directed along a straight path through the cavity passages. The electromagnetic interaction occurs along the electron beam and the microwave signal appearing at the cavity proximate the beam.
The beam is confined or focussed to the axial path by magnetic means to minimize spreading. So-called pole pieces define the cavities and walls of the slow wave structure while magnets positioned outside the vacuum region of the tube provide the magnetic flux. Protruding ferrules project from the front and back sides of the pole piece walls, serving to surround the electron beam passage and providing a concentrated, axially-extending magnetic field between the ferrule of one pole piece and that of an adjacent pole piece. The beam passage formed in the pole piece between the ends of the ferrules functions ascalifornia: hiking in mount tamalpais

Dave arrived in San Francisco after I had been there for about ten weeks, and one of the things he was most keen to do was hike at Mount Tamalpais. We drove there across the Golden Gate Bridge (which was exciting enough as it is!) and wound our way up to the visitor centre. It was such an amazing drive, we couldn’t believe how high it was. San Francisco looked tiny from right at the top.

Map in hand, we decided to do a four mile round trip, the Steep Ravine trail down the mountain, and the Dipsea trail back up. Steep Ravine was a steep (doh) trail through a magical redwood forest, with babbling brooks and waterfalls along the way. It felt like fern