Higher-Order Modes and Beam-Loading Compensation in CLIC Main Linac Oleksiy Kononenko BE/RF, CERN CLIC RF Structure Development Meeting, March 14, 2012

Outline Motivation Beam-loading compensation scheme Frequency Domain: HFSS/ACE3P benchmark Time Domain: HFSS/ACE3P/gdfidl benchmark Effect of the higher-order modes to the compensation scheme Conclusion 2

Motivation: CLIC Performance Issue *CLIC-Note-764, private conversations with Daniel Schulte (CERN) In order to have luminosity loss less than 1%, the RMS bunch-to-bunch relative energy spread must be below 0.03% 3

CLIC Drive Beam Generation Complex *CLIC-Note-764

Energy Spread Minimization Scheme Unloaded Voltage in AS - fix phase switch times in buncher - generate corresponding drive beam profile - take into account PETS (+PETS on/off) bunch response - calculate unloaded voltage Loaded Voltage in AS - calculate AS bunch response - calculate total beam loading voltage - add to unloaded voltage Energy Spread Minimization varying buncher delays 5

Beam-Loading Compensation Main results are published: O. Kononenko, A. Grudiev, Transient beam-loading model and compensation in Compact Linear Collider main linac, Physical Review, Special Topics on Accelerators and Beams, 2011, Vol. 14, Issue 11, 10 pages, 6

HFSS Simulation Setup Model: - 90 deg of the structure - copper outer walls H-plane Port 2 Port 1 H-plane Simulation profile: - second order basis functions - curvilinear elements enabled s-parameters accuracy leads to ~300K tet10 mesh 7

HFSS Port and Plane Wave Excitations Thanks to Valery Dolgashev from SLAC for the idea to use the plain wave source 8

s3p Simulation Setup Model: - 90 deg of the structure - copper outer walls H-plane Port 2 Port 1 H-plane 9 Simulation profile: - second order basis functions - curvilinear elements enabled - 2M tet10 mesh

Reflection Coefficient s 11 10

Complex Magnitude Ez, f=11.994GHz 11

E z (z) in a Complex Plane, f=11.994GHz 12

s3p/HFSS Benchmark Summary TD26 RF Design Remarkss3pHFSS f, GHz Filling time, ns Q-factor, Cu S 12, dB S 11, dB Voltage, V (Pin=4W) There is a very good agreement between the HFSS and s3p results 13

t3p Simulation Setup Model: - 90 deg of the structure - bunch sigma = 1mm - ABC/WG condition: couplers, beam-pipe, damping waveguides - PEC/copper outer walls Simulation profile: - second order basis functions - curvilinear elements enabled - 2, 3, 6, 12M tet10 meshes H-plane Beam 14

Bunch Passage through TD26 DC trail which is caused by the numerical errors can be observed, 2M mesh has been used 15

ACE3P Wake Convergence Study Different wake length is simulated because of the limited computer resources. Maximum 6hours x 2400 CPU per one run 16

gdfidl Simulation Setup Model: - 90 deg of the structure - bunch sigma = 1mm - PEC outer walls Simulation profile: - mesh planes fixed to the irises, thanks to Vasim - 100, 50 um uniform cubic meshes, 50x50x25um mesh Model: - 90 deg of the structure - bunch sigma = 1mm - PML condition: couplers, beam-pipe, damping waveguides - PEC outer walls H-plane Beam 17

gdfidl Convergence upon the Mesh Size Convergence is observed while wake rises at the tail for some reason 18

Beam Coupling Impedance HFSS/ACE3P/gdfidl Strange ACE3P Resonances Monopole band 19

Lowest Monopole Band Impedance HFSS/ACE3P/gdfidl 20

Fundamental Mode Impedance HFSS/ACE3P/gdfidl 21

HFSS/ACE3P/gdfidl Wakes HFSS and gdfidl are ok at the beginning, while ACE3P/gdfidl are ok after that because of the PEC boundary condition (copper in HFSS), also no ACE3P/HFSS wake rise is observed in the tail 22

HFSS wake shape vs BW This wake (wake function) for the delta function bunch is used for the compensation scheme, since bunch length in CLIC is only 44um. 23

Energy Spread vs BW Fixed optimized buncher delays and injection time for 1 GHz BW BW, GHzΔE/E,% Bunch Number 24

Conclusions Good agreement between ACE3P and HFSS in frequency domain Some difference has been observed between HFSS/ACE3P/gdfidl in time domain HOM’s taken into account don’t affect the developed beam-loading compensation scheme on the level of 0.03% Beam-loading compensation scheme should work 25

Acknoledgement I would like to thank my supervisor Alexej Grudiev, all of the members of the CERN CLIC RF team, SLAC ACD group. Thank you! 26