Breakdown Limits on Gigavolt-per-Meter Electron-Beam-Driven Wakefields in Dielectric Structures

First measurements of the breakdown threshold in a dielectric subjected to $\mathrm{GV}/\mathrm{m}$ wakefields produced by short (30–330 fs), 28.5 GeV electron bunches have been made. Fused silica tubes of $100\mu \mathrm{m}$ inner diameter were exposed to a range of bunch lengths, allowing surface dielectric fields up to $27\mathrm{GV}/\mathrm{m}$ to be generated. The onset of breakdown, detected through light emission from the tube ends, is observed to occur when the peak electric field at the dielectric surface reaches $13.8\pm 0.7\mathrm{GV}/\mathrm{m}$. The correlation of structure damage to beam-induced breakdown is established using an array of postexposure inspection techniques.

Figure 1

Conceptual drawing of the dielectric wakefield accelerator (DWA). A “drive” beam excites wakefields in the tube, while a subsequent witness beam (not shown) would be accelerated by the $E_z$ component of the reflected wakefields (bands of color).

Figure 2

Visible (color intensity map) light images of a $100\mu \mathrm{m}$ i.d. tube for three different ${\sigma }_z$ values (a),(b),(c). In a long pulse case, ${\sigma }_z\sim 45\mu \mathrm{m}$, the fiber end glows with incoherent optical emission (a). The fiber holder boundaries, viewed at a 45° angle from beam axis, are also displayed, as is the region where emission is blocked by the fiber’s Al coating, which starts $\sim 500\mu \mathrm{m}$ from the fiber end. For an intermediate pulse length (b) the incoherent optical emissions have increased. In a case of near minimum pulse length (c), ${\sigma }_z\sim 12\mu \mathrm{m}$, a breakdown event produces a large flash of light. This tube’s breakdown threshold is visible in a plot (d) of the pixel sum of each frame as a function of $I_{\mathrm{peak}}$. The red line is a fit to the breakdown-free region below 10 kA.

Figure 3

Microscope images of select dielectric tubes (a),(b),(c) matched with far-field intensity patterns observed downstream of each laser-illuminated fiber displayed with the same scaled-to-intensity color map (d),(e),(f). The visual appearance and laser transport characteristics of an unexposed control tube (a),(d) compare well to those of a tube exposed to 1000 long bunches (b),(e) for which no breakdown flashes were observed. Alternatively, a tube exposed to 1000 short bunches (c),(f) shows significant changes after suffering numerous breakdown events. These changes include microcracking and radiation-induced browning (c) along with reduced laser transport (f).