First Observation of the Point Spread Function of Optical Transition Radiation

We represent the first experimental observation of the point spread function (PSF) of optical transition radiation (OTR) performed at KEK-Accelerator Test Facility extraction line. We have demonstrated that the PSF vertical polarization component has a central minimum with a two lobe distribution. However, the distribution width varied significantly with wavelength. We assume that we observed a severe effect from spherical or chromatic aberrations which are not taken into account in any existing theoretical model. We believe that the result of this work will encourage theoreticians to continue developing the theory as it is important for various transition radiation applications. Nonuniform distribution of the OTR PSF creates an opportunity for developing a submicrometer transverse beam size monitor.

Figure 1

OTR source calculated using two approaches presented in 8, 9 (dashed line) and 10 Eq. (1) (solid line). ($\gamma =2500$, $\lambda =500\mathrm{nm}$, $x=0$, and $\mathrm{const}=1$.)

Figure 2

Experimental layout.

Figure 3

Calibration curve: A linear fit is shown on the picture. The magnification factor is $M=18.87\pm 0.18$ derived from the fit.

Figure 4

CCD Images taken with three different optical filters: (a) $-609\mathrm{nm}$; (b) $-558\mathrm{nm}$; (c) $-459\mathrm{nm}$.

Figure 5

OTR projections (left) demonstrated in Fig. 4a (open circles) and Fig. 4c (solid circles) and the distribution widths (right): solid circles—peak-to-peak distance and open circles—doubled rms width calculated using the second momentum method. The error bars are statistical obtained from averaging over 5 shots.

Figure 6

Two OTR distributions measured with 609 nm optical filter for two current settings of QD18X quadruple magnet. Open circles—$I_{QD18X}=29.61\mathrm{A}$ and solid circles—$I_{QD18X}=29.91\mathrm{A}$ corresponding to SAD calculation of the vertical beam size to be ${\sigma }_y=1.73\mu \mathrm{m}$ and ${\sigma }_y=4.03\mu \mathrm{m}$ respectively.