Experimental measurements of rf breakdowns and deflecting gradients in mm-wave metallic accelerating structures

We present an experimental study of a high gradient metallic accelerating structure at sub-THz frequencies, where we investigated the physics of rf breakdowns. Wakefields in the structure were excited by an ultrarelativistic electron beam. We present the first quantitative measurements of gradients and metal vacuum rf breakdowns in sub-THz accelerating cavities. When the beam travels off axis, a deflecting field is induced in addition to the longitudinal field. We measured the deflecting forces by observing the displacement and changes in the shape of the electron bunch. This behavior can be exploited for subfemtosecond beam diagnostics.

Figure 1

Solid model of the output section of the mm-wave traveling wave accelerating structure, including output coupler and waveguides (a), photo of the copper half structure (b), geometry of one quarter of the vacuum part of the regular cell (c).

Figure 2

Plot of the electric field (a) and magnetic field (b) of the fundamental mode over one quarter of the period of the regular cell ($\text{gap}=0.2\mathrm{mm}$), at synchronous frequency. These fields are calculated for a bunch charge of 3.2 nC and $50\mu \mathrm{m}$ long in an infinite traveling wave structure.

Figure 3

Deceleration voltage [(a) and (b)] and transverse voltage [(c) and (d)] as a function of the horizontal beam-structure displacement, for different gaps. The fields are generated by a 3.2 nC bunch with ${\sigma }_z=50\mu \mathrm{m}$ in the 10 cm long structure. The solid lines are calculated with cst, the “X” points with the code novo.

Figure 4

Schematic of the experimental FACET section.

Figure 5

Measurement of beam deflection in a horizontal scan with $\text{gap}=0.9\mathrm{mm}$. The blue line is the simulation.

Figure 6

Image of the beam recorded by the camera. The vertical axis is proportional to the energy, the zero is highest energy. The horizontal axis is the displacement. The dark points on the picture are the centroids obtained with the double Gaussian fit of each horizontal slice (a). Example of double Gaussian fit reconstruction of the horizontal slice, showing the two Gaussian centroids (b).

Figure 7

Horizontal deflection of the bunch tail indicated by the horizontal shift of the centroids of double Gaussian fit. The blue dots are for the maximum left deflection, the red dots for the maximum right deflection and the black dots are for the undeflected beam, far from the corrugations. The bunch tail is clearly observed.

Figure 8

Horizontal breakdown probability versus peak surface electric field obtained in a horizontal scan, 0.9 mm gap. The blue line is for increasing gradient and the red is for decreasing gradient. The electric peak field is calculated considering all modes.