Basics of RF beam diagnostics Basics of beam

Basics of RF beam diagnostics Ø Basics of beam monitoring with RF cavities Ø Example devices Ø Quantitative analysis ØMeasuring rms bunch length

Basics of RF cavities for beam measurements Field probe couples RF field to electronic B. P. filter RF front end converts HF to LF Charged particle Beam operator LF signal conditioning feedback systems Data acquisition system Depending on cavity field pattern and receiver electronics this allows retrieving information on • Bunch intensity • Bunch phase (longitudinal position) • Transverse bunch position RF cavity E-field of resonant modes couples to particle charge • Beam angle • Bunch length

Energy transfer from beam to cavity

Energy transfer from beam to cavity

Energy transfer for finite bunchlength Example: ω=2π∙ 3 GHz, σ b=10 ps Fb=0. 982

Dissipation of energy in cavity

Energy flow Field probe couples RF field to electronic Pe= signal power Measurement electronic PC= ohmic losses in cavity wall Charged particle Beam RF cavity E-field of resonant modes couples to particle charge

Dissipation of energy in cavity and external measurement electronic

Cavity driven by continuous bunch train

Signal power vs. coupling factor β for c. w. beams

Beam measurement types Measurement of beam intensity or beam timing/phase relative to some external RF reference Measurement of beam position beam Cavity mode with rotational symmetry and electric field maximum on beam axis Cavity mode with Zero electric field on axis, azimuthal field dependence like Cos(θ) and field strength dependence on beam position approximately linear with displacement r “Monopole mode” , “TM 010 like mode” “Dipole mode”, “TM 110” like mode

Signal path in RF frontend ω0 reference sync. with beam Phase shifter φ LO Field probe couples RF field to electronic B. P. filter RF IF Mixer Charged particle Beam Mode with Resonance at ω0 Digitizer

Example of RF monitors in MAMI phase & intensity monitor position monitor 4. 9 GHz Phase and Position Resonators in MAMI double sided Microtron (at Mainz University) Courtesy H. Euteneuer and O. Chubarov

MAMI monitors cont. Drawing of a similar cavity (but at 9. 8 GHz)

RF electronic for 9. 8 GHz RF BPM’s in MAMI Courtesy H. Euteneuer and T. Doerk

Example of RF front with IQ demodulation and switchable gain RF switch tree Beam – 83~+17 d. Bm attenuator array IQ demodulator – 50~+10 d. Bm I Amp Baseband +40 d. B BPF RF-BPM 4760 MHz RF switch tree – 60, -40, -20 and 0 d. B LO 4760 MHz Schematic from SCSS/Spring 8 0 90 – 6 d. B 0 d. B Q To ADC 238 MSPS

RF cavities as monitors for longitudinal and transverse beam position ØQuantitative analysis for “pillbox cavities” ØError sources for position monitors and countermeasures Ø Examples of cavity BPM electronic boards Ø Comparison with other BPM types

Properties of “Pillbox” cavity TMmn 0 modes d R z

Properties of “Pillbox” cavity TMmn 0 modes cont.

Properties of “Pillbox” cavity TMmn 0 modes cont.

Pillbox cavity with beam pipe, monopole mode d R a

Energy transfer for off axis beam, monopole modes I 0(x) J 0(x) valid for arbitrary cavity geometries with rotational symmetry :

Pillbox cavity with beam pipe, dipole mode d R a

Energy transfer for off axis beam, dipole modes I 1(x) J 1(x) valid for arbitrary cavity geometries with rotational symmetry :

Loss factors for Pillbox cavities with beam aperture ø=2 a

Scaling of Cavity properties with frequency Always stay below cut-off frequency of lowest waveguide mode in beam-pipe (TE 11) Example: R beampipe=2 cm stay well below 4. 4 GHz

Optimum cavity length for single bunch BPM 0. 371

Optimum cavity length for c. w. beam BPM 0. 453

Numerical example Position resolution and measurement range of a 3 GHz RF BPM with a=15 mm, d=25 mm, β=5 Material copper σ=5. 88∙ 107 for single bunch beam with q=100 p. C σt=3 ps T=100 Me. V (v≈c). RF front end can resolve Pe = -50 d. Bm.

Measurement troubles R Exp ect ed Me asu red |V| X-position a Moving beam In X- direction

Sensitivity to beam angle Exp ect ed Me asu red |V | Reasons: • Beam comes with an angle • Cavity is tilted Remedies • Improve cavity angle alignment • Shorten cavity length (at the expense of reduced sensitivity) • Use it as a feature (requires IQ demodulation) Moving beam In X- direction Im V X-position Moving in x 900 out of phase signal from beam angle Re V

Common mode signal from monopol signals Z Z/x TM 010 TM 110 + 110 TM Remedies: • Symmetric coupling with 1800 Hybrid • Mode selective couplers TM |V| 010 ω X-position

RF-BPM (similar designs for SCSS, European-XFEL, Swiss. FEL) Dual-resonator, coaxial connectors, mode-selective (E-XFEL, 3. 3 GHz) Reference cavity (1 connector): 3. 3 GHz signal ~ bunch charge Position cavity (4 connectors) : 3. 3 GHz signal ~ position * charge 100 mm D. Lipka/DESY, based on SCSS design Visible: Vacuum, couplers Mode-selective couplers suppress undesired other modes Beam Position = k * (VPos_Cav / VRef_Cav). Factor k: Not fixed, variable via attenuator. Courtesy B. Keil/PSI

Reject Monopole Mode Ref: V. Vogel Nanobeam 2005 Magnetic Field Electric Field Beam Coupling waveguides couples to TM 110 mode, not to TM 010 Courtesy D. Lipka/DESY 34

Reject Monopole Mode Dipole Mode Monopole Mode • propagation of dipole mode in waveguide • monopole mode no propagation in waveguide Courtesy D. Lipka/DESY D. Lipka, MDI, DESY Hamburg 35

PSI Cavity BPM Electronics for Eu-XFEL and Swiss. FEL BPM Unit (PSI Design): 2 -4 RFFEs, 1 FPGA Board. 3. 3 GHz RFFE 6 x 16 bit 160 Msps ADC Mezzanine Courtesy B. Keil/PSI FPGA Mezzanine Carrier Board (IBFB version)

Cavity BPM Electronics: RFFE M. Stadler Courtesy B. Keil/PSI

Cavity BPM Electronics: ADC Mezzanine Board Six 16 -bit 160 Msps ADCs 500 pol. High Speed Connector (Carrier Board FPGA Interface) Differential Inputs Low Jitter Clock Distribution (80 fs) On-Board Gain and Offset Calibration Control Unit Courtesy B. Keil/PSI M. Roggli

BPM Electronics: Digital/FPGA Carrier Board RFFE 2 ADCs • ADC/DAC Clock • Bunch Trig. • Bunchtrain Pretrigger Clocks & Trigger LVDS (0. 5 -1 Gbps) BPM FPGA 1 Cavity Pickup XFEL Control Sys. Link Buttons PSI Maintenance Link Buttons RFFE (I/Q Dem. ) 2 ADCs (Virtex 5 *XT) IBFB Link User Defined I/Os RAM RFFE Control (Gain, PLL Freq. , …) Courtesy B. Keil/PSI IBFB Link Contr. Sys. Link VME-P 2 Backplane Board Serial Bus Transceivers VXS (1 -5 Gbps) Rocket IOs Clocks & Trigger LVDS (0. 5 -1 Gbps) Backplane FPGA (“Low Cost”) 2 ADCs BPM FPGA 2 Config. FPGA System FPGA Compact Flash & Controller (Virtex 5 *XT) (Virtex 5 FXT) VME 64 x/2 esst Transceivers VMEbus RAM “GPAC” Board 2 SFP Fiber Optic Transceivers RAM 2 ADCs • ADC/DAC Clock • Bunch Trig. • Bunchtrain Pretrigger Piggyback Boards Buttons BPM data processing & storage, RFFE tuning, calibration, … Control system interface: VME, VXS, or front-panel fiber optic links (Ethernet, …). Power. PC in FPGA can run Linux/EPICS.

Comparison of different BPM types Pickup Transformer Button Matched Stripline RF Cavity Monopole Mode Suppression Modal (hybrid) / electronics Modal (coupler), frequency, Typical RMS Noise, 10 p. C, *20 mm pipe* >50μm >100μm ~60μm <1μm 0. 1… 200 MHz 300… 800 MHz 1 -12 GHz Spectrum Typical Electronics Frequency Pictures

Comparison of different BPM types cont. Transformer Button Stripline RF Cavity ++ + - - precision & resolution + ++ sensitivity for low charges + - + ++ difficulty of calibration + + + - impedance (collective instabilities) + + + - complexity of electronics + + + - size - ++ + + flexibility bunch spacing, train length, bunch length