MAIN LINAC DDS DESIGN Vasim Khan Bohr 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 /34

CLIC scheme e - e + collider C. M. Energy : 3 TeV Normal conducting technology Frequency : 12 GHz Acc. Gradient : 100 MV/m Luminosity : ~ 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 /34

CLIC parameters* ParametersDesigned valueunit C.M Energy3TeV Frequency GHz Acc. Gradient100MV/m No. of cells per structure24 luminosity5.9 x cm -2 s -1 Luminosity in 1% of energy2 x cm -2 s -1 No. of particles per bunch3.72 x 10 9 No. of bunches per pulse312 Bunch IP44μmμm Transverse emittance IP660, 20nm rad Beam size IP40,0.9nm Crossing IP20mrad Beam Power14MW Total site length48.4km Total site AC power392MW Overall wall plug-beam efficiency7.1% * H. Braun, et al., Updated CLIC Parameters, CLIC-Note 764, V. Khan Bohr seminar series, HEP group, The University of Manchester /34

CLIC complete layout SECTOR/LINAC: 24 PETS/SCTOR: 1491 No. of acc. Str./PETS: 2 Main Linac Ref: H. Braun, et al., Updated CLIC Parameters, CLIC-Note 764, V. Khan Bohr seminar series, HEP group, The University of Manchester /34

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 /34

Why ? 1) Linacs ? 2) Normal conducting ? 3) X-band frequency of 12 GHz ? Synchrotron radiation [1] Energy loss per revolution [1] CollidersParticleBeam energy Circumference∆E/rev. circularTeVkm LHCp-p keV LEPe - e MeV CLIC * e - e 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 /34

Is there any energy loss in linear acceleration ? CollidersParticleBeam energy Accelerator lengthP/(dE/dt) LinearTeVkm% CLICe - e CLIC*e - e *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. V. Khan Bohr seminar series, HEP group, The University of Manchester /34

Frequency scaling of rf parameters RF parametersRequirementNormal conducting Super conducting RF surface resistance (Rs)Low Power dissipated (P dis )Low Quality factor (Q)High Shunt impedance per unit length (R’) High  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 ? V. Khan Bohr seminar series, HEP group, The University of Manchester /34

X-band (8-12 GHz)? Initial proposal was 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 /34

What is wakefield ? V. Khan Bohr seminar series, HEP group, The University of Manchester /34

What is wakefield ?  Fields excited by the ultra relativistic (v~c) particles  Short range wake :tail of the bunch experiences field excited by the head of the bunch  Long range wake : trailing bunches experience fields excited by the leading bunches  Transverse wake : emittance dilution luminosity dilution  Longitudinal : energy spread ε= Emittance σ= Beam size β= Beta function L=Luminosity V. Khan Bohr seminar series, HEP group, The University of Manchester /34

Main Linac ~25 cm V. Khan Bohr seminar series, HEP group, The University of Manchester /34

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 ~ [2] Light line Syn. mode Ref: [1]: A. Grudiev and W. Wuenschs, LINAC08. [2]: R. Jones, et al., PRSTAB 9, , (2006). V. Khan Bohr seminar series, HEP group, The University of Manchester /34

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 /λ and Δa/ 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). 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, V. Khan Bohr seminar series, HEP group, The University of Manchester /34

Accelerating cells : Several designs 4.5 mm V. Khan Bohr seminar series, HEP group, The University of Manchester /34

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. V. Khan Bohr seminar series, HEP group, The University of Manchester /34

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 StructureCLIC_G Frequency (GHz)12 Avg. Iris radius/wavelength /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 cavity24 Bunch separation (rf cycles)6 No. of bunches in a train312 Ref: R. Jones, PRSTAB 12, , (2009). V. Khan Bohr seminar series, HEP group, The University of Manchester /34

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 V. Khan Bohr seminar series, HEP group, The University of Manchester /34

NLC/GLC DDS design High power rf coupler HOM coupler Beam tube Acceleration cells Manifold Ref: R. Jones, et al., PRSTAB 9, , (2006). V. Khan Bohr seminar series, HEP group, The University of Manchester /34

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, 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 (P min ), indicated by a black dashed line. V. Khan Bohr seminar series, HEP group, The University of Manchester /34

Key parameters for designing an accelerating 12 GHz & 100 MV/m Iris radii of the end cells Iris thickness /λ Group velocity No. of cells per structure Bunch spacing Bunch charge No. of bunches in a train => pulse length V. Khan Bohr seminar series, HEP group, The University of Manchester /34

Large bandwidth structure Error function distribution V. Khan Bohr seminar series, HEP group, The University of Manchester /34

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? V. Khan Bohr seminar series, HEP group, The University of Manchester /34

Small bandwidth structure : Zero crossing scheme Parameters closely tied to that of CLIC_G with two major changes 1)Gaussian distribution of cell parameters 2)Q= 500 V. Khan Bohr seminar series, HEP group, The University of Manchester /34

CLIC_ZC structure #ParametersZC1ZC2Unit 1 /λ IP/OP iris thickness1.6 / 0.7 mm 3IP / OP iris radii2.99 / /2.13mm 4IP / OP group velocity1.49 / /.83mm 5First / Last cell Q06366 / / First / Last cell Shunt impedance 107 / /138MΏ /m 7Filling time ns 8IP Power (peak)4847MW 9RF-to-beam efficiency % 10Bunch population3.0 x x Esur (max)285231MV/m 13∆T max2025.2K MW(ns)^1/3/mm V. Khan Bohr seminar series, HEP group, The University of Manchester /34

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. V. Khan Bohr seminar series, HEP group, The University of Manchester /34

Cell parametersCell # 1Cell # 24 Iris radius (mm) Iris thickness (mm) Ellipticity Q R’/Q (kΩ/m) vg/c (%) ∆f = 3.6 σ = 2.3 GHz ∆f/fc =13.75 % /λ=0.126 A 2.3 GHz Damped-detuned structure V. Khan Bohr seminar series, HEP group, The University of Manchester /34

Typical DDS cell Manifold Coupling slot Accelerating mode (monopole mode) Dipole mode Manifold mode E-field in a quarter symmetry DDS cell V. Khan Bohr seminar series, HEP group, The University of Manchester /34

24 cells No interleaving 48cells 2-fold interleaving ∆fmin = 32.5 MHz ∆tmax =30.76 ns ∆s = 9.22 m 24 cells No interleaving ∆fmin = 65 MHz ∆tmax =15.38 ns ∆s = 4.61 m 48cells 2-fold interleaving Spectral function* -----(IFT)  Wake function * Ref: R. Jones, et al., PRSTAB 9, , (2006). V. Khan Bohr seminar series, HEP group, The University of Manchester /34

96 cells 4-fold interleaving 192 cells 8-fold interleaving 96 cells 4-fold interleaving ∆fmin = MHz ∆tmax = ns ∆s = m 192 cells 8-fold interleaving ∆fmin = 8.12 MHz ∆tmax =123 ns ∆s = m Spectral function -----(IFT)  Wake function V. Khan Bohr seminar series, HEP group, The University of Manchester /34

For CLIC_G structure /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) /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). 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 V. Khan Bohr seminar series, HEP group, The University of Manchester /34

ParametersCLIC_G (Optimised) [1,2] CLIC_DDS (Single structure) CLIC_DDS* (8-fold interleaved) Bunch space (rf cycles/ns)6/0.58/0.67 Limit on wake (V/pC/mm/m) Number of bunches312 Bunch population (10 9 ) Pulse length (ns) Fill time (ns) Pin (MW) Esur max. (MV/m) Pulse temperature rise (K) RF-beam-eff Figure of merit (a.u.) [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 V. Khan Bohr seminar series, HEP group, The University of Manchester /34

V. Khan Bohr seminar series, HEP group, The University of Manchester /34 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 CERN and we anticipate a full design early next year which will be high power tested by the end of 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. Zennaro KEK : T. Higo SLAC : J. Wang and Z. Li V. Khan Bohr seminar series, HEP group, The University of Manchester /34

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 V. Khan Bohr seminar series, HEP group, The University of Manchester /34

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.