ESS-Bilbao Initiative Workshop. Front Ends for High Intensity

Front Ends For High Intensity

Alan Letchford
STFC RAL ISIS Injector Group
ESS-Bilbao Initiative Workshop
March 2009

Outline
• Front Ends

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

•Outlook

Front Ends For High Intensity Alan Letchford, ESS-Bilbao Initiative Workshop

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 Source Radio Frequency Linac
Drift Tube
H+ or H- Quadrupole
(for example)

Low Energy Medium Energy
Beam Transport Beam Transport
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 keV
H+ fraction > 80 %
Reliability > 95 %
Emittance < 0.2 mm.mrad
CW 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
Baseline LBNL source has
been developed to >35mA
at 4% duty factor. 40mA

The Large Volume
External Antenna Source
has demonstrated >60mA
but chamber heating an
issue.
2 week production run

RMS emittance ~0.2 mm
mrad


H- Ion Sources.

Eg RAL FETS Penning Surface Production Source
Development of the ISIS source
has demonstrated feasibility of
60

40

both >60mA and 7% duty factor. 20

0
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Time (us)
-20

On FETS power supplies will allow -40

both to be achieved
Discharge Current (A)
Beam Current (mA)
-60
Extract Volts (kV)

simultaneously.
-80

1.2ms 35mA beam at 50Hz

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

The Penning source can be
changed in ~2 hours.

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.
The RAL front end test
stand will employ laser
wire tomography for full
2D non destructive beam
density measurement. -5
1*10 hPa
-4
1*10 hPa
Laser
2
1 electrostatic
LEBT

A laser stripping based beam
axis

emittance measurement 1*TP
ion CC
D
source

system is being
ca

1*TP
2 einzel lenses

2*TP
me
ra
R=40mm
magnetic
coils
differentiell pumping
developed. tank
1, 2 slit position of emittance scanner
TP = Turbopump

Faraday cup Scintillator
Dumping system


Outlook.

High intensity front ends in operation or under
development include (but not limited to):
H- 30 mA 50 Hz 300 µs 0.6 MeV 202.5 MHz
ISIS
H- 60 mA 50 Hz 2 ms 3 MeV 324 MHz
RAL FETS
H- 30 mA 25 Hz 0.5 ms 3 MeV 324 MHz
J-PARC
H- 30 mA 60 Hz 1 ms 2.5 MeV 402.5 MHz
SNS
H- 20 mA 60 Hz 2 ms 3 MeV 350 MHz
PEFP
H+ 70 mA 4 Hz 36 µs 3 MeV 325 MHz
FAIR

H+/H- 20 mA 2.5 Hz 3 ms 2.5 MV 325 MHz
HINS
H- 40 mA 50 Hz 1.2 ms 3 MeV 352 MHz
SPL
D+ 125 mA CW CW 5 MeV 175 MHz
IFMIF
H+ 100 mA CW CW 6.7 MeV 402.5 MHz
LEDA


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

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