FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 1 Alternate Means of Wakefield Alternate Means of Wakefield Suppression in CLIC Main Suppression in CLIC Main Linac Linac Roger M. Jones Contribution from Cockcroft Institute and The University of Manchester X-Band Structures and Beam Dynamics Workshop 1 st – 4 th December 2008 The Cockcroft Institute, Daresbury
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Alternate Means of Wakefield Suppression in CLIC Main Linac
Alternate Means of Wakefield Suppression in CLIC Main Linac. Roger M. Jones Contribution from Cockcroft Institute and The University of Manchester. X-Band Structures and Beam Dynamics Workshop 1 st – 4 th December 2008 The Cockcroft Institute, Daresbury. - PowerPoint PPT Presentation
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Abstract of the planned activityThis work package will explore HOM damping in single multi-cell cavities and in groups of thereof. The features of both the long-range and short-range wake-fields will be explored. The consequences of the short-range wake-field on cavity alignment will be delineated. For the long-range wake-fields, trapped modes in particular will be focused upon. Global scattering matrix analysis will be employed in addition to current electromagnetic codes. The frequency sensitivity of the modes will be explored by exploiting a circuit analysis of the electromagnetic field and this will enable the sensitivity of the wake-field to fabrication errors to be evaluated over the complete collider.
At the University of Manchester and the Cockcroft Institute we are actively involved in simulating higher order modes of accelerating cavities and experimentally determining the structure of these modes with a purpose built stretched wire measurement set-up. We are actively involved in using intensive computer codes coupled with cascading of individual sections in order to rapidly compute the modal structure.
List of Goals and MilestonesGoal 1. Develop a circuit model and a generalized scattering matrix technique to obtain accurate calculations on the global electromagnetic field from small segments thereof. This is a study of mode excitation.
Milestones1.1 Sep 09: Write report on circuit model and globalised scattering matrix technique. This will include an analysis of the partitioning of dipole modes in CLIC structures1.2 Apr 110: Produce a report for the design of damping and detuning a CLIC module
Goal 2. Make an accurate simulation of the wake-fields and HOMS. This is expected to be broadly verified with initial experiments on CTF3 and more precisely verified with an experiment at the SLAC FACET facility and stretched wire measurements.
Milestones2.1 Apr 10: Experiments on the measurement of HOMs on CTF3. This will enable the predicted features of HOM damping to be verified although only the broad characteristics of the modes are expected to be measurable.2.2 Aug 10: Perform additional measurements on the wake-field at the SLAC FACET facility. This will facilitate a detailed comparison between the predicted decrement in the wake envelope and experimentally determined values. ASSET typically is accurate to better than 0.01 V/pC/mm/m.
2.3 Sept 10: Write up a report on the experimental measurement of modes.2.4 April 11: Conduct wire measurement on CLIC cavities to verify the distribution of frequencies and kick factors
Goal 3. Undertake beam dynamics simulations with Placet. These simulations will take into account both the long-range and short-range wake-fields. Simulations will be performed both with the baseline design and with relaxed fabrication tolerances. In addition to the standard wake-field the influence of x-y coupling of wake-fields from possible cavity distorsions will also be investigated. Milestones
3.1 April 11: Initial result on baseline beam dynamics simulations 3.2 June 11: Results on beam dynamics simulations with relaxed tolerances and initial simulations on transverse mode coupling3.3 August 11: Report on beam dynamics simulations including long and short range wakefields. 3.3 Sept 11: Report on beam dynamics simulations including transverse mode coupling
Integration of Task 9.2 within NC WP 9
Major goal: Design and measure wakefield suppression in module
Wealth of Experience on Detuned Structure and Manifold Wakefield Suppression
DDS1 DDS3
RDDS1 H60VG4SL17A/B
1. SLAC-PUB 7287 (1996), 2. SLAC-PUB 8174 (1999)3. Wakefield damping in a pair of X-band accelerators for linear colliders.R.M. Jones , et al, Phys.Rev.ST Accel.Beams 9:102001,2006.
Emittance dilution (illustrated by the red dashed curve) versus the percentage change in the bunch spacing. Also shown is the corresponding rms of the sum wake-field (by the solid blue curve).
Emittance we incorporate random frequency errors into a set of 50 accelerating structures and randomly distribute them along the entire linac. In all cases the beam is injected into the linac with an offset of approximately one y, with an energy of 5 GeV and the progress of the
beam is monitored as it traverses the entire linac. The final emittance dilution, together with the rms of the sum wake-field, is illustrated for small changes in the bunch spacing. The particular simulation illustrated includes a cell-to-cell frequency error with an rms value of 20 MHz. We chose this rather large frequency error in order to gain an understanding of the impact of relaxed
**Wakefield damping in a pair of X-band accelerators for linear colliders.R.M. Jones , et al, Phys.Rev.ST Accel.Beams 9:102001,2006.
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.
Band PartitioningBand 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 pi/2.
Kick factors versus phase advance for cells with an iris radius of ~ 4.23 mm.
FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009FP7 NC WP 9 Task 9.2, Kickoff Meeting, CERN, 26 Jan 2009 21EPAC, 26 June 2008 W. Wuensch, CERN
HOM damping waveguides
Magnetic field concentration –pulsed surface heating
High electric field and powerflow region - breakdown