R.M. Jones, XB Structures Collaboration Workshop, SLAC, 16 R.M. Jones, XB Structures Collaboration Workshop, SLAC, 16 th th – 18 – 18 th th May May 2011 2011 1 Status of Detuned and Status of Detuned and Manifold-Damped HG Linacs for Manifold-Damped HG Linacs for CLIC CLIC Roger M. Jones The University of Manchester and Cockcroft Institute b a a+a 1 R a 1 t/2 L R c a a R.M. Jones, XB Structures Collaboration Workshop, SLAC, 16 R.M. Jones, XB Structures Collaboration Workshop, SLAC, 16 th th – 18 – 18 th th May May 2011 2011
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Status of Detuned and Manifold-Damped HG Linacs for CLIC
Status of Detuned and Manifold-Damped HG Linacs for CLIC. Roger M. Jones The University of Manchester and Cockcroft Institute. R. t/2. b. a 1. a. R c. L. a. a. a+a 1. R.M. Jones, XB Structures Collaboration Workshop, SLAC, 16 th – 18 th May 2011. Overview. - PowerPoint PPT Presentation
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Band partitioning of kick factors in 206 cell DDS1 X-band structure (facc=11.424 GHz). Largest kick factors located in the first band. Third and sixth bands although, an order of magnitude smaller, must also be be detuned along with the 1st band.
CLIC design facc =11.9942 GHz shifts the dipole bands up in frequency.
1. Band Partitioning1. Band Partitioning
Ref: Jones et. al, 2003, SLAC-PUB 9467
The partitioning of bands changes with phase advance. Choosing a phase advance close to pi per cell results in a diminution of the kick factor of the first band and and enhancement of the 2nd and 3rd bands. A similar effect occurs close to /2.
Kick factors versus phase advance for cells with an iris radius of ~ 4.23 mm.
1. General Aspects of Detuning1. General Aspects of Detuning
Gaussian density distributions Kick factor weighted density function: Kdn/f ~ exp[-(-0
)2/2 2]
Ideally: W(t) ~ exp(- 2 t2/2)
Advantages over other methods1. It is non-resonant and hence it does not freeze collider operation a bunch
spacing other than the minimum bunch spacing.2. Wakefield decreases rapidly and monotonically3. It permits an error function interpolation with relatively sparse parameters
Disadvantages1. Gaussian distribution is not limited and thus eventually it is truncated. This
truncation gives rise to a sinc-like (=sin(x)/x) wake which curtails the rapid fall-off at a level dependent on the truncation point
2. The finite number of cells => finite number of modes => partial recoherence of wake-field starting at a time t ~ 1/fmax (where fmax is the maximum separation of modes). Also, with damping there is another coherence point, further out, at 1/ fmin (where fmin is the minimum separation of modes, which lies in the centre of the Gaussian)
Electron bunch serves as the witness bunchIn traversing the DUT, the witness bunch is deflected by the wake function generated by the positron drive bunch. Witness bunch passes though chicane and down linac where trajectory is recorded by BPMs The transverse wake function is determined by measuring the change in the witness bunch deflection per unit change in the drive bunch offset in the structure. W is the transverse wake function at time t
behind the drive bunch, Ew (~ 1.2 GeV) is the witness bunch energy and yd is the offset in the drive bunch from the electrical centre of the accelerating structure. Wake function units are transverse voltage per drive charge (end), drive offset and structure length (Ls), and
y d wW t y / E
2 2 2 2s de L n exp( / c )
Angular kick imparted to the witness bunch is found from ratio of the transverse to longitudinal energy:
Ref: R. M. Jones, Wake field Suppression in High Gradient Linacs for Lepton Linear Colliders, Phys. Rev. ST Accel. Beams 12, 104801, 2009
1. Determination of HOMs in Structure via Stretched Wire Measurement
Illustrated is an X-band Set-up at SLAC.Designed as part of the GLC/NLC programme.Able to accommodate 1.8m structures.Several other configurations in use internationally.Trapped modes not readily accessible (hence beam- -based verification of simulations needed)
Ref: F. Caspers, Bench methods for beam-coupling impedance measurement (Lecture notes in beams: intensity limitations vol 400) (Berlin, Springer, 1992)
Roger M. Jones (Univ. of Manchester faculty)Alessandro D’Elia (Dec 2008, Univ. of Manchester PDRA based at CERN)Vasim Khan (PhD student, Sept 2007)Nick Shipman (PhD student Sept 2010, largely focused on breakdown studies)Part of EuCARD ( European Coordination for Accelerator Research and Development) FP7 NCLinac Task 9.2
Major Collaborators: W. Wuensch, A. Grudiev, I. Syrachev, R. Zennaro, G. Riddone (CERN)
2. FP420 –RF Staff2. Wake Function Suppression for CLIC -Staff
V. Khan, CI/Univ. of Manchester Ph.D. studentgraduated April 2011 (now CERN Fellow)
A. D’Elia, CI/Univ. of Manchester PDRA based at CERN (former CERN Fellow).
N. Shipman, CERN/CI/Univ. of Manchester Ph.D. student
L. Carver Sept 2011CI/Univ. of Manchester Ph.D. student
2. Introduction –Present CLIC baseline vs. alternate DDS design
The present CLIC structure relies on linear tapering of cell parameters and heavy damping with a Q of ~10. Wake suppression is effected through waveguides and dielectric damping materials in relatively close proximity to accelerating cells. Choke mode suppression provides an alternative, but as shown for SW (V. Dolgashev et al), it will negatively impact Rsh -planned TW structures are worth investigating though (talk by J. Shi?)
A viable alternative is presented by our CLIC_DDS design - parallels the DDS developed for the GLC/NLC, and entails:
1. Detuning the dipole bands by forcing the cell parameters to have a precise spread in the frequencies –presently Gaussian Kdn/df- and interleaving the frequencies of adjacent structures.
2. Moderate damping Q ~ 500-1000
R.M. Jones, XB Structures Collaboration Workshop, SLAC, 16R.M. Jones, XB Structures Collaboration Workshop, SLAC, 16 thth – 18 – 18thth May 2011 May 2011 21EPAC, 26 June 2008 W. Wuensch, CERN
HOM damping waveguides
Magnetic field concentration –pulsed surface heating
High electric field and powerflow region - breakdown
1. Initial investigation of required bandwidth to damp all bunches (~3GHz) –succeeds to suppress wakes, fails breakdown criteria!
2. New design, closely tied to CLIC_G (similar iris a), necessitates a bandwidth of ~ 1 GHz. Geometry modified to hit bunch zero crossings in the wakefield -succeeds from breakdown perspective, tight tolerances necessary to suppress wakes!
3. Relaxed parameters, modify bunch spacing from 6 to 8 rf cycles and modify bunch population. Wake well-suppressed and satisfies surface field constraints. CLIC_DDS_C (f ~ 3.6, 13.75%) –SUCCESS (on suppressing wakes and meeting breakdown criteria)
Three designs
1. Initial investigation of required bandwidth to damp all bunches (~3GHz) –succeeds to suppress wakes, fails breakdown criteria!
2. New design, closely tied to CLIC_G (similar iris a), necessitates a bandwidth of ~ 1 GHz. Geometry modified to hit bunch zero crossings in the wakefield -succeeds from breakdown perspective, tight tolerances necessary to suppress wakes!
Three designs
1. Initial investigation of required bandwidth to damp all bunches (~3GHz) –succeeds to suppress wakes, fails breakdown criteria!
Dispersion curves for select cells are displayed (red used in fits, black reflects accuracy of model)Provided the fits to the lower dipole are accurate, the wake function will be well-representedSpacing of avoided crossing (inset) provides an indication of the degree of coupling (damping Q)
Optimisation of parameters based on manifold damped structures.Vary half-iris thickness. 3-cell simulations, with intermediate parameters obtained via interpolation.Choose parameters with minimal surface E-field, pulse temperature rise, and adequate efficiency.
Chosen optimisation (CLIC_DDS_E)
2. CLIC_DDS_E, Optimisation of:, ∆f and Efficiency
Rounding necessitates reducing this length (moves up)
Rounding
To facilitate machining of indicated sections, roundings are introduced (A. Grudiev, A. D’Elia).In order to accommodate this, Rc needs to be increased DDS2_ER.Coupling of dipole modes is reduced and wake-suppression is degraded. How much?
DDS2_E DDS2_ER
Rc
2. CLIC_DDS_E: Modified Design Based on Engineering Considerations
CLIC_DDS_E :Rc=6.2 - 6.8 mm (optimised penetration)
CLIC_DDS_ER : Rc=6.8 mm const (a single one of these structures constitutes CLIC_DDS_A, being built for HP testing)Wakefield suppression is degraded but still within acceptable limits.
4. CLIC_DDS_E vs CLIC_DDS_ER WakefieldSpectral Function Wakefunction
Info. on the ability of the 8-fold interleaved structure to sustain high e.m. fields and sufficient T can be assessed with a single structure.Single structure fabricated in 2010/1st quarter 2011, CLIC_DDS_A, to fit into the schedule of breakdown tests at CERN.
2. CLIC_DDS_A: Structure Suitable for High Power Testing
Design is based on CLIC_DDS_ERTo facilitate a rapid design, the HOM couplers have been dispensed with in this prototype.Mode launcher design utilisedSRF design complete!Mechanical drawings, full engineering design completed!Qualification end cells fabricated. Received Oct 15 2010 from VDL (Netherlands)
2. CLIC_DDS_A WakeWake of a non-interleaved 24 cell structure –first structure of 8-fold interleaved structure chosen.Motivated by high gradient testingWake is measurable and provides a useful comparison to simulations (but will not, of course, meet beam dynamics criteria)FACET (revive ASSET?) tests of wake?
Firstly, match-out either end of structure with regular cells: Structure for test will utilise a mode launcher Initially, simulate a structure with one regular cell and two matching cells at
either end and we study the minima in S11 as a function of the geometrical parameters of the matching cells (a, L –adopt L variation, rather than b, from space considerations)
Add additional (2, then 3) identical standard cells (const. imp) and follow the same procedure and modify parameters of matching cells to minimise S11
The matching condition (on a, L) is that which coincident with all 3 simulations. Secondly, once complete, match-out the full, tapered structure based on this
2. Cell Qualification of CLIC_DDS_A VDL (Netherlands) have machined and measured several cells –end cells. (recvd by CERN Oct 2010) Global profiles made with optical Zygo machine are illustrated for disk 24 Design, tolerance bounds and achieved profile shown Morikawa (Japan) will fabricate cells –rf test at KEK Fabrication and bonding of complete structure by last quarter of 2011 HP test of structure in 2012?
CLIC_DDS_A : RF (including mode launcher matching cells) and mechanical design has been completed.
Qualifications cells fabricated (VDL)–all cells by last quarter 2010 Structure will be subsequently bonded in the first quarter of 2011, --
ready for high power testing in 2011 at the CLIC test stand. CLIC_DDS_B: Includes HOM couplers and interleaving. HOM
coupler design in progress.
3. Final Remarks
Transfer cell fabrication to Morikawa inc. guided by KEK Cell ETA April 2011 -10 qualification cells, followed by full 24 cellsRecent events in Japan have affected schedule –full structure expected during last quarter of 2011 (T. Higo will update)RF cold test measurements (S21) at KEK
New CLIC_DDS R&D in progress:HPA: High phase advance design is being studied. It Allows optimisation of the remaining parameters –minimise surface fields, wakefields at stipulated vg
Further optimisation is being explored by implementing additional manifolds and with the potential for the insertion of SiC rods to reduce the Q further
I am pleased to acknowledge a strong and fruitful collaboration between many colleagues and in particular, from those at CERN, University of Manchester, Cockcroft Inst., SLAC and KEK.
Several at CERN and KEK within the CLIC programme, have made critical contributions: W. Wuensch, A. Grudiev, I. Syrachev, R. Zennaro, G. Riddone (CERN), T. Higo Y. Higashi (KEK).
3. Acknowledgements
1. R. M. Jones, et. al, PRST-AB, 9, 102001, 2006.
2. V. F. Khan and R.M. Jones, EPAC08, 2008.
3. V. F. Khan and R.M. Jones, LINAC08, 2008.
4. V. F. Khan and R.M. Jones, Proceedings of XB08, 2008.