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Daddy, where do beams come from High Power Proton Accelerator Development at the Front End Test Stan

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Title: Daddy, where do beams come from High Power Proton Accelerator Development at the Front End Test Stan


1
Daddy, where do beams come from? High Power
Proton Accelerator Development at the Front End
Test Stand
  • Simon Jolly
  • Royal Holloway PP Seminar
  • 1st October 2008

2
Abstract
Contrary to popular belief, the generation and
acceleration of high energy particle beams is not
the result of some sort of primitive voodoo, but
a little-known branch of high energy physics
called accelerator physics. The accelerator
physics group at Imperial College London is
currently involved in a number of projects,
including the development of future proton
accelerators. High power proton accelerators
(HPPA's) with beam powers in the megawatt range
have many possible applications, including
drivers for spallation neutron sources, neutrino
factories, waste transmuters and tritium
production facilities. These applications
typically propose beam powers of 5 MW or more
compared to the highest beam power achieved from
a pulsed proton accelerator in routine operation
of 0.2 MW at the ISIS spallation neutron source
at RAL. The Front End Test Stand (FETS) is an
accelerator test assembly currently under
development at RAL, in collaboration with IC and
Warwick. The aim of FETS is to demonstrate the
production of a high quality 60 mA, 2 ms, 50 Hz,
chopped H- beam at 3 MeV. This requires the
development of a high current H- source, an
accelerator section based on RadioFrequency
Quadrupoles (RFQ's), a fast beam chopper and
corresponding beam transport. Also under
development are a series of novel beam
diagnostics. This talk will focus on the
accelerator background behind FETS and where the
technical challenges lie. Plus some voodoo..."
3
From Luminosity to Emittance
High energy physics with colliding beams is like
banging two bags of potatoes together and trying
to get out chips…
The key quantity for the experiment is
Luminosity, L
kb number of bunches, Nb particles per
bunch fr revolution frequency, HD pinch
enhancement sx/sy beam size at IP
Luminosity is a measure of the interaction rate
of the collider. To get high luminosity, you
need low emittance…
4
Definition of Emittance
Define position of each particle in transverse
phase space ex(x,x), ey(y,y)
Make phase space plot of all particles
qy y
y
z
x
qx x
Each particle has coordinates in 6-D x, x, y,
y, z, E.
Area of ellipse gives ex ey.
5
Emittance Calculation
RMS emittance is defined as
Emittance is an invariant quantity…
position x, angle x, phase space cell density r
6
Liouvilles Theorem
x
x
Drift
x
x
Liouvilles Theorem states that, for a
conservative system (ie. an accelerator
beamline), phase space volume is conserved. In
other words things can only get worse!
7
High Power Proton Accelerators (HPPAs)
  • New generation of High Power Proton Accelerators
    (HPPAs) required for
  • neutron spallation sources.
  • neutrino factory.
  • Accelerator Driven Systems (ADS) transmutation,
    power reactor systems.
  • High power is difficult imperative to keep beam
    losses low (1 W/m)
  • ISIS only 0.2 MW, but 2 beam losses would make
    life very difficult (23 mSv annual dose limit).
  • Need good quality beam.
  • Beam must be chopped at low energy remove
    sections of the beam to prevent unwanted losses
    from transients etc.
  • This is where FETS comes in…

8
The Front End Test Stand (FETS)
  • FETS will demonstrate the early stages of
    acceleration (0-3 MeV) and beam chopping required
    for HPPAs.
  • FETS specification
  • 2 ms pulse length.
  • 50 pps rep. rate.
  • 60 mA H- beam current.
  • Perfect chopping.
  • H- beam used for early stages of acceleration to
    make ring injection easier.

9
FETS Chopping Scheme
10
FETS Layout
Ion Source
LEBT
RFQ
Chopper
Diagnostics
Chopper
RFQ
  • FETS main components
  • High brightness 70 mA H- ion source.
  • 65 keV 3 solenoid Low Energy Beam Transport
    (LEBT).
  • 324 MHz, 3 MeV Radio Frequency Quadrupole (RFQ).
  • Very high speed beam chopper MEBT.
  • Conventional and non-destructive diagnostics.

11
FETS Layout
Ion Source
Beam Diagnostics
Laserwire Tank
LEBT
RFQ
MEBT/ Chopper
12
Ion Source
13
FETS Ion Source
  • FETS ion source design based on Penning source
    used for ISIS
  • Surface Plasma Source (SPS).
  • 45 mA through 0.6?10 mm aperture (750 mA/cm2).
  • 200-250 ?s, 50 Hz ? 1 d.f.
  • Need higher current, better duty factor, longer
    pulse…

Ion Source Assembly
14
Ion Source Development Rig (ISDR)
Ion Source
Beam
15
Ion Source Targets (vs. ISIS)
16
Ion Source Mode of Operation
Time
17
Mica
10mm
Mounting Flange
18
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19
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20
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21
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22
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23
ISDR Diagnostics
24
ISDR Diagnostics
Movable Scintillator with Interchangeable
Pepperpot or Profile Head
25
The Pepperpot Emittance Scanner
  • Current slit-slit scanners give high resolution
    emittance measurements, but at fixed z-position
    and too far from ion source.
  • X and Y emittance also uncorrelated, with no idea
    of x-y profile.
  • Correlated, 4-D profile (x, y, x, y) required
    for accurate simulations.
  • Pepperpot reduces resolution to make correlated
    4-D measurement.
  • Moving stage allows measurement at different
    z-locations space charge information.
  • Possible to make measurements within a single
    pulse.
  • High resolution x-y profile measurements with
    second head.

26
Pepperpot Principle
  • Beam segmented by tungsten screen.
  • Beamlets drift 10mm before producing image on
    quartz screen.
  • Copper block prevents beamlets from overlapping
    and provides cooling.
  • CCD camera records image of light spots.
  • Calculate emittance from spot distribution.

Quartz screen
Copper block
Fast CCD Camera
H- Ion Beam
Tungsten screen
H- Beamlets
27
Pepperpot Components
  • Pepperpot head
  • Tungsten intercepting screen, 50mm holes on 3mm
    pitch in 41x41 array.
  • Tungsten sandwiched between 2mm/10mm copper
    support plates.
  • Quartz scintillator images beamlets.
  • Camera system
  • PCO 2000 camera with 2048 x 2048 pixel, 15.3 x
    15.6 mm CCD.
  • Firewire connection to PC.
  • 105 mm Micro-Nikkor macro lens.
  • Bellows maintains light tight path from vacuum
    window to camera.
  • Main support
  • Head and camera mounted at either end of 1100 mm
    linear shift mechanism, with 700 mm stroke.
  • All mounted to single 400 mm diameter vacuum
    flange.

28
FETS Pepperpot Design
Beam profile head
Tungsten mesh
Pepperpot head
Shutter
Bellows
Camera
Moving rod
Vacuum bellows
Mounting flange
29
Pepperpot Installation
30
Scintillator Problems
  • Pepperpot rapidly became scintillator
    destruction rig.
  • Scintillator requirements
  • Fast (down to 500ns exposure).
  • High light output.
  • Survives beam (lt1 micron stopping distance).
  • High energy density from Bragg peak causes severe
    damage.
  • Finally chose Ce-Quartz.

31
Pepperpot Data Image
Raw data
Calibration image
Colour enhanced raw data image, 60 x 60 mm2.
Calibration image use corners of 126 x 126 mm
square on copper plate to give image scaling,
tilt and spot spacing.
32
Pepperpot Emittance Extraction
Emittance profiles
X
Y
Pepperpot image spots hole positions (blue) and
beam spots (red)
33
Pepperpot/Profile Comparison
34
Pepperpot Quiver Plots
9 kV Extract
13 kV Extract
35
Profile Measurements for Different Extraction
Voltages
36
17 kV Extraction Voltage
35 kV Platform Voltage
18 kV Post Acceleration Voltage
47 mA Beam Current
37
16 kV Extraction Voltage
35 kV Platform Voltage
19 kV Post Acceleration Voltage
42 mA Beam Current
38
15 kV Extraction Voltage
35 kV Platform Voltage
20 kV Post Acceleration Voltage
40 mA Beam Current
39
14 kV Extraction Voltage
35 kV Platform Voltage
21 kV Post Acceleration Voltage
38 mA Beam Current
40
13 kV Extraction Voltage
35 kV Platform Voltage
22 kV Post Acceleration Voltage
35 mA Beam Current
41
12 kV Extraction Voltage
35 kV Platform Voltage
23 kV Post Acceleration Voltage
32 mA Beam Current
42
11 kV Extraction Voltage
35 kV Platform Voltage
24 kV Post Acceleration Voltage
28 mA Beam Current
43
10 kV Extraction Voltage
35 kV Platform Voltage
25 kV Post Acceleration Voltage
25 mA Beam Current
44
9 kV Extraction Voltage
35 kV Platform Voltage
26 kV Post Acceleration Voltage
21 mA Beam Current
45
8 kV Extraction Voltage
35 kV Platform Voltage
27 kV Post Acceleration Voltage
17 mA Beam Current
46
7 kV Extraction Voltage
35 kV Platform Voltage
28 kV Post Acceleration Voltage
13 mA Beam Current
47
6.5 kV Extraction Voltage
35 kV Platform Voltage
28.5 kV Post Acceleration Voltage
12 mA Beam Current
48
6 kV Extraction Voltage
35 kV Platform Voltage
29 kV Post Acceleration Voltage
10 mA Beam Current
49
5.5 kV Extraction Voltage
35 kV Platform Voltage
28.5 kV Post Acceleration Voltage
9 mA Beam Current
50
Ion Source MAFIA Model
MAFIA modelling indicates problems with Dipole
magnet field and extract geometry.
Large vertical beam spread at dipole exit due to
over-focussing within dipole field
51
Sector Magnet Pole Pieces
STANDARD ISIS POLES
n 1.4 wide
n 1.0
n 0.8
n 1.4
Scott Lawrie
52
Development Rig Results
Test new pole pieces
n 1.4 Old
53
Decrease Post Acceleration Gap
54
Ion Source Current Status
  • At normal operating conditions (17 kV Extraction
    Voltage) the beam is collimated into a round beam
    by the post acceleration electrodes.
  • The beam is asymmetrically focused in the
    horizontal plane.
  • Severe vertical defocusing present CST
    simulations show incorrect dipole field index.
  • Modifications to post-acceleration geometry
    reduce emittance.
  • More work required to understand effect of
    extract geometry.

55
FETS Layout
Ion Source
Beam Diagnostics
Laserwire Tank
LEBT
RFQ
MEBT/ Chopper
56
Diagnostics
57
FETS Beam Diagnostics
  • Conventional beam diagnostics currently used for
    FETS (eg. pepperpot, slit-slit) are destructive
    a bit like like sticking your finger in a plug
    socket to see if its live…
  • Need non-destructive diagnostics to make
    measurements while accelerator is running.
  • 2 types of beam diagnostic under development,
    based on photo-detachment by laser
  • 4-D emittance measurement ( longitudinal
    profile) downstream of chopper.
  • 2-D profile measurement, between ion source and
    LEBT.

58
Photo Detachment for Beam Diagnostics
Photodetachment
Threshold energy ED 0.754eV Maximum Ephoton2
ED
  • max 4.010-17 cm2

H0 no significant momentum transfer
Faraday Cup
-
Dipole
-
-
y
-
LASER
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
z
Photo
Charge
Detection of
detachment
separation
distribution
59
Laserwire Beam Profile Measurement
  • Non-destructive, non-invasive measurement of the
    X-Y beam profile.
  • Integrated into vacuum vessel after ion source.
  • Movable mirrors in the vacuum vessel enable many
    profiles to be measured.
  • Reconstruction of the 2D density distribution
    will be possible.

Laser photo-dissociation
Electron collection with Faraday Cup
60
Laserwire Profile Concept
Multiple mirror setup allows laser to sample beam
from all directions
61
Two orthogonal projections gives the X Y
profiles but coupling information is lost.
62
Using pairs of mirrors covering 90? sectors
allows 2D reconstruction.
63
Laserwire Vacuum Tank
Beam from Ion Source
Laserwire assembly
Beam
Start of LEBT
Vacuum pumps
64
Laserwire Vacuum Tank (2)
Detector
Rotary motors x4
Mirrors
Linear motors for vertical motion
Motor chassis
Door chassis
65
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66
Laserwire Electron Collector
Beam
Collector Field Map/Trajectories
Photo-detached electrons bent by dipole field…
…and collected by Faraday Cup
67
FETS Layout
Ion Source
Beam Diagnostics
Laserwire Tank
LEBT
RFQ
MEBT/ Chopper
68
LEBT
69
The Low Energy Beam Transport
  • Beam must be focussed from gt20mm at Ion Source to
    2-3mm at RFQ.
  • Large dynamic aperture required to handle beam
    size and space charge.
  • 3 solenoid design with weak focussing provides
    effective focussing with minimal emittance growth

Focussing in single solenoid
70
LEBT Layout
Beam
Solenoid
Beam
Vacuum pump
71
LEBT Design
Design based on ISIS LEBT - three solenoids
between drift areas. Typical set-up and
dimensions given below
Vary drift lengths d2 and d3 and the solenoid
lengths and B fields. Look for solutions where
the beam is focused (converging) into the
RFQ. Constraints B lt 0.6 T, solenoids long
enough to ensure flat axial field (d 25cm)
d1 25cm, d4 15cm (minimum for vacuum
equipment and diagnostics) Overall length must
not be too long (cost)
72
Beam Trajectories in 3-Solenoid LEBT
73
LEBT Performance for Ideal Beam
Vertical lines Drift and solenoid regions
RFQ Acceptance Ellipse
End of LEBT
74
First tank L.E.B.T.
L.E.B.T. support structure and rail system
currently being installed.
Vee blocks for remaining 5 support stands to be
delivered on 14th March.
75
FETS Layout
Ion Source
Beam Diagnostics
Laserwire Tank
LEBT
RFQ
MEBT/ Chopper
76
RFQ
77
Accelerators Go Small, Go Swift…
  • Two main aims of accelerator beamline
  • Focussing (go small).
  • Acceleration (go swift).
  • For relativistic beams, we can do this with a
    FODO lattice, interleaved with accelerating
    structures.

78
The FODO Lattice
…magnets to focus/bend
Cavities to accelerate…
79
Low Energy Acceleration/Focussing
  • However, at low energies things become more
    complex
  • Variation in b means RF cavity length must
    increase as beam is accelerated.
  • Space charge puts a premium on continuous
    focussing.
  • Perhaps we can accomplish the whole thing in one
    go…

80
The RadioFrequency Quadrupole (RFQ)
RFQs accelerate, bunch AND focus all at once!
2 types 4-rod and 4-vane
4-rod RFQ
4-vane RFQ
81
RFQ Focussing
  • RF field causes positive/negative charges on
    pairs of vanes.
  • Since field varies with time, alternate
    focussing/ defocussing mimics FODO.

RFQ E-field
Standard Quad
RFQ vane tips
82
RFQ Acceleration/Bunching
  • RFQ vane tips modulated longitudinally.
  • Curved field lines produce longitudinal field
    acceleration and bunching.

Alternate modulation gives acceleration
Single vane
83
RFQ Transverse Field Map
84
RFQ On-Axis Ez Field
85
Initial Conditions Z-Y, 5 bunches
86
Full FETS Simulation Z-Y, 5 bunches
87
Initial Conditions Z-E, full beam
88
Full FETS Simulation Z-E, full beam
89
RFQ Transmitted Current
90
RFQ Integrated Design
  • RFQ parameterised by a and m parameters for
    modulations and L for cell length.
  • These parameters generated using optimisation
    code, then handed to Frankfurt for RFQ
    manufacture.
  • Would like to have a method of designing RFQ
    where all steps are integrated
  • Engineering design.
  • EM modelling.
  • Beam dynamics simulations.

91
RFQ Integrated Design Step 1
  • Most FETS CAD modelling done using Autodesk
    Inventor, including the cold model.
  • Possible to draw vane modulations using spline
    interpolation.
  • Parameters read out from Excel spreadsheet can
    change modulations on the fly...

92
RFQ Integrated Design Step 2
  • EM modelling already carried out for cold model
    using CST Microwave Studio.
  • Export .sat file to MWS from Autodesk of 3D
    vane model only central 1cm x 1cm section.
  • Cut into 4 sections
  • Mirrors real assembly.
  • Easier for MWS meshing.
  • Output as E B field map.

93
RFQ Integrated Design Step 3
  • Import field map of central field region into GPT
    for particle tracking.
  • Optimise design based on RFQ transmission and
    feed back into engineering design.
  • We now have a method of producing a field map and
    carrying out simulations for the thing were
    going to build!

94
RFQ Development
Physics design
1st Engineering design
Manufacturing test
Manufactured RFQ sections
2nd Engineering design
Brazing test
95
Brazed RFQ in Mounting Frame
96
RFQ Current Status
  • Incremental progress on field flatness and
    resonant properties see EPAC08 paper THPP024,
    S. Jolly et al.
  • RFQ beam dynamics simulations in GPT very
    promising see bunching, acceleration,
    current-dependent transmission.
  • gt90 transmission for ideal beam, only 50 for
    real parameters.
  • Can (almost) run end-to-end simulations in GPT
    using pepperpot measurements from ion source,
    optimised LEBT parameters and field map for RFQ.
  • Integrating Autodesk, MWS and GPT design steps
    will reduce bifurcation of design.
  • Need to ensure CAM systems will understand our
    CAD models so we can manufacture what were
    designing (this is the point...).

97
FETS Installation (1)
70 kV HV Platform Construction
HV Cage Construction
98
FETS Installation (2)
99
FETS Installation (3)
100
FETS Installation (4)
101
The State of The Nation…
  • Installation in R8 (RAL) rails, ion source
    platform, LEBT tank and solenoids.
  • Klystron delivered (2 MW).
  • Power supply in production (depending on
    finances...).
  • LEBT solenoids and power supplies almost
    complete.
  • Vacuum vessel and laser diagnostic under
    construction.
  • For end of 2008 beam through LEBT.

102
The FETS Collaboration
  • I have shamelessly pilfered slides from all
  • members of the FETS Collaboration
  • John Back (Warwick).
  • Mike Clarke-Gayther, Adeline Daly, Dan Faircloth,
    Christoph Gabor, Scott Lawrie, Alan Letchford,
    Ciprian Plostinar (RAL).
  • Ajit Kurup, David Lee, Jürgen Pozimski, Pete
    Savage (Imperial).
  • …and probably a few more besides…
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