Permanent Magnet Quads
Permanent Magnet Quads
In order to potentially match a ramped drive beam into an actual plasma in a way which produces the desired enhancement of the wake-field transformer ratio, it must be focused to a sufficiently small transverse size to produce the requisite brightness. In particular, in order to operate in the so-called "blowout regime" the beam density must exceed the background plasma density, and in order to get a transformer ratio greater than 2 the bunch length must exceed two plasma skin depths. For a given charge and bunch length the first requirement imposes an upper limit on the transverse beam size, and the second imposes a lower limit on the plasma density. In combination, the result is that for the Neptune parameters, with a bunch charge of 600pC and a bunch length of 2 mm, the spot size must be less than 70 microns and the plasma density must be greater than 3e13 inverse cubic cm.
(Click image to enlarge)
To produce the small transverse beam size that is required, a permanent magnet quadrupole (PMQ) final focusing triplet is being developed. The PMQ design, originally developed for the Neptune Nonlinear Inverse Compton Scattering Experiment, uses NdFeB permanent magnet material to produce a focusing strength of 110 T/m. A PowerTrace simulation of the proposed final focus is shown above.
The longitudinal phase space (energy vs time) of an electron beam can be reconstructed by combining a deflecting cavity with a dipole spectrometer. If the setup is arranged such that the time-dependent kick delivered by the deflecting cavity is orthogonal to the bend-plane of the spectrometer magnet, then the transverse profile of the beam after both elements will display time on one axis and energy on the other. This is illustrated in the following figure.
The first plot on the left shows the longitudinal phase space distrubution from an ELEGANT simulation of the beam after propagation through the S-Bahn dogleg. The simulated reconstruction based upon the transverse profile downstream of the deflector and spectrometer is shown in the second figure. A cartoon diagram of the setup is shown on the right. A permanent magnet dipole (PMD) spectrometer with a 90 degree bend was recently constructed. This PMD was originally designed by A. Doyuran using the magnetostatics code RADIA as part of the Neptune Inverse Compton Scattering Experiment.