A single-shot method for measuring fs bunches in linac-based FELs

Z. Huang, K. Bane, Y. Ding, P. Emma

Growing interests in a few fs and sub-fs x-ray pulses

Introduction

We (and LCLS users) would like to know the compressed bunch length of the LCLS low charge (20 pC) beam

LCLS S-band transverse cavity resolution is limit at 10~20 fs (X-band TCAV resolution ~ 4x smaller)

Needs techniques with1-fs resolution (or even lower)

Traditional RF zero-phasing is insufficient in measuring very short bunches because of its sensitive to the initial energy spread

A longitudinal mapping technique developed by T. Smith’s group overcomes this limitation of RF zero-phasing

We propose to use this technique to measure fs bunches in LCLS (taking into account wakefield of a long linac, SLAC-PUB-14104, 2010)

Measurement of 60-mm FEL microbunching at Stanford, 2000

Initially proposed by E. Crosson et al., 1995

BC2 4.3 GeVBC2 4.3 GeV

add a diagnostic chicane R56’add a diagnostic chicane R56’

Run L3 at zero crossing (-90 deg) h3Run L3 at zero crossing (-90 deg) h3L2 (2)

To high-resolutionenergy spectrometerSlightly adjust BC2 R56

=0

Final energy spread/profile corresponds to short bunch length/profileWakefield of long linac must be taken into account

d

z

sz

Over-compression

sdZero-crossing

Apply this method to measure fs bunches

Diagnostic chicane can be part of BC2

Run LiTrack with 20 pC setup (L2 phase at -31 deg, under-compression)Run L3 at -90 deg (10 GeV over 4.3 GeV leads to h3 = 139 m-1)Increase BC2 R56 by R56’ = -1/ h3 = -7.18 mm

Turn off Linac-3 wake (discussed in next slides)

LCLS low charge example

After nominal BC2

After adjusted BC2 and L3

Needs to measure ~1e-4 energy spread with a high-resolution spectrometer

L3 wake introduces an additional energy spread to the measurement For very short bunches (<10 mm), wake-induced energy spread (primarily a linear chirp) is independent of bunch length

Linac Wakefield

d

z

sz

Over-compression

Zero-phasing

With wake

d

z

sz

More over-compression

Zero-crossingwith wake

Wakefield un-corrected Wakefield corrected

This simple wake-correction scheme works for almost arbitrary (short) bunch length we want to measure!

N: # of e- L: L3 lengtha: iris radius

Linac-3 wake can be corrected by a bit more over-compressionUsing stronger chirp in Linac-2

Or using stronger R56 in BC2

I2 is peak current in L2 (same for all BC2 compression settings)IA=17 kA, h3 is L3 chirp by RF zero-phasing

Wakefield compensation

Preferred wake-correction method is by shifting R56 of BC2, which needs to be increased by ~8.08 mm

R56’ (= -7.18 mm = -1/ h3 ) and R56 (≈ -0.9 mm for wake compensation)

Wakefield compensation by changing R56

R56’ = -8.08 mm

• Real bunch length• E-spread/chirp

Run LiTrack with 20 pC (L2 phase at -31 deg, under-compression)Run L3 at -90 deg (10 GeV over 553 m leads to h3 = 139 m-1)Turn on Linac-3 wake

Increase BC2 R56 by R56’=-1/ h3= -7.18 mm

Wakefield un-corrected

Increase BC2 R56 by R56’+R56 = -8.08 mm

Wakefield corrected

A-line as a high-resolution spectrometer

Spectrometer screen (PR18)x = -6.4 mx = 100 mEnergy resolution ~1×10-5

Elegant simulation (20 pC, L2 at -31.5 deg)

L3ENDBC2 END

A-line PR18

~ 2 mm

RMS bunch length (Elegant simulations)

Temporal resolution = Energy resolution (~1×10-5) divides by h3 ~ 100 m-1 = 0.1 um or 0.3 fs

Wakefield/CSR/LSC add a systematic error ~0.5 fs

Summary

A single-shot method for measuring fs bunches is studied

An experimental test at the LCLS using the A-line spectrometer is planned

The method requires no extra hardware (besides a high-resolution spectrometer) and may be applicable to other XFEL facilities

Thanks R. Iverson, J. Frisch, H. Loos et al. for reviving the A-line spectrometer and for many useful discussions

Backup slides

Phase shift agrees with theory

Wake effect can be corrected empirically by identifying full compression phase through CSR bunch length monitor

• Real bunch length

• E-spread/chirp• E-spread/chirp (shift 2 by 1°)

Wakefield compensation by shifting L2 phase

J. Frisch

R56’ = -7.18 mm