MAIN LINAC DDS DESIGN

MAIN LINAC DDS DESIGNMAIN LINAC DDS DESIGN

Vasim Khan

06.11.09Bohr seminar series, HEP group, The University of Manchester

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

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

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.


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.


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.


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.


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.


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 ?


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.


What is wakefield ?


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


Main LinacMain Linac

~25 cm


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).


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.


Accelerating cells : Several designs

4.5 mm


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.


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).


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


NLC/GLC DDS design

High powerrf coupler

HOM coupler

Beam tube

Acceleration cells

Manifold

Ref: R. Jones, et al. , PRSTAB 9, 102001, (2006).


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.


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


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 )


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?


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


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


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.


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


Typical DDS cell

Manifold

Coupling slot

Accelerating mode(monopole mode)

Dipole modeManifold mode

E-field in a quarter symmetry DDS cell


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).


96 cells4-fold interleaving



∆fmin = 16.25 MHz∆tmax = 61.52 ns∆s = 18.46 m


∆fmin = 8.12 MHz∆tmax =123 ns∆s = 36.92 m

Spectral function -----(IFT) Wake function


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


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



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....

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


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 ........


Additional slides

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.

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