceptive methods– Often indirect measurements– Suitable for (high current) rings

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Cooler Synchrotron COSY

- COSY accelerates (polarized) protons and deuterons between 300 and 3700 MeV/c for p 535 to 3700 MeV/c for d

- Kicker extraction, stochastic extraction

- 4 internal and 3 external experimental areas

- Electron cooling at low energy

- Stochastic cooling at high energies

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COSY-Cooler

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COSY-Cooler

Electron energy:Up to 100 kV

Electron current: 0.2 - 3 A

Operating at: 24,5kV

100-250 mA

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e+ trap

Septum

Cooling section

Quadrupole

Collectore-gun

BDetector

e+ source

LEPTA

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LEPTA

Electron gun

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Parameters of e-cooled ion beam

• Small transverse size/emittance

• High density• Small momentum

spread• During e-cooling

the ion beam is dc• Often unstable

Longitudinal Schottky spectra, uncooled and cooled proton beam

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• Pick-ups (at least two) are needed inside the cooling section to measure the position of both beams.– To measure the position of the e-beam

longitudinal modulation must be applied– Large dynamic range of preamplifiers (variable

gain)– Difficulties in mechanical design, bad service

possibilities (COSY, LEPTA…)

Diagnostics in the cooler section

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COSY BPMs

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• Count rate of the particles recombinating in the

cooler section can be used to find optimum alignment of the electron and ion beams and for fine tuning the energy of the electron beam.

• Measurement of the profile of recombination particles (e.g. MWPC) is the easiest way to determine the ion beam profile (only during cooling process)

Diagnostics in the cooler section

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Example of H0-profile measurement at COSY

Calculated from the measured H0- Profiles

Em

itta

nce

[m

rad

]B

eam

radiu

s [m

m]

horizontalvertical

Proton beam current

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• Looking at the signals of the pick-up located in the cooling section in frequency domain gives useful information about residual gas ions oscillating in the cooler section.

• Such a pick-up can be used also as a clearing electrode (experience at COSY, I.Meshkov, A.Sidorin). Applying ac-voltage to the clearing electrodes makes it possible to kick out the trapped ions, provided the frequency corresponds to resonant the frequency of a particular ion species.

Diagnostics in the cooler section

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Space charge field

To measure the space charge parabola of the electron beam a low intensity cold ion beam can be used.

In this case the ion beam is used as a probe which scans the e-beam.

Procedure:Inject ion beam in the machine, cool it, measure the

revolution frequency of the ion beam, make a parallel shift of the e-beam using the cooler magnetic system, measure the frev again, repeat the procedure several times shifting the e-beam in both directions from the initial position.

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Temperature of the e-beam

Longitudinal temperature can be derived from the Schottky spectrum of the cooled ion beam Beam heating effects should be taken into account

Transverse temperature can be measured by the pepper pot method Only in the pulsed mode Requires complex mechanical design

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The idea of T-measurement

The optical analysis of the electron beam temperature, V. Golubev et all., Proceedings of the Workshop on Beam Cooling and Related Topics, 1993.

The electrons move in the longitudinal magnetic field.

Method based on the measurement of transverse Larmor radius

Pulse duration – 20-50 s

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Profile of the ion beam

Can be based on:

• Ionization of residual gas • Laser induced luminescence• Laser induced photo-neutralization• Light radiation of residual gas,

exited by the beam particles • Wire scanner

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Ionization profile monitor

е-

H+

Proton beam

+

Y

X

Detector for ions

Detector for electrons (optional)

If collecting the electrons additional magnetic field is required.

Position sensitive detectors are usually based on the MCPs.

For dense beams MCP life time is a crucial issue.

IPMs are installed in:TSR, SPS, COSY,RHIC…

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IPM at COSYIon

Electrons

MCP in Chevron Geometry

Charge signalAnode

Channels

MCP1

MCP2

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Beam Profiles measured in COSY

Electron cooled proton beam

Profile measurement

The proton beam is not cooled

1,3·109 particles in the ring, 45 MeV.

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Experience with IPM at COSY

• For the W&S anode high amplification factor is necessary – Use of two MCPs in chevron geometry– High electron density in the second MCP– Short life time of the MCPs– Limitations on beam current– Protection screen is installed – Triggering of the MCP power supply is applied

• Using an MCP with a phosphor screen is probably the best way to build a position sensitive detector for IPM

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Laser profile monitor

• Laser induced luminescence (for ions) in connection with laser cooling (ASTRID…)– Watching the light using a camera

• Photo-neutralization for H- beam (LANL, BNL, ORNL…)– A tightly focused laser beam is directed

transversely through the beam, causing photo-neutralization.

– Scanning the ion beam with the laser and simultaneously measure the beam current

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PM based on light radiation of residual gas, exited by the beam

particles

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Spectral analysis of the beam signals

-signals of a pick-up in frequency domain give a lot of information– Exiting the beam und measuring the

betatron frequencies gives the tune– Stability information can be obtained

using the Beam Transfer Function (BTF) method

– Electron cooling improves S/N ratio

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Example of the beam spectrum at COSY

Vertical delta signal

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Vertical BTF

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Transverse stability diagram

Imaginary part[relative units]

Z

Real part[relative units]

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Longitudinal BTF at COSY

600 466.67 333.33 200 66.67 66.67 200 333.33 466.67 6001300

1033.33

766.67

500

233.33

33.33

300

566.67

833.33

1100

1366.67

1633.33

1900

2166.67

2433.33

0,8 mA2,7 mA4,5 mA

Stability Diagram

Real part

Imag

inar

y pa

rt

0

0

For different

proton beam

currents:

0,8 mA

2,7 mA

4,5 mA

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Transverse BTF at COSY

2.01 104

1.41 104

8174.4 2216.6 3741.2 9699

4.66 104

2.41 104

1672

2.08 104

4.32 104

6.57 104Stability Diagram

Real Part[Ohm/m]

Imag

inar

y pa

rt [

Ohm

/m]

Beam current:3,2 mA2,5 mA

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Summary

• Electron cooling gives much better S/N ratios– Schottky diagnostics is a very powerful method– Schottky spectra of an e-cooled ion beam

might be strongly distorted– BTF, longitudinal and transverse– Online BTF measurement should be further developed

• Profiles of an e-cooled ion beam are difficult to measure– Better resolution is needed– Life time of MCPs

• For new machines diagnostic must be planed together with the machine design

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Thank you