Linearly and circularly polarized radiation with a low on-axis heat load from an asymmetric magnet pole undulator

The heavy heat load on optical components from the normal planar undulator has become a serious problem in the synchrotron radiation light source, especially for generating the low-energy photon in a high-energy storage ring and/or a diffraction-limited storage ring. The asymmetric magnet pole undulators have different period lengths between the upper and lower magnet poles. Taking the advantage of the asymmetric magnet pole undulators in building various magnetic fields, this paper proposes a new class of undulators to obtain the specific magnetic fields which can scatter the electron velocity away from the undulator axis. It consists of four magnet arrays and has a similar structure compared to the DELTA/APPLE-X undulator; however, each couple of magnet arrays has a different period length with the period ratio of $n_1:n_2$. Taking the one-to-two and two-to-three period ratios as examples, a novel Figure 8 undulator and a novel Knot undulator are demonstrated. The polarization state can be flexibly switched from the linear polarization to the circular polarization by tuning the phase shifts of magnetic fields, with the on-axis heat load greatly suppressed. The heat-load suppressions of linear polarization at two predetermined orthogonal directions are almost the same and fully effective. Analysis and simulations of undulator designs with the corresponding radiation characteristics are reported.

Figure 1

Schematic illustrations of (a) the typical Figure 8 undulator and (b) the typical APPLE-Knot undulator.

Figure 2

The electron orbits (solid left) and velocities (dotted right) in the $x\text{−}y$ plane, in which the upper subfigures (blue) and lower subfigures (red) are corresponding to the Figure 8 undulator and the linear mode of APPLE-Knot undulators, respectively.

Figure 3

Schematic illustrations of the proposed AMPU under DELTA-type (a) and APPLE-X-type (b) transverse structures with the period ratio $n_1:n_2$ at each direction.

Figure 4

The magnetic fields (a) of the linear and circular mode in the Fig. 8 AMPU, with the corresponding electron orbits (b), (d) and velocities (c), (e). The blue lines (b), (c) are corresponding to the 135° linear mode and the red lines (d), (e) are corresponding to the right-hand circular mode, respectively.

Figure 5

The photon flux (solid blue) and polarization degree (dotted red) depend on photon energy for the linear mode (a) and the circular mode (b) in the Fig. 8 AMPU, with the pinhole radius of 0.3 mrad. The lines are from the spectra code and the scatter symbols are based on Eqs. (3) and (5).

Figure 6

The photon flux (solid blue) and polarization degree (dotted red) depend on pinhole radius at 7 eV for the linear mode (triangle symbol) and circular mode (square symbol) in the Fig. 8 AMPU.

Figure 7

The transverse distribution of power density for the linear mode (upper) and circular mode (lower) in the Fig. 8 AMPU.

Figure 8

The magnetic fields (a) of the linear and circular mode in the Knot AMPU, with the corresponding electron orbits (b), (d) and velocities (c), (e). The blue lines (b), (c) are corresponding to the 135° linear mode and the red lines (d), (e) are corresponding to the right-hand circular mode, respectively.

Figure 9

The photon flux (solid blue) and polarization degree (dotted red) depend on photon energy for the linear mode (a) and the circular mode (b) in the Knot AMPU, with the pinhole radius of 0.3 mrad. The lines are from spectra code and the scatter symbols are based on Eqs. (3) and (5).

Figure 10

The photon flux (solid blue) and polarization degree (dotted red) depend on pinhole radius at 7 eV for the linear mode (triangle symbol) and circular mode (square symbol) in the Knot AMPU.

Figure 11

The transverse distribution of power density for the linear mode (upper) and circular mode (lower) in the Knot AMPU.

Figure 12

The flux (left), polarization degree (middle) and power (right) depend on the deviation of phase shift ${\varphi }_2$. Upper subfigures are corresponding to the Fig. 8 AMPU and lower subfigures are corresponding to the Knot AMPU, respectively.

Figure 13

The flux (left), polarization degree (middle) and power (right) depend on the deviation of field amplitude $B_2$. Upper subfigures are corresponding to the Fig. 8 AMPU and lower subfigures are corresponding to the Knot AMPU, respectively.