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Imperial College

Progress at the RAL Front End Test Stand

J. Pozimski

Talk outline :

• Overview

• Ion source development

• LEBT

• RFQ

• Beam Chopper

• MEBT

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Imperial CollegeOverview – Experimental Parameters

Aim is to demonstrate a 60 mA, 2 ms , 50 pps chopped beam at 3 MeV

It is a collaboration of several physics institutes: RAL/ISIS (chopper, ion source, engineering) RAL/ASTeC (beam dynamics, RF cavities)

Imperial College HEP group (engineering, RFQ, diagnostics, LEBT)Warwick University HEP group (LEBT)

RFQ, 324 MHz, 3 MeV

MEBT, chopper

Ion source 70 mA, 2 ms, 10 % dc

3 Soleniod LEBT

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Imperial CollegeIon source

D. Faircloth - ISIS

• Double output current :35mA → 70mA

• Increase pulse length :200μs → up to 2 ms

• Improve (preserve) emittance • Maximize lifetime

Parameters of the ISIS source

Development Goals :

• Penning H- ion source

• Surface Plasma Source (SPS)

• 35 mA through 0.610 mm aperture ( 600mA/cm2)

• 200-250 s, 50 Hz 1% duty cycle

• 0.17 mm mrad (@ 665 keV, 35 mA, rms)

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Imperial College

Ion source development -resultsIn the experiments the nominal beam current of 70 mA was already

reached and an pulse length of up to 1750μs achieved

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60

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0 200 400 600 800 1000 1200 1400Time (us)

Beam Current (mA)

Discharge Current (A)

Extract Volts (kV)

70mA beam current !

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Imperial College

A 3 solenoid LEBT like in ISIS will be used. The parameter space was scanned by the use of a envelope simulation to search for possible solutions which fulfil the

RFQ input requirements for the given source parameters. Now further simulations with GPT for a more detailed analyses of the found solutions to further optimize

the beam transport are planned.

no o

f su

cces

sfu

l ru

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LEBT length [m]

LEBT J. Back – Warwick; S. Jolly - Imperial

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Imperial CollegeElectromagnetic modelling of the RFQ using Mafia

(A. Letchford – ISIS; A. Kurup, S. Jolly – Imperial)

Distribution of the magnetic fields of the quadrupole mode

in the 4 vane RFQ.

Theoretical Q value of the 4 vane RFQ ~12000

Theoretical Q value of the 4 rod RFQ model ~6400

Model of the 4 rod RFQ and electric field distribution of the

quadrupole mode near the electrodes

• 4-rod or 4-vane ?• 4- rod 324MHz ?• start of particle

dynamics calculations

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Imperial CollegeBeam Chopper

(M. A. Clarke-Gayther – ISIS)

To avoid a partial chopped beam, the fast chopper (<2 ns) removes 3 bunches while the slow chopper (t<12 ns) risesto its max voltage and then chops for 240 ns-0.1ms

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Imperial CollegeBeam Chopper – Pulse generator -

measurements

Measurements show that the built fast pulse generator system already fulfils

the requirements on the voltage and the rise- and fall time (<2ns).

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Imperial CollegeMEBT

FG - AsTec

Improvements compared to the ESS scheme :

•chopped beam from fast & slow chopper is collected in 2 dedicated beam dumps, designed to take full beam power (no need for 2 identical chopper sections)

• deflection of fast & slow chopper is optically increased by defocusing quadrupoles

•now only 30% of original plate voltage for the slow chopper is required (6 kV -> 2 kV), -> faster rise time for slow chopper

•larger apertures for all elements

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Imperial College

Conclusions

• Encouraging progress at the ion source development• Particle dynamics design of the LEBT will finish this autumn

and mechanical design started• Particle and electrodynamic design of RFQ started, cold

models will be built next spring• Design of beam chopper and MEBT develops strongly• Electronics for the fast beam chopper on the test rig• Definition of new beam diagnostics started