SLAC - PUB - 6342 sLAc / ssRm 55

We present a planar helical undulator designed to produce elliptically polarized light. Helical magnetic fields may be produced by a variety of undulatory with four parallel cassettes of magnets. In our design, all cassettes are mounted in two planes on slides so that they maybe moved parallel to the electron beam. This allows us to produce x-rays of left-or right-handed elliptical or circular polarization as well as horizontal or vertical linear polarization. In model calculations, we have found that by sliding the top pair of rows with respect to the bottom pair, or the left pair with respect to the right pair, we retain the polarization setting but change the magnetic field strength, and hence the x-ray energy. This allows us to select both energy and polarization by independent phase adjustments alone, without changing the gap between the rows. Such a design maybe simpler to construct than an adjustable gap machine. We present calculations that model its operation and its effects on an electron beam. J Experiments on magnetic and biological materials which exhibit circular dichroism are now being performed using circularly polarized soft x-rays. The most common sources of such radiation are electron storage kng bending magnets, where the different senses of polarization are obtained above and-below the plane of the electron trajwtory. For increased intensity, new sources are being built using insertion devices such as wigglers and undulatory [1]. One class of undulator is the planar helical design, where rows of magnets are arrayed above and below the electron beam in two planar jaws; generally each jaw comprises two rows of magnets [2]. We are building a device of this type, which is a combination of a magnet design by Sasaki [3], and the adjustable phase undulator (APU) concept [4]. The elliptically-polarizing undulator (EPU) has four identicd rows of Halbach sinusoidal pure permanent magnet blocks [5] arranged in the four quadrants about the axis of the electron beam. If the rows in quadrants 2 and 4 are moved longitudinally in the same direction by the same distance with respect to the rows in quadrants 1 and 3, a helical magnetic field is created. Electrons in this field execute a helical trajectory, and emit elliptically-polarized x-rays. By moving in the opposite direction, the other sense of helicity is obtained. We call this motion a " row phase " shift. One period (of length lu) of the EPU magnet lattice …