Transcript Slide 1 - ILC - International Linear Collider

International Linear Collider (ILC)
Linac Basics
Part I: General Design Constraints
Part II: ILC Design Choices
Chris Adolphsen
SLAC
Part I: General Design Constraints
• Linear collider design
is complex due to the
interrelationships
among the various
parameters and the
soft constraints on
their values.
• I will give one or many
possible descriptions
of the rationale behind
the ILC linac design.
Bob Palmer
1990
1.3 GHz
TESLA
Cavities
‣ Made with solid, pure niobium – it has the highest Critical Temperature
(Tc = 9.2 K) and Thermodynamic Critical Field (Bc ~ 1800 Gauss) of all
metals.
‣ Nb sheets are deep-drawn to make cups, which are e-beam welded to
form cavities.
‣ Cavity limited to ~ 9 cells (~ 1 m Long) to reduce trapped modes, input
coupler power and sensitivity to frequency errors.
‣ Iris radius (a) of 35 mm chosen in tradeoff for low surface fields, low rf
losses (~ a), large mode spacing (~ a3 ), small wakes (~ a-3.5 ).
The Basics
• The low rf surface losses in superconducting
cavities allow essentially 100% RF-to-beam
transfer efficiency in steady state:
RF Input Power = Cavity Voltage * Beam Current
• First look at what limits cavity voltage (gradient *
cavity length)
– Note: highest gradient not always cost optimal !
Cavity Operating Parameters
• Operate at 2K (in super-fluid He)
to reduce resistivity:
Niobium Surface Resistance
• 1.3 GHz frequency (f) chosen to
reduce power loss, sensitivity to
thermal instabilities, wakes, and
cavity size and to match
available sources.
Qo ~ 2e10
at T = 2K
• At Qo = 1e10, cavity time
constant (Qo/w) ~ 1 sec, and at
35 MV/m and 0.1% duty factor,
average power loss ~ 1 W/m
(but it takes ~ 1 kW/m of AC
power to remove heat).
4.6
3.1
2.3
1.8
Temperature (K)
1.5
1.3
Operating Gradient
• Qo varies with gradient due to a number of mechanisms.
• In recent years, gradients are approaching fundamental
limit: Bc * (Grad / B surface) ~ 1800/41.5 ~ 43 MV/m
Best 9 Cell Cavity Result to Date:
CW Performance of a Cavity ElectroPolished at DESY
Qo
Gradient (MV/m)
The Basics (2)
• The low rf wall losses in superconducting cavities
allow essentially 100% RF-to-beam transfer
efficiency in steady state:
1.3 GHz Input Power = 25-50 MV/m * 1m * Beam Current
• Next look at beam structure
– Beam Current = Bunch Charge / Bunch Spacing
– Bunch Train Length = Number of Bunches * Bunch
Spacing
Bunch Charge and Length
• Nominal Bunch Charge (N = 2e10) and Length (sz = 300 mm)
– Mainly determined by damping ring, linac energy spread
and IP considerations.
• Bunch length reduced from 6 mm to 300 microns prior to
linac injection
– Also constrained by
• Short-Range Transverse Wake Kicks (N sz / a 3.5 )
• Short-Range Loading (N / sz / a 2 )
Wakefield in a PETRA cavity
Iris
Radius
a
Bunch
Number of Bunches per Pulse, Repetition
Rate, Luminosity and AC Power
• Luminosity ~ Rep Rate  Number of Bunches per Pulse
– Repetition rate (5 Hz) constrained by damping ring store
time (see next slide).
– Number of Bunches per Pulse constrained by
• Train Length > Cavity Fill Time: for > 50% rf-to-beam efficiency,
minimum number of bunches is1870 (at 35 MV/m and 2e10
e/bunch) independent of bunch spacing !
• Length and cost of damping rings (newer, smaller designs
assume smaller bunch separations).
• Train Length < Max klystron pulse length (limited by pulse
heating).
– Product constrained by practical site power (few hundred
MW). Would limit rep rate even damping rings did not.
Damping Ring Constraints
• Optimal bunch train length very long (>> linac length), so
– Minimize bunch spacing in the damping ring – limited by separation
(3-20 ns) required to extract individual bunches with a kicker magnet.
– Still, damping ring is long (17 km circumference in TDR, 6 km in
current design), which makes the required store time long (200 ms),
even with a few hundred meters of wigglers.
– RF pulse length (1.4 ms) << store time so ring needs to hold full
bunch train.
– Store time limits machine repetition rate (5 Hz). Could increase
wiggler length, but already about 20% of damping ring cost.
Dog Bone Damping Ring
TESLA Design Choices
(2001 TDR)
Gradient = 23.4 MV/m
Bunch Charge = 2e10 e
Rep Rate = 5 Hz
# of Bunches = 2820
Bunch Spacing = 337 ns
Beam Current = 9.5 mA
Input Power = 230 kW
Fill Time = 420 ms
Train Length = 950 ms
Bunch Spacing
• Non-linac constraints on minimum spacing
– Peak positron target heating
– IP bunch separation
• Weak linac constraints
– Bunch coupling from long-range transverse wakefields
– Steady state RF-to-Beam efficiency: w/Qo very high
(much different than for a warm machine)
• Strong linac constraints
– Power source costs ~ 1 / Bunch Spacing
– Cryogenic costs ~ Bunch Spacing
Klystron Economics
• Cost of 1.3 GHz Klystron + Modulator (crude approximation)
– Similar for similar average power
– Independent of peak power.
• However, rf energy per pulse differs greatly, for example,
– 20 MW peak, 10 msec pulses (300 Hz, 60 kW)
– 5 MW peak, 2 msec pulses (10 Hz, 100 kW)
– 0.1 MW peak, CW
• Number of klystrons
~ Cavity Input Power / Peak Klystron Power
~ 1 / Bunch Spacing (with fixed peak power)
• With smaller bunch spacing, would not use full
average klystron average power capability.
Bunch Spacing and Fill Time
• Adjust Qext to match cavity impedance (R/Qo * Qext) to the beam
impedance (Gradient / Current). So Fill Time ~ Bunch Spacing
• For TDR parameters, Qext = 3e6 so cavity BW = 430 Hz.
• Need to achieve < 0.1% energy gain uniformity.
Other Bunch Spacing Considerations
• Input coupler power limitations
– Power ~ 1 / Bunch Spacing
Coaxial Power Coupler
– Baseline TTF3 design processed to 1 MW and tested up to 600 kW
for 35 MV/m operation (1000 hours): long term reliability for required
operation at 350 kW not known.
Input
Power
TESLA Design Choices
(2001 TDR)
Gradient = 23.4 MV/m
Bunch Charge = 2e10 e
Rep Rate = 5 Hz
# of Bunches = 2820
Bunch Spacing = 337 ns
Beam Current = 9.5 mA
Input Power = 230 kW
Fill Time = 420 ms
Train Length = 950 ms
Where Does the Power Go
(NLC/GLC vs TESLA TDR Efficiencies and Average Power)
Cost Optimization
• Major cost components that depend on Gradient (G) and
Bunch Train Length (Tb).
Cooling
~ A  Tb  G
+
Power
+
Length
+
B  Tb-1
+
C  G -1
• Cost Study: Compute Cost vs G and Tb for fixed
Luminosity (L)
– Assume charge per bunch and number of bunches constant
– Cavity fill time (Tf) scales as G * Tb
– RF pulse length (Trf) = Tb + Tfill
Relative Total Project Cost* (TPC)
-vs-
Linac Gradient
1.2
1.18
Relative Cost
1.16
1.14
1.12
1.1
1.08
1.06
1.04
1.02
1
0.98
20
25
30
35
40
45
50
55
60
Gradient ( MV/m)
* TPC is for 500 GeV machine in US Options Study but does not include additional unpowered tunnel sections.
Contributions to TPC (One Linac)
Cryo Plant
Cost (B$)
Cryomodules
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
20
30
40
50
Gradient ( MV/m)
60
0.1
20
30
40
50
Gradient ( MV/m)
60
Relative TPC -vs- Bunch Train Length
1.05
Relative Cost
1.04
1.03
1.02
G = 35 MV/m
1.01
23.4 MV/m
1
0.99
400
600
800
1000
1200
Bunch Train Length (us)
1400
Relative TPC -vs- Luminosity
(35 MV/m, Fixed Bunch Charge, Linac Changes Only)
1.04
1.02
Relative Cost
Reduce Beam Current
1
Reduce Rep Rate
0.98
0.96
Reduce Train Length
0.94
0.92
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Relative Luminosity
0.8
0.9
1
Part II: ILC Design
Choices
ILC Layout
(not to scale)
Initial
500 GeV CMS
Future Upgrade
1000 GeV CMS
Operational Parameter Plane
• Rather than a specific machine design, the
baseline configuration is for an operating ‘plane’.
• Four parameters sets proposed to achieve
design peak luminosity:
– Nominal.
– Low bunch charge (Q).
– Large vertical IP beam size (Large Y).
– Low Beam Power (Low P).
• Not strictly fixed sets -> used to define
necessary operational flexibility.
Parameter Plane
N
nb
gex,y
bx,y
sx,y
tb
dBS
sz
Pbeam
L
nominal
low Q
large Y
low P
2
1
2
2
2820
5640
2820
1330
mm, nm
9.6, 40
10,30
12,80
10,35
cm, mm
2, 0.4
1.2, 0.2
1, 0.4
1, 0.2
nm
543, 5.7
495, 3.5
495, 8
452, 3.8
ns
308
154
308
462
%
2.2
1.8
2.4
5.7
mm
300
150
500
200
MW
11
11
11
5.3
1034
2
2
2
2
1010
Main Linac Design
• Baseline Configuration Document (BCD) distilled from
Snowmass Working Group recommendations in August
2005.
• Major differences from 2001 Tesla TDR 500 GeV Design.
– Higher gradient (31.5 MV/m instead of 23.4 MV/m) for cost
savings.
– Two tunnels (service and beam) instead of one for improved
availability.
• The Linac Area Group of the Global Design Effort (GDE)
is continuing to evolve design.
1.3 GHz
TESLA
Cavities
‣
For ILC, would accept only ‘vertically’ tested cavities (using CW rf without high
power couplers) that achieve gradients > 35 MV/m and Q > 8e9 (discard or
reprocess rejects).
‣
When installed in 8 cavity cryomodules, expect stable operation at an average
gradient of 31.5 MV/m and Q = 1e10 (rf system designed for 35 MV/m).
‣
Derating due to desire for overhead from quench limit, lower installed
performance and limitations from using a common rf source.
‣
For a 1 TeV upgrade, expect average gradient = 36 MV/m, Q = 1e10 for new
cavities (the TDR 800 GeV design assumed 35 MV/m and Q > 5e9).
ILC Linac RF Unit (1 of ~ 600)
Gradient = 31.5 MV/m
Bunch Charge = 2e10 e
Rep Rate = 5 Hz
# of Bunches = 2967
Bunch Spacing = 337 ns
Beam Current = 9.5 mA
Input Power = 311 kW
Fill Time = 565 ms
Train Length = 1000 ms
(8 Cavities per Cryomodule)
Achieved Gradients in Single and 9-Cell Cavities
• In recent years, single-cell cavity gradients approached fundamental limit:
Bc * (Grad / B surface) ~ 1800/41.5 ~ 43 MV/m for Tesla-shape cavities.
• During past 2.5 years, DESY has produced 6 fully-dressed cavities with
Gradients > 35 MV/m and Q > 8e9. Yield for such cavities < 30%.
Test Results for Dressed-Cavities that will be
used in a ’35 MV/m’ Cryomodule
Main Production Problem Has Been
Poor Reproducibility
45
40
35
BCP
EP
10 per. Mov. Avg. (BCP)
10 per. Mov. Avg. (EP)
ILC Goal
Eacc[MV/m]
30
25
20
15
10
5
Gradients achieved over time in DESY cavities
0
Jan-95 Jan-96 Jan-97 Jan-98 Jan-99 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04 Jan-05 Jan-06
Achieved Gradients in Tesla Test Facility (TTF)
8-Cavity Cryomodules
(Cavities not Electro-Polished)
Diamonds and Error Bars = Range of Gradients Achieved in
Individual CW Cavity Tests.
Gradient (MV/m)
 = Average Gradient Achieved in Cryomodule
Cryomodule Number
High Gradient R&D: Low Loss (LL) and ReEntrant (RE) Cells with a Lower Bpeak/Eacc Ratio
Single Cell Results: Eacc = 47 - 52 MV/m
Fabricated
at Cornell
/Ichiro
Studies also underway using single or
large grain Nb – could eliminate need
for Electro-Polishing (EP)
CEBAF Single cell Chinese Large Grain
Q0 vs. Eacc
BCP + 120C Baking
1.00E+11
Test#5a,after 1250C,3hrs,in situ baked
Test #2,no bake
Test#5,after 1250C,3 hrs, no bake
Test #2/5/5a
Q0
1.00E+10
Quench @ 36.6 MV/m
1.00E+09
0
5
10
15
20
Eacc [MV/m]
25
30
35
40
Tuning the Cavities
•
Both slow (500 kHz over minutes) and fast (2.5 kHz
during the 1.6 ms pulse) tuning required – achieve
by compressing the cavity (~ 1 micron per 300 Hz).
•
Want tuners located away from cavity ends to
minimize cavity spacing.
•
‘Blade Tuner’ shown below. To date, have not
achieved more than ~1kHz range of fast tuning.
Final design for BCD not yet chosen.
RF Fill Dynamics
• For ILC, Qext = 4e6 so cavity BW = 325 Hz (DL = 1 micron).
• Need to achieve < 0.1% energy gain uniformity with Low Level RF
(LLRF) system
– Feedback to maintain constant ‘sum of fields’ in 24 cavities
Low Level RF Feedback Control
RF Distribution Math
(for 35 MV/m Max Operation)
10 MW Klystron
35 MV/m * 9.5 mA * 1.038 m = 345 kW (Cavity Input Power)
× 24 Cavities
× 1/.93 (Distribution Losses)
× 1/.89 (Tuning Overhead)
═ 10.0 MW
Modulators (115 kV, 135 A, 1.5 ms, 5 Hz)
(~ 2 m Long)
To generate pulse, an array of capacitors
is slowly charged in parallel and then
discharged in series using IGBT switches.
Pulse Transformer Style
Will test full prototype in 2006
ILC Baseline
Pulse Transformer
Modulator
IGBT’s
Marx Generator Modulator
12 kV Marx Cell (1 of 16)
• IGBT switched
• No magnetic core
• Air cooled (no oil)
Modulators
• Baseline: Pulse Transformer
– 10 units have been built over 10 years, 3 by FNAL and 7 by industry.
– 8 modulators in operation – no major reliability problems (DESY
continuing to work with industry on improvements).
– FNAL working on a more cost efficient and compact design, SLAC
building new dual IGBT switch.
• Alternative: Marx Generator
– Solid state, 1/n redundant modular design for inherent high
availability, reliability.
– Highly repetitive IGBT modules (90,000) cheap to manufacture.
– Eliminating transformer saves size, weight and cost, improves energy
efficiency.
Other
Alternative
Modulators
SNS High Voltage Converter
Modulator (Unit installed at SLAC)
RECTIFIER
TRANSFORMER
AND FILTERS
SCR
REGULATOR
ENERGY
STORAGE
SWITCHING
BOOST
TRANSFORMER
HV RECTIFIER
AND FILTER
NETWORK
4mH
400A
13.8KV
3Ø
CØ
-HV
-HV
-HV
10ohm
20mH
BØ
.03uF
3Ø
(ON/OFF)
6 EACH
50mH
.05uF
AØ
BØ
CØ
.03uF
VMON
RTN
6 EACH
AØ
INPUT
LINE CHOKE
5th
HARMONIC
TRAP
7th
HARMONIC
TRAP
RECTIFIER
TRANSFORMER
AND FILTERS
HV
OUTPUT
4mH
400A
SCR
REGULATOR
HVCM
EQUIPMENT
CONTROL RACK
Series Switch Modulator
(Diversified Technologies, Inc. )
IGBT Series Switch
140kV, 500A switch shown at left
in use at CPI
As a Phase II SBIR, DTI is building
a 120 kV, 130 A version with a
bouncer to be delivered to SLAC at
the end of 2006
Klystrons
Baseline: 10 MW Multi-Beam Klystrons (MBKs) with ~ 65%
Efficiency: Being Developed by Three Tube Companies in
Collaboration with DESY
Thales
CPI
Toshiba
Status of the 10 MW MBKs
• Thales: Four tubes produced, gun arcing problem occured and seemed
to be corrected in last two tubes after fixes applied (met spec). However,
tubes recently developed other arcing problems above 8 MW. Thales to
build two more without changes and two with changes after problem is
better diagnosed.
• CPI: One tube built and factory tested to 10 MW at short pulse. At DESY
with full pulse testing, it developed vacuum leak after 8.3 MW achieved –
has been repaired and will be tested again.
• Toshiba: One tube built and achieved operation spec but developed
arcing problems above 8 MW – being shipped to DESY for further
evaluation.
• These are vertically mounted tubes – DESY will soon ask for bids on
horizontally mounted tubes for XFEL (also needed for ILC).
Alternative Tube Designs
10 MW Sheet Beam
Klystron (SBK)
5 MW Inductive Output
Tube (IOT)
Low Voltage
10 MW MBK
Parameters similar to
10 MW MBK
Output
Voltage 65 kV
Current 238A
More beams
Klystron
Perhaps use a Direct
Switch Modulator
IOT
Drive
SLAC
CPI
KEK
Klystron Summary
• The 10 MW MBK is the baseline choice – continue to
support tube companies to make them robust (DESY needs
35 for XFEL although will run at 5 MW).
• SLAC funding design of a 10 MW sheet-beam klystron (will
take several years to develop).
• Backup 1: Thales 2104C 5 MW tube used at DESY and
FNAL for testing – it appear reliable (in service for 30 years)
but has lower effiency compared to MBKs (42% vs 65%).
• Backup 2: With increased DOE funding next year, propose
to contract tube companies to develop high efficiency,
single-beam, 5 MW klystron.
RF Distribution
Baseline choice is the
waveguide system used at
TTF, which includes offthe-shelf couplers,
circulators and 3-stub
tuners (phase control).
Need more compact design
(Each Cavity Fed 350 kW, 1.5 msec Pulses at 5 Hz)
Two of ~ 16,000 Feeds
And should consider simplifications
(circulators are ~ 1/3 of cost)
Baseline
Alternative Design with No Circulators
Adjustable Tap-Offs Using Mode Rotation
load
4
1
rotatable
joints
2
3
feed
Rotatable section with central
elliptical region, matched for both
polarizations of circular TE11 mode
with differing phase lengths.
a
2a
C. Nantista
Proposed RF Distribution Layout
• Adjustable power to pairs of cavities
• No circulators
• Pairs feed by 3 dB hybrids (requires nl +/- 90 degree
cavity spacing – only 7 mm longer than TDR/BCD spec)
loads
flexible waveguide
cavity couplers
diagnostic
directional
couplers
1.326 m
three-stub
tuners
beam direction
3-dB hybrids
C. Nantista
Cryomodules
TTF Module
Installation
date
Cold time
[months]
CryoCap
Oct 96
50
M1
Mar 97
5
M1 rep.
Jan 98
12
M2
Sep 98
44
M3
Jun 99
35
M1*
MSS
Jun 02
30
8
M3*
M4
M5
Apr 03
19
19
19
M2*
Feb 04
16
Cryomodule Cross Section
Multipacting Simulation of TTF3 Coupler
“Cold Side”
Bellows
Primaries -Green, Secondaries- Red
Cryomodule Design
Relative to the TTF cryomodules
– Continue with 8 cavities per cryomodule based on experience and
minimal cost savings if number increased (12 in TDR).
– Move quad / corrector / bpm package to center (from end) to
improve stability.
– Increase some of cryogenic pipe sizes (similar to that proposed for
the XFEL).
– Decrease cavity separation from 344 mm to 283 mm as proposed
in the TDR.
Beam-Related Design Issues
• Optics / Tolerances / Operation similar to that in TDR:
– One quad per rf unit (three, 8-cavity cryomodules).
– Few hundred micron installation tolerances for cavity, quad and BPM
(demonstrated with TTF cryomodules).
– Cavity BPM resolution of a few μm (should be achievable).
– Use quad shunting and DFS tuning algorithms for dispersion control
(need to better understanding systematic effects).
– Assume beamline will follow Earth’s curvature.
– XFEL will serve as a benchmark although emittance much larger.
• Alternatives for cost savings.
– Larger quad spacing at high energy end of the linac where wake and
dispersion effects smaller.
– Halve quad and bpm aperture to allow superferric quad and higher
resolution BPMs (increases wakes by 10%).
Quad / Corrector / BPM Package
887 mm
66
666 mm
77
BPM
78
Quad and
Correctors
ILC
Proposal
TDR
Quad / Corrector / BPM Package
S-Band BPM Design
(36 mm ID, 126 mm OD)
SC Coils
Iron Yoke
Block
Al Cylinder
SC ‘Cos(2f)’ Quadrupole Magnet
Dipole Design: Flux density and Flux Lines
Vertical Quad Vibration at TTF
(ILC Goal: < 100 nm uncorrelated rms motion > ~ 0.2 Hz)
Noise
Integrated RMS Motion (nm)
100
10
1
0.1
1
10
100
Frequency (Hz)
1000
Vertical Displacements of Cryomodule
4 and 5 after Cooldown Relative to Positions
Just Prior to Cooldown
EPAC04
Alternative Quad Location
TTF
Cavity
Cavity
Quad
Alternative
Cavity
Quad
Cavity
S-Band BPM Triplet Results
(0.8 micron resolution, 1.4e10 electrons, Q of 500 for clean bunch separation)
Cryogenic System
Assume static heat leaks based
on TTF measurements instead of
the smaller values assumed in
the TDR
TESLA cryogenic unit
To Cryoplant
Cryoplant Layout
For ILC 500, require 57 MW of AC power for Cryoplants
ILC Tunnel Layout
For baseline, developing deep underground (~100 m)
layout with 4-5 m diameter tunnels spaced by 5 m.
TESLA Tunnel (TDR)
ILC Availability Challenge
• The ILC will be an order of magnitude more complex
than any accelerator ever built.
• If it is built like present HEP accelerators, it will be
down an order of magnitude more (essentially
always down).
• For reasonable uptime, component availability must
be much better than ever before. Must do R&D and
budget for it up-front.
Lifetime Improvements
Device
magnets - water cooled
power supply controllers
flow switches
water instrumention near pump
power supplies
kicker pulser
coupler interlock sensors
collimators and beam stoppers
all electronics modules
AC breakers < 500 kW
vacuum valve controllers
regional MPS system
power supply - corrector
vacuum valves
water pumps
modulator
klystron - linac
coupler interlock electronics
Tom Himel
Required MTBF
Improvement Factor
20
50
10
10
5
5
5
5
10
10
5
5
3
3
3
MTBF from Present
Experience (khours)
1,000
100
250
30
200
100
1,000
100
100
360
190
5
400
1,000
120
50
40
1,000
SC Linac Summary
• A superconducting linac allows for efficient
acceleration – for the ILC with a 35 MV/m gradient,
– 60% RF-to-Beam Efficiency
– 15% AC-to-Beam Efficiency
• A low beam current is required to make this approach
cost competitive (matches rf source capabilities).
• The resulting long bunch trains require large damping
rings with untested designs.
ILC Design Summary
• Basic linac design complete: converging on details
– Tradeoffs of operability, availability and cost.
• Major cost and technical risks
– Producing cryomodules that meet design gradient at a reasonable
cost (cost model still in development, XFEL will provide a reference,
and will get new industry-based estimates).
– Producing a robust 10 MW klystron.
• Potential Cost Savings
– Adopt Marx Modulator
– Use simpler rf distribution scheme
– Have one tunnel although ‘the additional cost is marginal when
considering the necessary overhead and equipment improvements
to comply with reliability and safety issues.’
– Reduce cavity aperture to 60 mm for 21% reduction in dynamic
cryo-loading and 16% reduction in cavity fill time.