Measurements of emittance at low energy
Ideally, experimental measurements of all beam parameters should be made at the exit of the injector. This has not been possible on DARHT Axis-II for various practical reasons. In particular, the emittance has not been measured there. This note reviews the methods we have used for emittance measurements on the linear induction accelerator (LIA) at low-energy (< 10 MeV). It includes a discussion of some of the issues associated with making those measurements on a beam from the injector. A successful plan for emittance measurements at any energy should incorporate measures to deal with these issues. To begin with, it should be recognized that emittance measurements at the injector are not critical for beam physics in the accelerator. The emittance at the injector has very little effect on accelerator beam dynamics except for an increase in size near the end, where the beam becomes emittance dominated. Therefore, imperfect knowledge has little effect on the design and implementation of a tune. This point is illustrated in Fig. 1, which shows beam envelope calculations for our present tune over a wide range of emittances. Compare this with using the wrong diode voltage in the tune design. Figure 2 shows the effect on the beam envelope of a 2% error in the measurement of diode voltage. Therefore, a few years ago, when we made plans for beam measurements at the diode while we were upgrading the injector cells, our highest priority was on magnetic spectrometer measurements of energy. These measurements were to be used to cross calibrate the diode-voltage monitor, so that accurate diode voltage measurements could be made on every shot after the spectrometer was removed. If emittance measurements at the injector are required, then three widely used methods should be considered. These are focusing magnet scans, the use of an emittance mask (pepper pot), and optical transition radiation (OTR). We have used all three of these on our accelerators. These are invasive methods. There are noninvasive methods that we have not yet used. For example, under some conditions the beam size obtained with diamagnetic loops can be used in conjunction with the envelope equation to infer emittance. To date our diamagnetic loop data has not been of sufficient quality to do this. Another technique is to measure the beam-distribution quadrupole moment with BPMs dispersed in a quadrupole magnet lattice. Our BPMs have this capability, but the only quad lattice is in the downstream transport (DST), so this method is deferred until we operate the DST. All three methods reviewed here are invasive, and require the insertion of a target into the beam. Therefore, there is always the accompanying risk of inadvertent destruction of the target, and scattering of target material throughout the accelerator or injector. The target damage mechanism is target heating to a temperature hot enough to evolve gas, which is impact ionized by the beam, which is then ion focused to a small enough spot for the energy density to damage the target material, often destructively. Target heating is directly proportional to beam current density and pulse length. Therefore, the key to minimizing risk of target damage is to design for a large beam at the target, low current from the diode, and the shortest possible current pulse. The large-beam requirement compromises emittance measurements with OTR or focal scans, but not with the pepper pot technique. Finally, to ensure a short pulse even if the crowbar fails, the Marx generator should be reconfigured to produce the shortest possible pulse.