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MAIN LINAC DDS DESIGNMAIN LINAC DDS DESIGN
Vasim Khan
06.11.09Bohr seminar series, HEP group, The University of Manchester
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Outlook• CLIC scheme
• Two Beam Acceleration
• Optimised parameters
• What is wakefield
• Main Linac
• Design constraints
• Present structure
• Our DDS design
• Comparison
• Forthcoming V. Khan Bohr seminar series, HEP group, The University of Manchester 06.11.09 1/34
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CLIC schemeCLIC scheme• e- e+ collider
• C. M. Energy : 3 TeV
• Normal conducting technology
• Frequency : 12 GHz
• Acc. Gradient : 100 MV/m
• Luminosity : ~ 1034 cm-2 s-1
• Novel technique : Two beam acceleration
• Overall site length 48 km........(compact ?)
V. Khan Bohr seminar series, HEP group, The University of Manchester 06.11.09 2/34
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CLIC parameters*
Parameters Designed value unit
C.M Energy 3 TeV
Frequency 11.9942 GHz
Acc. Gradient 100 MV/m
No. of cells per structure 24
luminosity 5.9 x 1034 cm-2 s-1
Luminosity in 1% of energy 2 x 1034 cm-2 s-1
No. of particles per bunch 3.72 x 109
No. of bunches per pulse 312
Bunch length@ IP 44 μm
Transverse emittance (x,y) @ IP 660, 20 nm rad
Beam size (x,y) @ IP 40,0.9 nm
Crossing angle @ IP 20 mrad
Beam Power 14 MW
Total site length 48.4 km
Total site AC power 392 MW
Overall wall plug-beam efficiency 7.1 %
* H. Braun, et al. , Updated CLIC Parameters, CLIC-Note 764, 2008.
V. Khan Bohr seminar series, HEP group, The University of Manchester 06.11.09 3/34
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CLIC complete layout
SECTOR/LINAC: 24PETS/SCTOR: 1491No. of acc. Str./PETS: 2
Main Linac
Ref: H. Braun, et al. , Updated CLIC Parameters, CLIC-Note 764, 2008.
V. Khan Bohr seminar series, HEP group, The University of Manchester 06.11.09 4/34
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Two Beam Acceleration• 140,000 main linac structures.
• It is difficult to supply power using conventional RF source i.e. Klystron......it will require 10,000’s of such klystrons.
• A low energy high current beam (drive beam) running parallel to the main beam.
• Drive beam interacts with the impedance of the Power Extraction and Transfer Structures.
• Drive beam is thus decelerated.
• The decelerated energy is used to accelerate main beam.
V. Khan Bohr seminar series, HEP group, The University of Manchester 06.11.09 5/34
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Why ? • 1) Linacs ?
• 2) Normal conducting ?
• 3) X-band frequency of 12 GHz ?
Synchrotron radiation [1] Energy loss per revolution [1]
2
4γ
4
synρ
E
2π
cCβP ρ
ECβU
4γ
3
0
Colliders Particle Beam energy
Circumference ∆E/rev.
circular TeV km
LHC p-p 7.0 27 6.0 keV
LEP e- e+ 0.1 26.7 2.8 MeV
CLIC* e- e+ 1.5 106 2.8 MeV
* Assume a circular CLIC collider (very impractical)
[1] S.Y. Lee, Accelerator Physics.
V. Khan Bohr seminar series, HEP group, The University of Manchester 06.11.09 6/34
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Is there any energy loss in linear acceleration ?
dx
dE
mc
r
dtdE
P e23
2
Colliders Particle Beam energy
Accelerator length P/(dE/dt)
Linear TeV km %
CLIC e- e+ 1.5 21 10-12
CLIC* e- e+ 55 20 1.0
*Assume if CLIC was proposed to accelerate with accelerating gradient of ~2.75 GeV/m(an impossibly large gradient)
In Linac we consider the power radiated to the power supplied by an external source [1]
[1] S.Y. Lee, Accelerator Physics.
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Frequency scaling of rf parametersRF parameters Requirement Normal
conductingSuperconducting
RF surface resistance (Rs) Low
Power dissipated (Pdis) Low
Quality factor (Q) High
Shunt impedance per unit length (R’)
High
2
1
1 21 1
Need high gradient for a feasible site length High gradient cavities will have high surface fields Super conducting (SC) cavities can be operated up to ~ 40 MV/m High gradient in SC cavities will quench the superconductivity. Possible option is Normal Conducting (NC) cavities.
Normal conducting ?
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X-band (8-12 GHz)?
• Initial proposal was 150 MV/m @ 30 GHz
• Operation with these parameters will suffer major breakdown issues
• The optimisation procedure has resulted in 100 MV/m gradient with 12 & 14 GHz frequency option.
• 12 GHz frequency was chosen to utilise more than two decades of R & D in the NLC/GLC project which was also proposed at 12 GHz.
V. Khan Bohr seminar series, HEP group, The University of Manchester 06.11.09 9/34
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What is wakefield ?
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What is wakefield ?
Fields excited by the ultra relativistic (v~c) particlesShort range wake :tail of the bunch experiences field excited by the head of the bunchLong range wake : trailing bunches experience fields excited by the leading bunchesTransverse wake : emittance dilution luminosity dilutionLongitudinal : energy spread
yxσσ
1αLεβσ
ε= Emittanceσ= Beam sizeβ= Beta functionL=Luminosity
V. Khan Bohr seminar series, HEP group, The University of Manchester 06.11.09 10/34
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Main LinacMain Linac
~25 cm
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Fundamental concepts
• Synchronous mode : Most dominating mode in an accelerating cell, its phase vel. is in synchronous with speed of light
• Bandwidth : Difference between the synchronous frequencies of the end cells (lowest dipole )
• Large BW : 3.3 GHz
• Small BW : 1 GHz
• Moderate BW : 2.3 GHz
• Heavy Damping : Q ~10[1]
• Moderate Damping : Q ~500-1000[2] Light line
Syn. mode
Ref: [1]: A. Grudiev and W. Wuenschs, LINAC08 .
[2]: R. Jones, et al. , PRSTAB 9, 102001, (2006).
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Constraints RF breakdown constraint [1],[2]
1)
2) Pulsed surface heating
3) Cost factor
Beam dynamics constraints [1],[2]
1)For a given structure, no. of particles per bunch N is decided by the <a>/λ and Δa/<a>2)Maximum allowed wake on the first trailing bunch
Rest of the bunches should see a wake less than this wake(i.e. No recoherence).
260MV/mEmaxsur
K 56ΔTmax
mmnsMW 18CτP 3in
3pin
N
10 4 mm/m6.667V/pC/W
9
t1
Ref: [1]: A. Grudiev and W. Wuensch, Design of an x-band accelerating structure for the CLIC main linacs, LINAC08 . [2]: H. Braun, et al. , Updated CLIC Parameters, CLIC-Note 764, 2008.
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Accelerating cells : Several designs
4.5 mm
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Wakefield suppression in CLIC main linacs
To minimise the breakdown probability and reduce the pulse surface heating, we are looking into an alternative scheme for the main accelerating structures:
• Detuning the first dipole band by forcing the cell parameters to have Gaussian spread in the frequencies
• Considering the moderate damping Q~500
The present main accelerating structure (WDS) for the CLIC relies on linear tapering of cell parameters and heavy damping with a Q of ~10. The wake-field suppression in this case entails locating the dielectric damping materials in relatively close proximity to the location of the accelerating cells.
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CLIC_G: Present baseline waveguide damped design
Ref: A. Grudiev, W. Wuensch, Design of an x-band accelerating structure for the CLIC main linacs, LINAC08
Structure CLIC_G
Frequency (GHz) 12
Avg. Iris radius/wavelength <a>/λ
0.11
Input / Output iris radii (mm) 3.15, 2.35
Input / Output iris thickness (mm)
1.67, 1.0
Group velocity (% c) 1.66, 0.83
No. of cells per cavity 24
Bunch separation (rf cycles) 6
No. of bunches in a train 312
pp
N
1ppt 2Q
i1tiωExpK
N
2W
Ref: R. Jones, PRSTAB 12, 104801, (2009).
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Damped and detuned design
• Detuning: A smooth variation in the iris radii spreads the dipole frequencies. This spread does not allow wake to add in phase
• Error function distribution to the iris radii varion results in a rapid decay of wakefield.
• Due to limited number of cells in a structure (trunated Gaussian) wakefield recoheres.
• Damping: The recoherence of the wakefield is suppressed by means of a damping waveguide like structure (manifold).
• Interleaving neighbouring structure frequencies help enhance the wake suppression
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NLC/GLC DDS design
High powerrf coupler
HOM coupler
Beam tube
Acceleration cells
Manifold
Ref: R. Jones, et al. , PRSTAB 9, 102001, (2006).
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Advantages• Moderate damping scheme: Breakdown probability is reduced
• Pulse temperature rise is reduced
• Manifolds can be used for beam position monitoring and remote measurements of cell alignments*.
Disadvantages• Need bigger bandwidth for adequate detunig and hence more input power
to achieve desired accelerating gradient
* Ref: R. Jones, et al. , SLAC-PUB 7388, 1996. R. Jones, et al. , SLAC-PUB 7539, 1997
Cell offsets of DDS1 obtained by coordinate measurement machine (CMM), indicated by red connected dots and, inferred from the energy radiated from the HOM ports (Pmin), indicated by a black dashed line.
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Key parameters for designing an accelerating structure @ 12 GHz & 100 MV/m
• Iris radii of the end cells
• Iris thickness
• <a>/λ
• Group velocity
• No. of cells per structure
• Bunch spacing
• Bunch charge
• No. of bunches in a train => pulse length
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Large bandwidth structure
df
dnK α WT
Error function distribution
Re erf n 4i t / 2 2where : (t, f )
erf n / 2 2
22 t
t
Truncated Gaussian :
W 2Ke (t, f )
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Eight fold interleaved structure
3.3 GHz structure does satisfy beam dynamics constraints but does not satisfy RF breakdown constraints.
Finite no of modes leads to a recoherance at ~ 85 ns.But for a damping Q of ~1000 the amplitude wake is still below 1V/pc/mm/m
Why not 3.3 GHz structure?
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Small bandwidth structure : Zero crossing scheme
p
pp
N
1ppt 2Q
tωexptωexpK
N
2ImW
pp
N
1ppt 2Q
i1tiωExpK
N
2W
Parameters closely tied to that of CLIC_G with two major changes
1)Gaussian distribution of cell parameters2)Q= 500
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CLIC_ZC structure# Parameters ZC1 ZC2 Unit
1 <a>/λ 0.102 0.1 -
2 IP/OP iris thickness 1.6 / 0.7 1.6/0.7 mm
3 IP / OP iris radii 2.99 / 2.13 2.87/2.13 mm
4 IP / OP group velocity 1.49 / 0.83 1.45/.83 mm
5 First / Last cell Q0 6366 / 6643 6408/6668 -
6 First / Last cell Shunt impedance
107 / 138 108/138 MΏ /m
7 Filling time 56.8 58.6 ns
8 IP Power (peak) 48 47 MW
9 RF-to-beam efficiency 27.09 26.11 %
10 Bunch population 3.0 x109 2.9 x 109 -
11 Esur (max) 285 231 MV/m
13 ∆T max 20 25.2 K
14 14.07 14.36 MW(ns)^1/3/mminpin CP 3
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Why not zero crossing scheme ?
• Though RF breakdown constraints are satisfied it will be very challenging to achieve zero crossing scheme due to tight tolerances.
• It may not be feasible to build a structure based on zero crossing scheme.
• Need many beam dynamics simulations with realistic offsets and random errors.
• Possible option is a moderate bandwidth.
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Cell parameters Cell # 1 Cell # 24
Iris radius (mm) 4.0 2.3
Iris thickness (mm) 4.0 0.7
Ellipticity 1.0 2.0
Q 4771 6355
R’/Q (kΩ/m) 11.64 20.09
vg/c (%) 2.13 0.9
∆f = 3.6 σ = 2.3 GHz∆f/fc =13.75 %<a>/λ=0.126
A 2.3 GHz Damped-detuned structure
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Typical DDS cell
Manifold
Coupling slot
Accelerating mode(monopole mode)
Dipole modeManifold mode
E-field in a quarter symmetry DDS cell
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24 cellsNo interleaving
48cells2-fold interleaving
∆fmin = 32.5 MHz∆tmax =30.76 ns∆s = 9.22 m
24 cellsNo interleaving
∆fmin = 65 MHz∆tmax =15.38 ns∆s = 4.61 m
48cells2-fold interleaving
Spectral function* -----(IFT) Wake function
* Ref: R. Jones, et al. , PRSTAB 9, 102001, (2006).
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96 cells4-fold interleaving
192 cells8-fold interleaving
96 cells4-fold interleaving
∆fmin = 16.25 MHz∆tmax = 61.52 ns∆s = 18.46 m
192 cells8-fold interleaving
∆fmin = 8.12 MHz∆tmax =123 ns∆s = 36.92 m
Spectral function -----(IFT) Wake function
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A 1.19
GHz 11.9942
6101.61072.3
I19-9
For CLIC_G structure <a>/λ=0.11, considering the beam dynamics constraint bunch population is 3.72 x 10^9 particles per bunch and the heavy damping can allow an inter bunch spacing as compact as ~0.5 ns. This leads to about 1 A beam current and rf –to-beam efficiency of ~28%.
For CLIC_DDS structure (2.3 GHz) <a>/λ=0.126, and has an advantage of populating bunches up to 4.5x10^9 particles but a moderate Q~500 will require an inter bunch spacing of 8 cycles (~ 0.67 ns).
A 1.13
GHz 11.9942
8101.6104.75
I19-9
V/pc/mm/m 1.71072.3150
10410010W
9
9limitT
V/pc/mm/m 5.6104.75150
10410010W
9
9limitT
Though the bunch spacing is increased in CLIC_DDS, the beam current is compensated by increasing the bunch population and hence the rf-to-beam efficiency of the structure is not affected alarmingly.
CLIC_G vs CLIC_DDS
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Parameters CLIC_G (Optimised)
[1,2]
CLIC_DDS(Single
structure)
CLIC_DDS*(8-fold
interleaved)
Bunch space (rf cycles/ns) 6/0.5 8/0.67 8/0.67
Limit on wake (V/pC/mm/m) 7.1 5.6 5.3
Number of bunches 312 312 312
Bunch population (109) 3.72 4.7 5.0
Pulse length (ns) 240.8 273 272.2
Fill time (ns) 62.9 42 40.8
Pin (MW) 63.8 72 75.8
Esur max. (MV/m) 245 232 224
Pulse temperature rise (K) 53 47 51
RF-beam-eff. 27.7 26.6 26.7
Figure of merit (a.u.) 9.1 8.41 8.29
[1] A. Grudiev, CLIC-ACE, JAN 08[2] H. Braun, CLIC Note 764, 2008* Averaged values of structure #1 & #8
CLIC_G vs CLIC_DDS
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Closing remarks• We have observed an error in the modelling software (several ver.
available) and in interpretation of meshing the geometry .
• We are re-examining the simulations in order to verify the accuracy of the results and calculations based on these results.
• Mechanical design with power couplers.
• Beam dynamics simulations of complete 21 km linac.
• The DDS design will result in reduced surface fields and comparable efficiency with respect to CLIC_G.
• We have a strong collaboration with CLIC group @ CERN and we anticipate a full design early next year which will be high power tested by the end of 2010.
• .................................Thesis writing....
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Acknowledgements
Firstly my acknowledgment goes to my supervisor Roger Jones for his patience guidance. I thank members of our MEW group for their suggestions throughout my work.
I would like to thank our collaborators for their involvement in discussions and many useful suggestions from
CERN : W. Wuensch, A. Grudiev, D. Schulte and R. ZennaroKEK : T. HigoSLAC : J. Wang and Z. Li
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Many of life’s failures are people who did not realise how close they were to success when they gave up.
.................Thomas Edison
Thank you ........Thank you ........
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Additional slides
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List of Publications
• Khan and Jones, Investigation of an alternate means of wakefield suppression in the main linacs of CLIC, PAC09, Canada.
• Khan and Jones, An alternate design for CLIC main linac wakefield suppression, XB08, U.K.
• Khan and Jones, Beam dynamics and wakefield simulations for the CLIC main linacs, LINAC08, Canada.
• Khan and Jones, Wakefield suppression in hte CLIC main linac, EPAC08, Italy.