Beam emittance diagnostic for the DARHT second axis injector

Summary form only given, as follows. Low beam emittance is key to achieving the required spot size at the output focus of the DARHT Second Axis. The nominal electron beam parameters at the output of the injector are 2 kA, 4.6 MeV, 2-microsec pulse width and an rms radius less than 1 cm. Emittance is measured by bringing the beam to a focus in which the emittance is a dominant influence in determining the spot size. The spot size is measured from Cerenkov or optical transition radiation (OTR) generated from a target intercepted by the beam. The current density in the focused DARHT beam would melt this target in less than 1/2 microsec. To prevent this we have designed a DC magnetic transport system that defocusses the beam on the target to prevent overheating, and uses a 60-ns FWHM pulsed solenoid to selectively focus portions of the beam. The fast focus must produce /spl sim/1 kilogauss field over an effective length of /spl sim/50 cm to bring the beam to a focus on the target. The fast focus field is generated with a 12-turn coil located inside the beam-transport vacuum chamber with the entire fast coil structure within the bore of the DC magnet. The pulsed coil diameter of /spl sim/15 cm is dictated by the return current path at the nominal vacuum wall. Since the drive system is to use 40 kV to 50 kV technology and much of the inductance is in the drive and feed circuit, the coil design has three 120 degree segments. We will present the coil, driver, and feed system design, as well as beam envelope calculations based on this design. Target heating will also be discussed. Operation of the OTR imaging system will be discussed in a separate publication.