Figure 3.

Available gradient for a standard Apple undulator as a function of the undulator field parameter on-axis. The gradient is obtained by shifting longitudinally the arrays of the Apple device. The different lines define areas with a certain level of field linearity, defined as the maximum deviation of the gradient in a region of ±1 mm with respect to the on-axis gradient. For instance, for = 1, we can obtain a gradient between 30 and 150 m⁻¹ with a linearity better than 20% (see text for more details).