Cavity BPM With Dipole-Mode Selective Coupler Zenghai Li Advanced Computations Department Stanford Linear Accelerator Center Presented at PAC23 Portland, Oregon. May 12-16, 23 Work supported by the U.S. DOE Contract No. DE-AC3-76SF1.
Contents Selective coupling scheme for cavity BPM Resolution limit Bandwidth Tolerance Test results of a NLC prototype Summary
Authors Zenghai Li Ronald Johnson Stephen R. Smith, SLAC, USA Takashi Naito, KEK, Japan; Jeffrey Rifkin, Lyncean Technologies, USA
NLC Q-BPM Almost all quadrupoles supports a BPM Function is to align beam to quads (or vice versa) Quantities ~28 Q BPM requirements: Parameter Value Conditions Resolution 3 nm rms @ 1 1 e - single bunch Position Stability 1 µm over 24 hours (!) Position Accuracy 2 µm With respect to the quad magnetic center Position Dynamic ±2 mm Range Charge Dynamic Range 1 8 to 1. 1 1 e - per bunch Number of 1-9 or 1-19 bunches Bunch spacing 2.8 ns or 1.4 ns Chose to use cavity BPM
Conventional Cavity BPM design Coupler couple to both TM 1 and TM 11 modes Need hybrid to cancel common mode Tight tolerances on geometry schematic Sum Port Sum diff Difference Port
More On Using Magnetic Coupling T. Shintake, C-band structure design. Vladimir Vogel, BINP, for ATF (from a paper by Marc Ross) NLC DDS structure. Using slots to damp dipole wakefields. Signal used for SBPM. Micron resolution.
Cavity BPM With TM 11 -mode Selective Coupler signal Dipole frequency: 11.424 GHz Dipole mode: TM 11 Coupling to waveguide: magnetic Beam x-offset couple to y port Sensitivity: 1.6mV/nC/µm (1.6 1 9 V/C/mm) signal Couple to dipole (TM 11 ) only Does not couple to TM 1 Low Q with narrow cavity gap May need to damp TM 1 OR, use stainless steel to lower Q
TM 11 Selective-coupling Scheme Port to coax Beam pipe M coupling NO coupling Slot modes
R or Z Waveguide Orientation R-orientation signal Z-orientation signal R or Z waveguide orientation to fit into different space.
6 8 1 12 14 16 18 2 Waveguide Signal With Beam Excitation Cavity height Guide height Guide R pos Pipe radius Q dipole W L amplitude 3mm 3mm 8mm 6mm 1 1.3E12 Beam impedance Y-port spectrum 4 3 2 1 6 4 3 1 6 8 1 12 14 16 18 2 F (GHz) Beam off in x plane y-port only picks up dipole signal Total rejection of TM 1 mode achieved by the selective coupling scheme
Intrinsic Resolution Resolution of BPM limited by signal/noise ratio Signal voltage determined by energy loss by the beam and the external coupling V( q, x) = q Z β ω k 1+ β Q 2 loss L x 2 k loss, tm11 (V/nC/mm 2 ) 3 3 2 2 1 1 Cavity gap (mm) 8 3 3. 3. 4. 4... 6. Pipe radius (mm) K loss, TM1 (V/nC/mm 2 ) 2 2 1 1 Cavity gap (mm) 8 3 3. 3. 4. 4... 6. Pipe radius (mm) Thermal noise determined by temperature and bandwidth V = Z kt F Signal/noise ratio independent of bandwidth Resolution limit.1-nm at room temperature for 1-nC charge beam Signal/thermal noise ratio 2 1 1 Cavity gap (mm) 8 3 1nm beam offset 3. 3. 4. 4... 6. Pipe radius (mm)
Bandwidth Adjust depth to design for different Q ext dipole Q ext 1 4 2 1 3 2 1 2 cavity gap (mm) 3 2 1 1 4. 4... 6. 6. 7. 7. 8. 8. 9. r-guide (mm) Q ext of dipole mode sensitive to depth of coupling slot (r-location of the waveguide), insensitive to beam pipe size and cavity gap. Wall loss Q about 4 for 3-mm cavity gap at X-band Q ext can very within a wide range by changing the slot depth Flexibility for design to meet different coupling β requirements. Broadband cavity BPM can be realized with this scheme.
Tolerance Frequency: -.7MHz/µm in cell radius. Can be made tunable, but need to keep cavity symmetry We consider the impact of machining errors in coupling slots: x, θ and r x θ r Both x, θ errors result in non-zero projection of the azimuthal magnetic field of the TM 1 mode along side the slot opening, causing coupling of the TM 1 mode to the waveguide and x-y coupling. r error may shift electric center of modes, results in potential TM 1 leakage and x-y coupling.
6 8 1 12 14 16 18 2 8 1 12 14 16 18 2 Tolerance On Slots x/ θ misalignment x X-port spectrum Y-port spectrum 4 3 2 1 1..7..2. x=.mm 6 8 1 12 14 16 18 2 F (GHz) Q ext 2 2 1 1.. 1. 1. 2. y-port offset in x (mm) V TM1 /V TM11 3 3 2 2 1 1 beam offset=1-nm x=.1 mm Cavity gap (mm) 8 3 3. 3.7 4..2 6. Pipe radius (mm) TM 1 /TM 11 signal ratio is about 2 for x=1µm, 1nm beam offset r misalignment r 6 Y-port spectrum X-port spectrum 4 3 2 1..4.3.2.1. r=-.mm 6 8 1 12 14 16 18 2 F (Hz) For a misalignment in x of 1µm, one can achieve resolution near thermal noise limit of about.1nm with modern electronics. Tolerance on r Significant looser than on x/ θ.
NLC Cavity BPM Cold Test Model Phase cavity & pickup Ω coax selective coupler -3-4 - -6-7 -8-9 11.4 1.61 19.8 8 12 16 2 Frequency (GHz) Cavity gap R beam pipe Waveguide r slot x/y BPM cavity 3-mm 6-mm 3mm 18mm 8-mm Frequency response measured using antenna excitation A resolution better than 1-nm obtained.
Conclusion The selective coupling scheme can reject common mode effectively Provides clean signal for position measurement. Advantages: Simplicity of geometry Loose tolerance. Nanometer resolution can be achieved with loose tolerance requirements. Can be designed for both narrow and broadband applications.
Phase-Cavity With 2mm Feedthrough.2mm