Nonintercepting electron beam size monitor using optical diffraction radiation interference

In recent years, the use of diffraction radiation (DR), emitted when a charged particle beam passes through a rectangular slit, has been proposed and successfully tested as a nonintercepting diagnostic of high brightness beams. However, some problems related to the control of the particle trajectory through the slit still remain. If an additional slit is placed in front of the first one, at a distance shorter than the radiation formation length, interference between the forward diffraction radiation from the upstream slit and the backward diffraction radiation from the downstream slit can be observed. In this paper we report the first experimental observation of this effect, which we call here optical diffraction radiation interference (ODRI). If the two slits have different dimensions and are not aligned on the same axis, the properties of the ODRI pattern can be effectively used for nonintercepting beam diagnostics, especially for the unambiguously determination of the beam size. Indeed, the advantage of ODRI compared with a single aperture DR screen is due to the reduction of synchrotron radiation background, the increase of sensitivity for transverse beam dimensions, and the possibility to separate effects caused by the beam size and by beam offset within the slit.

Figure 1

Sketch of the two-slits setup. Left: side view; center: top view; right: definition of the coordinates system. Dimension $a$ is the aperture size of the first slit, $b$ the size of the second slit, and $d$ the distance between slits center.

Figure 2

Comparison between ODR and ODRI angular distributions. Parameters of calculation are indicated in the text.

Figure 3

Comparison between ODRI angular distribution with different values of the angular divergence for a pointlike beam passing in the center of both slits.

Figure 4

Angular distribution produced by two different beams passing through a system of two collinear slits: a pointlike beam ($\sigma =0$, black curve) and a parallel beam, i.e., a beam without angular divergence (${\sigma }^{\prime }=0$, red curve).

Figure 5

Angular distribution produced by three different beams passing through a system of two noncollinear slits: a parallel beam with a size of $150\mu \mathrm{m}$ (black curve), a pointlike beam with angular divergence of $200\mu \mathrm{rad}$ (red curve), and a pointlike beam with angular divergence of $250\mu \mathrm{rad}$ (blue curve).

Figure 6

FLASH layout during our experiment (not to scale). The site of the experiment is indicated.

Figure 7

Sketch of the screen setup.

Figure 8

The best fit parameter set is determined by using a chi-square minimization. Left: chi square vs beam size. Right: chi square vs angular divergence.

Figure 9

Images of ODRI angular distribution for different positions of the beam with respect to the center of the 0.5 mm slit. A: $-125\mu \mathrm{m}$; B: $-50\mu \mathrm{m}$; C: center; D: $+50\mu \mathrm{m}$; E: $+125\mu \mathrm{m}$.

Figure 10

Measured ODRI angular distributions for two different vertical beam sizes. The beam sizes obtained from the fit are $82\mu \mathrm{m}$ and $94\mu \mathrm{m}$. Corresponding beam sizes measured by an OTR monitor are $78\mu \mathrm{m}$ and $90\mu \mathrm{m}$ rms, respectively. The associated angular divergences from the ODRI fit are 220 and $200\mu \mathrm{rad}$, respectively.

Figure 11

ODRI angular distribution of a beam going through the center of the second slit. Both experimental data (black squares) and a fit curve, based on theoretical model (red dashed line), are shown.

Figure 12

ODRI angular distribution of a beam going through a slit displaced by $50\mu \mathrm{m}$ with respect to the former case. Both experimental data (black squares) and a fit, based on the theoretical model (red dashed line), are shown.