AUP HL-LHC RFD Cavity RF Design Zenghai Li HL-LHC RFD Design Freeze Review – FNAL – March 15, 2018
Outline § Issues to be addressed for the HL-LHC crab cavity to meet HiLumi operation requirements High HOM beam power at 760 MHz mode § § High octupole current for compensating HOM effects § RFD design Improvements § Cavity shape - to minimize HOM beam power at 760MHz HOM couplers - to minimize HOM impedance § § Dimension sensitivity analysis Summary § Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 2
LARP Prototype RFD Crab Cavity LARP Prototype RFD Crab Cavity Frequency (MHz) 400.79 OperaBng Mode TE11 E V-HOM H-HOM Lowest dipole HOM (MHz) 633 (selective FPC (Hi-pass coupling) Lowest acc HOM 715 filter) High R/Q acc HOM 760.9 Iris aperture (diameter) (mm) 84 B Transverse dimension (mm) 281 Vertical dimension (mm) 281 Longitudinal dimension (w/o 556 couplers) (mm) R T (ohm/cavity) 433 V T (MV/cavity) 3.34 • No lower order mode Bs (mT) 55.5 Es (MV/m) 32.6 • Compact, clears the second beam pipe • Minimal multipole fields with shaped pole face Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 3
760 MHz Mode Issue of the LARP Prototype Cavity RFD"HOM"Impedance" RFD cav17f Beam HOM Power 1.0E+07& R_acc& 1.0E+05 R"[acc(ohm/cav),"dip(ohm/m/cav)]" R_dip_X& 1.0E+06& 1.0E+04 R_dip_Y& International Review of the Crab Cavity acc_req& Performance for HiLumi 1.0E+03 1.0E+05& dip_req& P (W/A^2) April 3-5, 2017 CERN 1.0E+02 1.0E+04& 1.0E+01 1.0E+03& 1.0E+00 1.0E+02& 1.0E-01 0.50 0.75 1.00 1.25 1.50 1.75 2.00 1.0E+01& F (GHz) 0.4& 0.6& 0.8& 1.0& 1.2& 1.4& 1.6& 1.8& 2.0& F"(GHz)" § Impedance meets beam dynamics • Acc. HOM mode at 760.94 MHz too close requirements (2016) to beam resonance at 761.50 MHz § (Elias Métral, Joint LARP CM26/Hi- • Resulting in beam HOM power >10kW Lumi Meeting, SLAC, 19/05/2016) • Design spec for beam power: < 1kW Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 4
Requirements from recent beam dynamics study § In order not to affect the operational § Most HOMs require negligible octupole scenarios we need to keep the CC current, even if they fall on the couple- HOMs below 1 MΩ/m bunch line Ultimate scenario (Nov. 2017) (no other sources of impedance) 2760 b, 2.3 x 10 11 ppb, b * = 40 cm, e n = 2.1 µ m 2.01 GHz 1.47 GHz Require attention S. An&pov, D. Amorim, N. Biancacci, X. Buffat, L. Carver, F. Giordano, G. Mazzacano, A. MeregheA, E. Metral, S. Redaelli, B. Salvant, 7th HL-LHC Collabora&on mee&ng, CIEMAT, Madrid – 15.11.17 (Nov. 2017) Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 5
Requirement Summary § 760 MHz beam HOM power <1kW Ω /m Transverse impedance < 1M § ≤ 1.5 & at 400.79 +,- § HOM Filters Output Power . /0 < 200 kΩ § Longitudinal HOM shunt impedance Field multipole of operating mode § Most important , sextupole 5 6 < 1000 mT/m 2 at total 10MV deflection voltage Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 6
Minimizing 760 MHz Mode beam power, by detuning HOM frequency farther off beam resonance Beam line spacing: 40.079 MHz, 19 th harmonic at 761.5 MHz R=~102 dR=-1.76mm Various options explored to detune the 760 mode § § Operating mode frequency § Approach adopted: reducing gap volume around § tuned to 400.79 MHz by adjusting beam pipe region rounding “R” by -1.76mm § 400 MHz dipole and 760 MHz monopole modes opposite in dF sensitivity § lowers frequency of 760 mode (target: |dF| > 6MHz) Achieved -9 MHz detuning of the 760 MHz mode from beam resonance of 761.5MHz New frequency of this longitudinal HOM : 752.2 MHz Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 7
Beam HOM Power Reduced from ~10kW to ~ 500W Beam HOM Power (RFD12 cav4g, run9) Beam HOM Power Extraction via HOM Ports 1 .0E+03 1000 VHOM port HHOM port Beam Power (W) (1AM beam) Beam HOM Power (W/1AM^2) 1 .0E+02 100 1 .0E+01 10 1 .0E+00 1 0.400 0.900 1.400 1.900 0.5 1 1.5 2 F (GHz) F (GHz) HOM power calculated for 1 -AM beam § § σ z =76mm Beam HOM power below 1 kW. Meet design requirement. Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 8
New Design Parameter Comparison RFD Crab Cavity LARP Prototype New Design Frequency (MHz) 400.79 400.79 Lowest dipole HOM (MHz) 633 636 Lowest acc HOM 715 699 High R/Q acc HOM 760.9 752.2 Transverse dimension (mm) 281 281 Vertical dimension (mm) 281 281 R T (ohm/cavity) 427 431 V T (MV/cavity) 3.34 3.34 Bs (mT) 55.5 55.1 Es (MV/m) 32.6 35.0 • Frequency of high beam power mode (760MHz) 9.3 MHz below beam resonance • Good RF parameters maintained Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 9
Multipole B 3, B 5, B 7 Assume Def mode symmetry (only cos term) Value n r n n ) ∑ cos( n ϕ ) ( e j ω z . c included in E acc Component E acc ( r , ϕ ) = E acc (total VT=10 MV) n Δ ( n ) ( r , ϕ ) = 1 je b3 429 mT/m 2 ω nr n − 1 ˆ n ( z ) dz L L ∫ ( ) ∫ u r cos( n ϕ ) + ˆ p ⊥ F ⊥ dz = u ϕ sin( n ϕ ) E acc c 0 0 b5 -1.8e6 mT/m 4 1 nj B ( n ) dz = ( n ) dz = ( n ) dz L L L ∫ ∫ ∫ b n = F E acc b7 -4.9E+08 mT/m 6 ⊥ 0 ec 0 ω 0 Δ ( n ) ( r , ϕ ) = e ∑ b n r n − 1 ( ) u r cos( n ϕ ) + ˆ ˆ p ⊥ V T ( r , ϕ ) = e u ϕ sin( n ϕ ) n V def = b 1 • Multipole components barely changed as compared with LARP prototype design • b3 < 1000 mT/m 2 , meet design requirement Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 10
HOM Impedance Improvements § Cavity shape modification altered the damping of HOM § Re -optimized both H-HOM and V-HOM couplers Simplified port interface § H-HOM (Hi-pass filter) V-HOM (selective FPC coupling) Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 11
H-HOM Coupler Cutoff waveguide stub + high-pass filter Hi.pass$Filter$Transmission$ 0.0% !10.0% !20.0% S12$(dB)$ !30.0% !40.0% !50.0% !60.0% !70.0% 0.4% 0.6% 0.8% 1% 1.2% 1.4% 1.6% 1.8% 2% F$(GHz)$ rejection of operating mode § HHOM coupler and filter in low field region – minimizes RF heating § Waveguide stub add additional rejection of operating mode – loosening tolerance on filter dimensions Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 12
HHOM Coupler Modifications to Enhance Damping Transition +10mm higher Larger waveguide stub size (170x40) H-HOM width H-HOM hook +5mm wider 35 degree angle § a Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 13
V-HOM Coupler Cutoff waveguide stub + hook pickup § Waveguide stub selectively couples to acc and vertical HOMs – no filter needed § Hook provides both electric and magnetic coupling, improving damping of HOMs at higher frequencies § Waveguide stub dimension slightly larger, (same as HHOM) , to enhance coupling Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 14
FPC and Field Pickup Port (minor location adjustment from the LARP prototype ) Pickup port: FPC: § On the V -HOM side of cavity § Waveguide stub + hook § Need to pickup ~ 1.5W. Qext ~ 2x10 10 § Qext : 5x10 5 Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 15
Port Interface Simplified to Same Dimension § Diameter for all ports, HHOM, VHOM, field pickup: 37.879 mm § One feed through design for all ports pickup V-HOM H-HOM Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 16
HOM Impedance of the New Design RFD12 cav4g, run9c 1.0E+08 Acc dip_X 1.0E+07 dip_Y 1 MΩ/m 1.0E+06 1.0E+05 R 1.0E+04 1.0E+03 1.0E+02 1.0E+01 1.0E+00 0.400 0.600 0.800 1.000 1.200 1.400 1.600 1.800 2.000 F (GHz) HOM impedance below 1 MΩ/m up to 2 GHz Longitudinal shunt impedance < 200 k Ω Meet requirement Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 17
RF Heating of Coupler Elements RFD11 vs. RFD12 § Power [W/(3.4MV/cavity] RFD11 RFD12-cav4g-run9 FPC hook 69 73 Copper NC H-HOM hook 0.0007 0.0014 Nb SC V-HOM (probe) HOOK 0.47 0.51 Copper NC Field pickup probe 0.09 Copper NC Low RF heating on coupler elements No thermal issue Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 18
Multipacting LARP prototype (RFD11-cav17f) New design Multipacting mostly (RFD12-cav4g) on end-plate Impact Energy Enhancement Counter Similar to LARP prototype cavity. Will not be a problem Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 19
Cavity Dimension Sensitivity to HOM Impedances Transverse dimension Trim End plate tilt x/ytank (+delta) Pole depth (unchanged) gap (+delta) • Assume depth of the pole kept unchanged • Pole gap change same amount as cavity transverse dimension • Resulted in very small frequency deviation Zenghai Li - HL-LHC RFD Design Freeze Review - Mar. 15, 2018 20
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