ess-bilbao initiative workshop. front ends for high intensity

Front Ends For High Intensity

Alan Letchford

STFC RAL ISIS Injector GroupESS-Bilbao Initiative Workshop

March 2009

Alan Letchford, ESS-Bilbao Initiative WorkshopFront Ends For High Intensity

• Front Ends

• Challenges• Ion sources• LEBTs• RFQs• MEBTs• Choppers• Funnels• Diagnostics

•Outlook

Outline


Front Ends

The ‘Front End’ is not precisely defined. Rarely taken to mean anything above 10-20 MeV. Often refers to just the first 2-3 MeV.

Ion SourceH+ or H-

Low Energy Beam Transport

Radio Frequency Quadrupole

Medium Energy Beam Transport

Drift Tube Linac (for example)

Funnel


Front Ends

Rather obviously, no linac can operate without a front end.

Getting the front end right is important as it defines the available current for the machine.

The front end defines the emittance for the whole linac.

Beam artefacts generated here may propagate along the linac and lead to loss.

Chopping and funnelling are challenging and essential in some scenarios.


Challenges

H+ Ion Sources.

For a long pulse neutron source with only a linac, an H+ ion source can be used.H+ sources can deliver >100mA at duty factors up to 100%.

Eg CEA SILHI ECR source:

H+ Intensity > 100 mA at 95 keVH+ fraction > 80 %Reliability > 95 %Emittance < 0.2 � mm.mradCW or pulsed mode


H- Ion Sources.

For a neutron source with synchrotron or compressor ring an H- ion source in required for charge exchange injection.

H- source performance does not match that of H+ sources.

Currents up to 60mA and duty factors approaching 10% have been demonstrated but not simultaneously for extended periods.

High currents require caesium which can limit lifetimes.


H- Ion Sources.

Eg SNS RF driven multicusp source

40mA

2 week production run

Baseline LBNL source has been developed to >35mA at 4% duty factor.

The Large VolumeExternal Antenna Source has demonstrated >60mA but chamber heating an issue.

RMS emittance ~0.2 mm mrad


H- Ion Sources.

Eg RAL FETS Penning Surface Production SourceDevelopment of the ISIS source has demonstrated feasibility of both >60mA and 7% duty factor.

On FETS power supplies will allow both to be achieved simultaneously.

State of the art diagnostics and modelling will lead to reduced emittance.

The Penning source can be changed in ~2 hours.

1.2ms 35mA beam at 50Hz-80

-60

-40

-20

0

20

40

60

0 200 400 600 800 1000 1200 1400 1600 1800 2000Time (us)

Discharge Current (A)Beam Current (mA)Extract Volts (kV)


Low Energy Beam Transport.

There are two approaches: solenoids or einzel lenses.

Space charge effects are very high at these particle velocities. Einzel lenses are short whereas solenoidal LEBTs allow for space charge compensation through background gas ionisation.

Both systems can introduce aberrations if the full aperture is used.


LEBT.

Electrostatic solutions may be problematic when operated close to a caesiated ion source.

Space charge compensation in negative hydrogen beams is less well understood than for positive beams. >90% compensation is expected but gas pressures can also lead to beam stripping.

Compensation takes time leading to an initially mismatched part of the beam.


LEBT.

Eg SNS electrostatic H- LEBT incorporating pre-chopper

Beam experiences full space charge but design is very compact.

HV sparking has limited performance of LEBT chopper.


LEBT.

Eg SILHI 2 solenoid H+ LEBT Almost 100% compensation is possible.Higher gas pressures are required to achieve full compensation in solenoids.

Large emittance growth can occur for some operating points


Radio Frequency Quadrupole.

The RFQ is the default accelerating structure from 10s of keV up to 2-5 MeV due to its strong focussing and efficient bunching.

Although the beam dynamics is quite mature the diversity of manufacturing methods suggests an optimum way of engineering the structure has not yet been found.

High surface fields can make RFQs prone to field emission issues.


RFQ.

High transmission and low emittance growth for high intensity beams leads to relatively long structures.

4-rod and 4-vane types are both feasible although 4-vane is possibly easier to cool at high duty factor.

4-vane structures can be bolted, brazed, electron beam or laser welded.


RFQ.

Eg ISIS RFQ

>95% transmission for >30mA but low frequency and low duty factor.

Approaching 5 years of almost faultless operation.

Matching to DTL is not optimal.


RFQ.

Eg J-PARC RFQ

30mA H- at 3%.

Employs Pi mode stabilising loops.

Cavity is inside an external vacuum tank.

Experiencing sparking issues.


RFQ.

Eg LEDA RFQ

100mA H+ up to CW

6.7MeV, 8m long.

Output current dropped during pulsed operation requiring up to 110% electrode voltage to cure –trapped ions may be the cause.


Beam Chopper.

For injection into a ring at high intensity, chopping the linac beam at the ring revolution frequency is essential for low loss acceleration.

Ideally there should be no partially chopped bunches in the linac which requires extremely fast switching times.

High voltage switching limits mean chopping has to be done at low energy in the Medium Energy Beam Transport.


Beam Chopper.

Even for a H+ linac with no ring, chopping may still be necessary.

Reducing average current without reducing bunch charge requires chopping.Alternative would be to reduce source output and retune whole linac for lower current.

A chopper may be required to remove slow beam transients at the beginning and end of pulse or ramping current at switch on.


Medium Energy Beam Transport.

Placing the chopper in the MEBT places constraints on the MEBT design.

Large drifts necessary for the deflectors and beam dumps and a relatively parallel beam through the chopper results in quite low phase advance in the MEBT.

Matching between the MEBT and RFQ and following structure – which have relative large phase advances –and controlling emittance growth can be challenging.


Beam Chopper.

Eg CERN Linac4 Chopper

Uses a meander type deflector mounted inside a quadrupole.

UP to 30% emittance growth in MEBT seen in simulations.


Beam Chopper.

Eg J-PARC Chopper

Uses 2 RF deflectors in MEBT plus induction gap pre-chopper in LEBT.

Low Q deflector cavities allow ~10ns rise times.


Beam Chopper.

Eg RAL FETS Chopper

Two stage chopping to achieve fast rise time and long flat-top.

Discrete deflector plates and delay lines instead of meander.

Sub 2ns rise and fall


Funnel.

Beam funnelling has been proposed as a solution to achieving higher currents than available from a single ion source (mainly applicable to H-) or to reduce space charge in the front end.

A low energy for funnelling reduces the amount of duplicated equipment. A higher energy may be preferable to control dispersion effects.


Funnel.

Eg Frankfurt 2 beam RFQ

Novel concept of two convergent RFQs and RF deflector in a single cavity.

Funnelling has been experimentally demonstrated.

It isn’t clear if dispersion is controlled.


Funnel.

Eg Los Alamos half funnel.

A 5 MeV H- beam was successfully ‘funnelled’ with good transmission and emittance growth.

Proof of principle that funnelling can be achieved.


Diagnostics.

Even at 3 MeV the beam power in a high intensity front end can be significant: nearly 20 kW on RAL FETS for example.

Non destructive diagnostics are an attractive proposition and can be applied throughout the linac.

For H- beams laser photo detachment techniques allow for online profile and emittance measurement.


Diagnostics.

Eg RAL FETS laser diagnostics.

Laser

beamaxis

CCD camera

12

Faraday cup

electrostaticLEBT

2 einzel lensesR=40mm

magneticcoils

Dumping system Scintillator

ionsource

differentiell pumpingtank

2*TP

1*TP

1*TP

1*10 hPa-4 1*10 hPa-5

TP = Turbopump 1, 2 slit position of emittance scanner

The RAL front end test stand will employ laser wire tomography for full 2D non destructive beam density measurement.

A laser stripping based emittance measurement system is being developed.


Outlook.

High intensity front ends in operation or under development include (but not limited to):

ISIS H- 30 mA 50 Hz 300 µs 0.6 MeV 202.5 MHz

RAL FETS H- 60 mA 50 Hz 2 ms 3 MeV 324 MHz

J-PARC H- 30 mA 25 Hz 0.5 ms 3 MeV 324 MHz

SNS H- 30 mA 60 Hz 1 ms 2.5 MeV 402.5 MHz

PEFP H- 20 mA 60 Hz 2 ms 3 MeV 350 MHz

FAIR H+ 70 mA 4 Hz 36 µs 3 MeV 325 MHz

HINS H+/H- 20 mA 2.5 Hz 3 ms 2.5 MV 325 MHz

SPL H- 40 mA 50 Hz 1.2 ms 3 MeV 352 MHz

IFMIF D+ 125 mA CW CW 5 MeV 175 MHz

LEDA H+ 100 mA CW CW 6.7 MeV 402.5 MHz


Discussion.

• Use H- even for long pulse to enable laser wire diagnostics.

• Design a dismantleable/repairable RFQ and have a spare.

• Include fast chopper even if there is no ring.

ess-bilbao initiative workshop. front ends for high intensity

Technology