Longitudinal cross-section of the PWT linac. The beam axis is the centerline of the drawing. The irises or disks, seen in cross-section, are supported by the water tubes from each end cap as shown. There are actually four water tubes, equally spaced in azimuthal angle. Within each end cap is a water reservoir. The entire structure is constructed in two halves which were then fitted together in the center and brazed; each half has a separate water supply.

This figure shows the field profile on axis, as measured during construction by R. Zhang, for half of the tube, starting from the center.

Photo of the PWT linac in place in the Neptune beamline. The electron beam travels from upper right to lower left in this picture.

PWT Linac

The small RF linac in use on the Neptune beamline, which accelerates the beam from roughly 4 MeV to roughly 12 MeV over 42 cm, is an example of a plane-wave transformer or PWT. This washer-loaded cylindrical cavity is the third in a series of PWT linacs designed and tested at UCLA. It was constructed in the early 1990s, partly in the Physics Department Machine Shop and partly at SLAC.

This so-called PWT-Mark III consists of 7 full cells and 2 half-cells separated by irises which are "floating", not attached to the outer cylindrical wall. Instead, the irises are supported on four longitudinal cooling-water lines equally spaced in azimuth, which extend into the cavity from each end. RF power is coupled in through the center cavity, and an RF loop antenna is installed on a side port.

As an RF structure, a PWT is unusual in that the cell-to-cell coupling is extremely strong and almost independent of the operating conditions. Unlike conventional linacs, it operates in a TEM-like hybrid mode in the coaxial region outside the irises, so that the fields there are similar to those of a plane wave in free space and propagate at the speed of light; meanwhile, each individual cell supports a longitudinal electric field that can accelerate charged particles.

The structure is designed to resonate at 2856 MHz, with detailed tuning accomplished by changing the temperature of the water, which affects mostly the longitudinal dimension of the cells. The tuning coefficient is about 50 kHz per degree C. The outer wall has no direct temperature control, though it is cooled slightly by a water manifold which compensates for RF heating during operation. Calculations and experience have shown that the resonance frequency and fields are relatively insensitive to the outer wall temperature.