David, Check out Ross Engineering for high quality vacuum spark gaps. Get a 2nd mortgage if you plan on buying any. (fax) 1408 377 5182 http://home.navisoft-dot-com/rossengineering/index.htm X-rays from a vacuum spark gap would be very difficult to tame. A tube is safer because most of the x-rays leave perpendicular to the tube. Easier to collimate. Ralph Zekelman