Laser Stripping and H 0 monitor systems 10/18/2011B.Cheymol, E. Bravin, U. Raich, F. Roncarolo...

Laser Stripping and H0 monitor systems

10/18/2011 B.Cheymol, E. Bravin, U. Raich, F. Roncarolo BE/BI 1

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Outline

• Motivation• SNS Laser wire beam profile system• Adaptation to LINAC4 parameters• H0 detection• Outlook

10/18/2011 B.Cheymol, E. Bravin, U. Raich, F. Roncarolo BE/BI

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Motivation

• Emittance measurement required at 160 MeV• A Slit&Grid system (as in LINAC2 ) not suitable for higher

energy.– High thermal load

– Slit thickness > 15 cm => Complex slit geometry

• Possibility of using a laser wire• Can measure full beam power• Non destructive measurement• No space charge effect• Can go to higher intensity, energy, repetition rate• Can achieve high resolution and accuracy


Principle of beam profile and emittance measurement with Laser Wire

• If the laser station is positioned before a dipole magnet, emittance and profile measurement can be done at the same time

10/18/2011 B.Cheymol, E. Bravin, U. Raich, F. Roncarolo BE/BI 4

Profile measurement

Emittance measurement

Stripping

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SNS Laser wire beam profile system

• Used in the Superconducting part of the LINAC for profile measurement.• Stripped electron are detected by a faraday cup.• Q‐Switched Nd:YAG laser (commercial type):

• 30 Hz repetition rate• Maximum energy 1 J• 10 ns beam pulse• Wavelength 1064 nm


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SNS Laser wire beam profile system (* )


Profiles of H- beam along the SCL measured during 2010. The measurement was conducted on full power (1 MW) neutron production beam(*). (beam sizes from 0.9 to 3.5 mm)

* LASER BASED DIAGNOSTICS FOR MEASURING H- BEAM PARAMETERS Y.Liu et al. (PAC 2011 Proceedings WEOCN1)

Measured emittance: ~ 0.2 mm mrad ⋅

200 MeV

1 GeV

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SNS Laser wire beam profile system (* )


* LASER BASED DIAGNOSTICS FOR MEASURING H- BEAM PARAMETERS Y.Liu et al. (PAC 2011 Proceedings WEOCN1)

• Beam power is not reduced => no effect on SNS Neutron production.

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Adaptation to LINAC4


Photoneutralization Cross section

Proposal : use similar Nd:YAG laser in LINAC4

Wavelength in the laboratory frame 1064 nm

In the beam rest frame, the photon energy is:

Egamma=1.6 eV i.e a wavelength of 780 nm

With Lorentz boost at 160 MeV and θL=90°

• At 160 MeV the cross section is close to the maximum

=> should use a Laser with similar wavelength

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Adaptation to LINAC4

• SNS beam vs. LINAC4 beam• Energy LINAC4 < SNS

=> Higher photo neutralization cross section

• Bunch length LINAC4 > SNS=> Saturation effects lower

=> LINAC4 beam parameters less critical

=>The system can be used with better stripping efficiency


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LINAC4 case

• During the acceleration along the linac and the transfer to the PS booster, the second electron can be stripped from black body radiation, magnetic field and residual gas.

• neutral beam can reach the H0 detector and perturb the measurement.• Simulations have been done to estimate the background

10/18/2011B.Cheymol, E. Bravin, U. Raich, F. Roncarolo

BE/BI

•Background from the linac removed by dipole 1

•Losses depends on the energy=> Constant energy=> Loss rate constant

•Simulations done in order to check the feasibility of the system .

Possible implementation of emittance measurement (Old TL design)

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LINAC4 case-background


ProccessProbability of loss per

meterBlack Body radiation neg.Magnetic field 1.10E-05rest gas 6.50E-07

• If laser beam size around 100 μm and assuming a full stripping => number of stripped particles ~107

Background and signal comparable if gating the detector over the all linac pulse.

Drift length [m]

Number of particles

14 6.87E+080.5 5.80E+070.3 4.59E+070.2 3.94E+07

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• If we assume that the probability of stripping by residual gas interaction or magnetic field stripping is independent of time, the background particles are generated over a pulse, i.e 400 μs. For the signal, the particles are generated over a laser pulse, i.e. 10 ns.

• By gating the signal with a short time window, the background effect can be reduced. Assuming a window of 20 ns, the number of background particles can be reduced by a factor 20000


LINAC4 case-Background

Drift length [m] Number of particles

14 3.44E+04

0.5 2.90E+03

0.3 2.30E+03

0.2 1.97E+03

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H0 detector

• H0 detector must have a fast time response.• must also have a good resolution and good signal/noise ratio

• The number of particles is low


microstrip Silicon or Diamond detector

Radiation hardness

detector Silicon diamondRadiation hardness [p/cm2] 10^12 10^15 10^17

lifetime 0.5 h 152 days 42 years

Silicon can not be used for this application

Diamond detectors

• A Diamond detector is more suitable than Silicon

– Can detect single particles (sensitive)– Collection times ~10ns (fast)– Up to 1017 MIPs/cm2 (rad-hard)

• Need to investigate the possibility of using a segmented pCVD diamond detector for profile measurement

10/18/2011 14B.Cheymol, E. Bravin, U. Raich, F. Roncarolo BE/BI

Main questions

• Are diamond substrates sufficiently uniform?

• Is the metallization of the different channels reproducible (junctions formation etc.)?

• Is the ageing effect sufficiently small to allow a long life?

• What is the effect of sustained radiation dose?

10/18/2011 15B.Cheymol, E. Bravin, U. Raich, F.

Roncarolo BE/BI

R&D with CIVIDEC

• A contract with CIVIDEC has been signed

– CIVIDEC will provide a 20x20x0.5 mm3 pCVD diamond detector with 5 readout strips on one side and a common BIAS plane on the other

– CIVIDEC will provide 5 analog fast front end amplifiers (40 dB 2GHz)


• CERN will have to provide the counting/sampling electronics with the corresponding software

R&D with CIVIDEC


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Outlook

Emittance measurement based on laser stripping.• Parasitic measurement• Flexibility• No interaction between H- beam and matter• Profile measurement with the full beam intensity

• Cost• Development of H0 detector• Background (can be reduce with fast electronic)


• NEXT• The background in the new transfer line should be calculate with more accuracy• Use precise beam parameters to determine the resolution of detector.• Beam test with Diamond detector provided by CIVIDEC• Laser test in the 3 MeV test stand

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Outlook

Integration and test in the transfer line:

• Without an intermediate dump in TL:» No separation of H0 and H- => no H0 detector test (can be done with over

beam)» 1 laser station can be installed after the PIMS:

• Commissioning and test of the laser station• Stripping efficiency test• Profile measurement with full LINAC4 Power.

• With an intermediate dump in TL:» Installation of a laser station after a dipole» Test of the full system


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EXTRA SLIDES


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SNS LASER System


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LINAC4 case-background issues

• Spectral density of thermal photon calculated with Planck formula.


Doppler shift applied for the energy spectrum in the bean rest frame

Stripping by black body radiation is negligible

Black body radiation

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• Spectral density of thermal photon calculated with Planck formula.


Doppler shift applied for the energy spectrum in the bean rest frame

Stripping by black body radiation is negligible

Black body radiation

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• For β=0.52, B=1T, and a distance in a magnetic field of 1 m, the stripping probability is 1.1.10-5

• For the background particles reaching the detector, the path in Magnetic field is less than the length of the dipole

• Assuming a target of 4*4 cm, 2 meter downstream the dipole exit, and a perfect magnetic field and trajectory, the path is 15 cm


Magnetic field stripping.

Ions lifetime in a magnetic field

Probability of stripping

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• The probability of stripping is given by :

10/18/2011B.Cheymol, E. Bravin, U. Raich, F. Roncarolo

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Residual gas stripping

l

eP

1

With λ the mean free path P

kT

The stripping cross section depends on the beam energy. Measurements are available in the literature at 400 and 800 MeV.The stripping cross section in N2 gas has been scaled and used for the calculation.The stripping probability per meter is: 6.5.10-7

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Possible tests at the 3 MeV diagnostic bench

• Slit tank is equipped with a laser window:

– Test of the stripping efficiency

• Use the full laser power to strip the beam (without focusing)• Detect the current drop with BCT• Measure current with H0/H- current monitor prototype (foreseen

for PSB injection)


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H0 detector-e- effect

• Error is less than 0.1%• Nevertheless FLUKA only track electron above 1 keV

• Possible problem with these very low energy electrons


Fluka simulations has been used to simulated the effect of the electron of the measurement, a simple geometry has been used (500 μm Diamond foil).

Bragg curve of 90 keV electron in Diamond Reconstructed profile

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H0 detector

• The range of 90 keV electron in Tungsten is around 10 μm.• A stripping foil van be used as electron dump upstream a diamond detector.• The effect on a proton beam of a 100 μm Tungsten foil has been simulated.

• Proton beam have no divergence and energy spread


Distance [cm] 0 1 2 3 4 5 10

error [%] 0.01 0.74 2 3.88 6.42 9.36 30

• Error on the beam profile increase with the distance.• The detector can not be installed in Air.

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H0 detector

Silicon

diamond single

crystal

diamond polycrystal

Band gap [eV] 1.1 5.47 5.47

Breakdown field [MV.cm-1] 0.3 10 10

Electron mobility [cm2V-1s-1] 1450 4500 1800

Hole mobility [cm2V-1s-1] 480 3800 1000

Ionization energy [eV] 3.62 13 13


Mobility is higher with diamond, no large difference between electron and holeCan use both for measurementCollection efficiency is higher with singe crystal (up to 100%). Efficiency of polycrystal has to be checked.Amplification in the detector: for every H0 entering the detector 1.2.104 e-/hole pairs are created.Assuming 90% of collection efficiency signal in the order of hundredth of mA.

Depending on the beamlet size and cost single crystal is a better solution.

Laser Stripping and H 0 monitor systems 10/18/2011B.Cheymol, E. Bravin, U. Raich, F. Roncarolo...

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