Longitudinal Profile Monitors

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Randy Thurman-Keup

Fermilab

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Albert Einstein

Outline

• Introduction
• Wall Current Monitors
• Cavities
• Zero Phasing
• Deflecting Mode
• Streak Cameras
• THz Interferometers
• Electro-optical Sampling
• Feschenko Monitor

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Introduction

• Bunch Length (longitudinal equivalent of transverse profile)
• Linacs
• Hadron – 10s of ps to 100s of ps
• Electron – 10s of fs to 10s of ps
• Synchrotrons
• Proton – 100s of ps to 10s of ns
• Electron – 10s of picoseconds

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R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Outline

• Introduction
• Wall Current Monitors
• Cavities
• Zero Phasing
• Deflecting Mode
• Streak Cameras
• THz Interferometers
• Electro-optical Sampling
• Feschenko Monitor

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Wall Current Monitor

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Wall Current Monitor

• Unfortunately, the beampipe on the two sides of the resistive break are never isolated 🡪 Both grounded
• The image current has no interest �in traversing the resistive break
• So we must force it via inductance

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Ferrite

Ceramic Gap

Resistive�Flex Board

Equivalent

Circuit

Wall Current Monitor

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Want

Small R

Large L

Small R

Large L

Small R

Wall Current Monitor

• Resistors are placed all around the gap to reduce the impact of off-center beams
• FNAL version has 80 resistors, each 120 ohms

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R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Wall Current Monitor

• Cable dispersion will stretch the signal
• Can be corrected by FIR filter
• Impulse response of a coax cable �(only skin depth losses)

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• Output signal is convolution of impulse �response with input
• FIR is inverse of impulse response

R. Thurman-Keup --- USPAS Hampton, VA

9

January 31, 2024

Tevatron

7/8” Heliax�cable

Undershoot from LRC??

Outline

• Introduction
• Wall Current Monitors
• Cavities
• Zero Phasing
• Deflecting Mode
• Streak Cameras
• THz Interferometers
• Electro-optical Sampling
• Feschenko Monitor

R. Thurman-Keup --- USPAS Hampton, VA

10

January 31, 2024

Zero Phasing Cavity

• A normal accelerating cavity is timed such that the beam passes through the accelerating gap near the peak of the rf
• Zero phasing is the process of timing the cavity so that the beam passes through at the zero crossing of the rf
• Synchrotrons run this way with stored beam
• The beam gets chirped, i.e. given E – t correlation
• Beam traverses a dispersive section (e.g. spectrometer dipole) imparting an E – x correlation
• Transverse profile is obtained

R. Thurman-Keup --- USPAS Hampton, VA

11

January 31, 2024

More Energy

Less Energy

rf

Dispersion

Transverse Profile

Zero Phasing Cavity

• Time resolution depends on
• Gradient of energy gain (rf)
• Dispersion of spectrometer�(needs to be high!)
• Initial energy spread�(needs to be low!)
• Non-dispersive beam size�(Beta function of beam at profile device)
• Systematic errors from
• Initial E – t correlation

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

W. Graves et al., PAC 2001, Chicago, 2224

Transverse Deflecting Cavity

• Beam is deflected by a higher order mode of a specially designed cavity imparting a y – t correlation
• Avoids the dispersive middle person of the zero-phasing technique
• But… requires a special cavity

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R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Higher frequency and voltage,

lower energy and emittance,

larger beta function at deflector are preferred.

Transverse Deflecting Cavity

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Fields of TM_1,1 mode

LOLA Cavity from SLAC

• 3-m-long S band 2856 MHz iris-loaded ”LOLA-type” RF waveguide structure built in 1960s
• Named for three inventors initials
• Designed to provide hybrid deflecting �modes HEM_1,1
• Linear combination of TM_1,1 and TE_1,1 dipole�modes resulting in transverse forces
• Used for beam separators and �RF deflectors

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R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Transverse Deflecting Cavity

• Flash FEL at DESY

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R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Transverse Deflecting Cavity

• LCLS

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Outline

• Introduction
• Wall Current Monitors
• Cavities
• Zero Phasing
• Deflecting Mode
• Streak Cameras
• THz Interferometers
• Electro-optical Sampling
• Feschenko Monitor

R. Thurman-Keup --- USPAS Hampton, VA

18

January 31, 2024

Streak Camera

• Provides time profile of incoming light pulse
• Light from a prompt source (OTR, OSR) enters the streak camera optics and is converted to photoelectrons
• The photoelectrons are swept by a time varying electric field to an MCP transforming t to y
• The amplified signal terminates on a phosphor screen for imaging by a camera

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Sweep Generator

OTR / OSR

Entrance

Slit

Beam�Pulse

Input�Optics

Photocathode

Accelerating

Grid

Sweep�Electrodes

MCP

Phosphor�Screen

Camera

Streak Camera

• Hamamatsu versions
• Synchroscan: Can sync sweep with external rf to allow accumulation of images with minimal jitter
• Dual Sweep: Slow sweep in the direction transverse to time; display many individual pulses

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Mirror Optics

Lens Optics

Streak Camera

• Source is generally broadband for bunch length measurements (OTR, OSR)
• Recall the index of refraction varies with wavelength
• But… index of refraction = 1 / phase velocity of light
• Lenses spread the time distribution of broadband light

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• Use mirror optics for the best resolution
• Or… use a bandpass filter to narrow the variation of n
• Entrance slit is a direct resolution contribution
• Variation of photoelectron energies can alter the time distribution
• Can measure bunch lengths down to ~300 fs

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R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Streak Camera Data

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Internal organs of a streak camera

Streak Camera Data

• Dual Sweep data from FAST at FNAL
• Hamamatsu C5680 streak camera

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Calibration = 0.38 ps/pix

σ =13.3pix

Streak-on

Streak-off

Resolution contribution of input slit is determined with the streak off

σ =3.9pix

Outline

• Introduction
• Wall Current Monitors
• Cavities
• Zero Phasing
• Deflecting Mode
• Streak Cameras
• THz Interferometers
• Electro-optical Sampling
• Feschenko Monitor

R. Thurman-Keup --- USPAS Hampton, VA

24

January 31, 2024

Terahertz Interferometry

• Method for extracting the spectrum of a pulse of light (Frequency domain bunch length measurement)
• Properly called Fourier-Transform Spectroscopy
• Want a pulse of light with a spectrum that matches the spectrum of the beam bunch
• Use coherent transition / synchrotron radiation
• Need wavelengths much longer than bunch
• Works at shorter bunch lengths than streak camera

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R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Incoherent

Coherent

Phases�not�correlated

Phases correlated

Coherent Radiation

• Coherent addition of waves

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Sum over radiation fields from N particles

Intensity spectrum

Incoherent

Coherent

Coherent radiation�spectrum is

bunch spectrum

Coherent Interferometry

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Coherent

d₁

d₂

Cosine Transform!!

Coherent Interferometry

R. Thurman-Keup --- USPAS Hampton, VA

28

January 31, 2024

Kramers-Kronig Relation

Coherent Interferometry

• Typically used for sub-ps bunch lengths
• Bunch length rms of 500 fs implies a frequency extent �of ~300 GHz (almost THz)
• Hence the often-used name Terahertz interferometry
• Typical detectors and collection systems do not work below ~100 GHz
• No spectrum below 100 GHz
• Must extrapolate spectrum to 0 Hz before inverse Fourier transform!

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R. Thurman-Keup --- USPAS Hampton, VA

29

January 31, 2024

Coherent Interferometry

• DESY ‘Single Shot’ CTR Spectrometer
• Temporal profiles measured with the TDS (-) and the CTR spectrometers [ CRISP 4 (-) and 4-2 (-)]

R. Thurman-Keup --- USPAS Hampton, VA

30

January 31, 2024

FWHM ~180 fs

Outline

• Introduction
• Wall Current Monitors
• Cavities
• Zero Phasing
• Deflecting Mode
• Streak Cameras
• THz Interferometers
• Electro-optical Sampling
• Feschenko Monitor

R. Thurman-Keup --- USPAS Hampton, VA

31

January 31, 2024

Electro-optical Sampling

• Encodes the coulomb field of the beam onto a laser pulse by modifying the polarization of the laser pulse

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Laser is

linearly

polarized

Output after polarizer is�zero unless polarization has�been rotated

Electro-optical Sampling

• Electro-optical crystal is a crystal that becomes birefringent when subjected to an external electric field
• Birefringence is the state of having multiple indices of refraction along different axes in the crystal
• The dielectric constant is a tensor quantity in these crystals
• Light polarized along one axis moves at a different speed than that polarized along a different axis

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R. Thurman-Keup --- USPAS Hampton, VA

33

January 31, 2024

Electro-optical Sampling

• Spectral Decoding
• Chirped laser pulse passes through crystal and has the bunch structure imprinted on it (t_bunch – 𝜆_laser correlation)
• Chirped is wavelength separation in time
• Imprinted pulse passes through grating spreading the wavelengths in space (𝜆_laser – x_screen correlation)

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R. Thurman-Keup --- USPAS Hampton, VA

34

January 31, 2024

Electro-optical Sampling

• Spatial Encoding
• Spatially spread laser pulse passes through crystal at some angle to the beam (t_bunch – x_laser correlation)
• Field may not be uniform across the�crystal which is correlated with time�because of the angle

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R. Thurman-Keup --- USPAS Hampton, VA

35

January 31, 2024

Electro-optical Sampling

• Temporal decoding
• Stretched laser pulse passes through crystal and has the bunch structure imprinted on it (t_bunch – t_laser correlation)
• Imprinted pulse is mixed in non-linear crystal (e.g. BBO) with unchirped laser pulse with a crossing angle (t_laser – x_screen correlation
• The mixing produces a new photon

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Electro-optical Sampling

• Temporal decoding

R. Thurman-Keup --- USPAS Hampton, VA

37

January 31, 2024

BBO Crystal

Bunch

Time

Unchirped Laser Propagation

Chirped Laser Propagation

Electro-optical Sampling

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024

Example: Bunch length at FEL test facility FLASH

Results at FLASH, Hamburg, see B. Steffen et al., FEL Conf. Stanford, p. 549, 2005.

Scanning of the short laser pulse relative to bunch:

Outline

• Introduction
• Wall Current Monitors
• Cavities
• Zero Phasing
• Deflecting Mode
• Streak Cameras
• THz Interferometers
• Electro-optical Sampling
• Feschenko Monitor

R. Thurman-Keup --- USPAS Hampton, VA

39

January 31, 2024

Feschenko Monitor

• Combination of SEM and deflecting mode cavity, or perhaps streak camera
• A wire at some negative voltage (e.g. -10 kV) acts as a secondary electron emitter
• The density of secondaries�in time encodes the�bunch profile in time
• Those secondary �electrons are swept�by an electrostatic rf�deflector thus encoding�the bunch profile spatially
• Slit in front of detector selects�particular time

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R. Thurman-Keup --- USPAS Hampton, VA

40

January 31, 2024

Detector

Extras

R. Thurman-Keup --- USPAS Hampton, VA

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January 31, 2024