Exit Window

The aim of the exit window is to allow the electron beam to leave the electron accelerator in order to bombard the target (e.g. electrolytic solution for radiation chemistry). This requires a special type of exit window that will allow the electrons to pass by, while maintaining the pressure difference and not allowing air to leak into the system.

In actual accelerator designs, these windows are usually made of a thin metal foil. The two most commonly used foils are aluminum and a material known as Havar, composed of iron, chromium, cobalt, nickel and a few other additive metals. Havar is advertised as a high-strength non-magnetic alloy that keeps its strength even at very high temperatures. It is commonly used in high current beam lines where the number of accelerated particles is high enough to produce large amounts of heat. As the beam current for MEGA is very low, strength at high temperatures is not required. Additionally, one sheet of Havar foil can cost up to $1000. At this price, the team decided to use a 99% pure aluminum sheet as the metal exit window.

Based on numerical physics simulations, electrons of 100 keV are completely stopped by as little as 0.05 mm of aluminum. The thickness selected was 0.02mm, and the aluminum foil was sandwiched between two flanges with centered holes of 0.4inches and 0.2inches. The holes are not made of the same size in order to avoid rupturing of the thin foil at points of contact.

Exit window: aluminum foil sandwiched between two flanges. Stresses on the middle of the foil can be seen.