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LEBTBeam neutralization, H-injection forspace charge compensation, magnets

LEBT=Low Energy Beam Transport

R. Scrivens, JB Lallement, T Zickler, T Fowler.

Linac4 Source External Review, 7 June 2011

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LEBT

Transfers and matches the beam from the source to the RFQ.Allows placement for diagnostics.Position for the Beam-Stoppers and Pre-chopper.

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Layout of the first 3MeV Building 152 LEBT

LEBT contains:

2 Solenoids2 H+V Steerers2 Beam stopper1 Faraday Cup1 Harp Profile MonitorPump / GaugeSector valveGas InjectionPre-ChopperBeam Current TransformerIon Trapping Electrode

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Almost fully assembled “3MeV Test Stand” LEBT

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Optics - computing

LEBT beam dynamics are modelled with the multi-particle PATH code.CERN code, maintained in BE/ABPSpace charge (radial mesh or point to point)Magnetic field maps (3D or R-Z)It is fast (1 minute per simulation)Batch processing of runs (scan solenoid settings automatically)IBSimu code now has a PATH Manager output file option.

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Solenoid

SourceEmittance meter

Comparison of the simulation and measurement through 1 solenoidMeasurements made before and after the solenoid.

Optics - computing

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Shown is a phase space plot of measured protons, H2+ H3+ beam, superimposed with a simulation.Shows good agreement of the main features of the beam -> Confidence in PATH.

Optics - computing

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X'(r

ad)

X(cm)

Single Solenoid 1000A -H+ H2+ H3+

10mA

X’ (mrad)

X (mm)

Protons

H2+

H3+

H0

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Optics

Based on the common two solenoids approach, as used at Linac2, and many other LEBTs.Original starting point was a measurement of the emittance from DESY (at 35keV).100mm aperture has been maintained (with the new solenoids).With significant changes to the extraction, LEBT input conditions change.Modifications to extraction will have to be iterated through the LEBT layout to assure consistency (aim is to not redesign the LEBT for each source modification).

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x rm

s (m

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Distance (m)

Example beam profile along the LEBT

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The decision has been made to use large aperture solenoids (more flexibility, better field quality in the usable aperture). Max 140mm aperture, 340mm total length.DC powering (estimates are than the DC option is cheaper overall).MedAustron (a CERN collaboration partner) is ordering these solenoids now. Linac4 can piggy back on the order.LEBT was designed to allow 320mm long solenoids. New length and final positioning need to be checked.Magnet follow up by TE/MSC (Magnets).Power Convertors from TE/EPC (Power Convertors)

Solenoids

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0 10 20 30 40 50 60

Bz (k

G)

Position (cm)

Bz field plot

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Steerers

2 sets of window steerer corrector magnets. Up to 50 mrad steering if required.Positioning made into “free space” in the LEBT.Laminated cores. Will allow shot-to-shot variation for testing.Ambient air cooled. DC power convertors.Magnets for the 3MeV test stand / Linac4 are operational (with power convertors)Spare unused magnets from the 3MeV line could be reused. Situation to be followed up.

Power Convertors by TE/EPC

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Pre-Chopper

The pre-chopper is an electrostatic kicker, deflecting the beam away from the RFQ input.Shortens the rise time of the beam, limiting the unwanted beam into the RFQ.Limits the fall time, reducing the beam sent to the tail dump.Acts as part of the beam interlock system, inhibit accelerated beam in Linac4, but allow the source to pulse. It is not considered to have a high safety “rating”.An initial, 10kV deflector was designed (request to increase voltage to 20kV came after production was launched).

Can be tested with protons, and DC powering in 2011.Driver will be produced by TE/ABT group.Driver specification is being written. Delivery is for mid 2012, to test with H- and the RFQ at the test stand.

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Space Charge Compensation

Space charge of the low energy beam is compensated by ions/electrons, created by projectile/rest gas collisions.Independent injection of gas into the LEBT is foreseen. Hydrogen and Xenon should be tested.Higher pressure means faster compensation, but more gas stripping of H-. ~3% stripping loss at 45keV at 1x10-5 mbar pressure.Optimisation of the two processes of this will be in the range 10-7 to 5x10-5 mbar.

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2 LEBT Installations

3MeV Test Stand – Future Linac4For June 2012• New Solenoids• Diagnostic tank modification• Pre-Chopper -> 20kV

Source Test Stand – Building 152Critical Items for September 2012• New Installation of all equipment.

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3MeV Test Stand LEBT modificationsObjective is to be ready to modifications in a 3 month period in 2012, when the proton source is stopped. Design office needs to start in August 2011 to meet this schedule.Full planning in progress.

General Schedule

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Items completed and installed

Solenoids 3MeV versions recovered from Linac1 and Linac2.

To be replaced by larger aperture DC solenoids.

Steerers

Faraday Cup

SEMGrid

Pumping 1 x 600 l/s turbo pump + group

Pressure Penning + Pirani Gauge set

Beam Current Transformer

Heavily shielded design

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Still to be completed

Beam Stoppers Under construction

Gas Injection Gas delivery to be made and tested with beam

Ion Trapping electrode

Produced, but not yet installed

In design Phase

Solenoids To be replaced by larger aperture DC solenoids.

Pre-Chopper Pre-chopper to be redesigned for higher voltages.Driver for specification starting.

Layout modification

Reorder diagnostics box to allow beam measurement in access conditions.

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Cost (kchf)

Original LEBT cost 650

Still to do for existing LEBT. 250

Extra pair of solenoids and steerers 110

Diagnostics See presentation U Raich

Pre chopper (Driver plus modifications to the design)

Power Convertors See presentation D Nisbet

LEBT modifications for new source prototypes 100

Additional b357 source test stand costs 200

Manpower (FTEy)

Finalising 3MeV TS LEBT – Co-ord Source TS LEBT 1.0

Pre Chopper Driver

Commissioning LEBT 1.0

Optics Simulations (for different source variants) 0.5

Commissioning (with different source variants) 0.75

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• Planning for modifications for the 3MeV test stand LEBT are very tight. • Time for beam testing will be VERY limited.• For the source test stand, prioritisation will need to be given after feedback from the workshop.

Feasibility

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The test stand version of the LEBT is running (with some missing parts).Several updates are needed to 3MeV Test Stand LEBT.A new Source Test Stand LEBT is needed. Planning very tight and prioritisation needed.

Summary