Micron-scale laser-wire scanner for the KEK Accelerator Test Facility extraction line

A laser-wire transverse electron beam size measurement system has been constructed and operated at the Accelerator Test Facility (ATF) extraction line at KEK. The construction of the system is described in detail along with the environment of the ATF related to the laser wire. A special set of electron beam optics was developed to generate an approximately $1\mu \mathrm{m}$ vertical focus at the laser-wire location. The results of our operation at the ATF extraction line are presented, where a minimum rms electron beam size of $4.8\pm 0.3\mu \mathrm{m}$ was measured, and smaller electron beam sizes can be measured by developing the method further. The beam size at the laser-wire location was changed using quadrupoles and the resulting electron beam size measured, and vertical emittance extracted.

Figure 1

Geometry of the laser and electron beam interaction.

Figure 2

ATF extraction line from kicker to dump.

Figure 3

Horizontal and vertical betatron $({\beta }_x,{\beta }_y)$ and horizontal dispersion (${\eta }_x$) functions for the ATF extraction line. The vertical line marks the location of the wire scanner (${\mathrm{S}}_{\mathrm{WS}}$), close to the LWIP.

Figure 4

ATF extraction line measured and simulated dispersion. The upstream quadrupole focusing strengths were $l{k}_{\mathrm{QD}4\mathrm{X}}=-1.52{\mathrm{m}}^{-1}$ and $l{k}_{\mathrm{QF}4\mathrm{X}}=1.58{\mathrm{m}}^{-1}$, where $lk=\frac{l}{B\rho }\frac{d{B}_z}{dx}$, where $l$ is the length of the quadrupole, $\frac{1}{B\rho }$ is the magnetic rigidity, and $\frac{d{B}_z}{dx}$ is the field gradient. $s=0$ corresponds to the beginning of the extraction line. The vertical dotted line indicates the laser-wire location.

Figure 5

Schematic of the laser-wire interaction region installation.

Figure 6

Horizontal section of the laser-wire system, showing the interaction chamber, focusing lens, collimation lens, and the final scanning mirror.

Figure 7

Diagram of the final focus lens.

Figure 8

Results of zemax simulation of the $W_0$ of the custom lens for different input beam sizes and grid sizes. The minimum and maximum of the model are shown. The input laser has $M_0^2=1$.

Figure 9

Results of zemax simulation of the $M^2$ after propagating through the custom lens of a laser beam with input $M_0^2=1$ for different input beam sizes and grid sizes. The minimum and maximum of the model are shown.

Figure 10

View of the interaction chamber with the laser exit side flange removed, showing the 45° screen/knife edge.

Figure 11

Cross section of the window clamping and sealing arrangements.

Figure 12

The bdsim [23] simulation results. Solid line, energy spectrum of Compton photons produced from the laser wire. Dashed line, the energy spectrum of the electrons or positrons from the electron or positron pairs entering the Aerogel. The vertical line indicates the Cherenkov threshold of the aerogel.

Figure 13

The locations of the beam size parameters $W$ and the definition of rotation angle $\varphi$.

Figure 14

A profile measurement of the pulsed laser.

Figure 15

An IP profile measurement in the horizontal axis of the CW laser after the custom final focus lens.

Figure 16

Compton signal maximum amplitude as a function of laser pulse arrival time.

Figure 17

QD4X quadrupole scan. QF4X current was kept constant at 73.58 A.

Figure 18

QS1X skew quad vs ${\sigma }_s$ scan. QD4X was set to 74.06 A.

Figure 19

The smallest rms ${\sigma }_s$ laser-wire scan measured using the laser-wire system.

Figure 20

Waist scan. The laser focus position is $x$ in electron beam coordinates. The fit function is the laser propagation function, Eq. (2).

Figure 21

The smallest ${\sigma }_s$ laser scan performed using the laser-wire system, according to both a Gaussian and full integral fit. The integral fit function [Eq. (11)] is shown.

Figure 22

Fit results of Fig. 21 as a function of the $x\mathrm{\text{−}}y$ electron beam aspect ratio.

Figure 23

${\chi }^2/NDF$ of the fit to Fig. 21 for different values of ${\sigma }_{ex}/{\sigma }_e$.

Figure 24

Laser wire measured ${\sigma }_e^2$ vs QD4X lk value.