Characterization and long term operation of a novel superconducting undulator with 15 mm period length in a synchrotron light source

A new cryogen-free full scale (1.5 m long) superconducting undulator with a period length of 15 mm (SCU15) has been successfully tested in the ANKA storage ring. This represents a very important milestone in the development of superconducting undulators for third and fourth generation light sources carried on by the collaboration between the Karlsruhe Institute of Technology and the industrial partner Babcock Noell GmbH. SCU15 is the first full length device worldwide that with beam reaches a higher peak field than what expected with the same geometry (vacuum gap and period length) with an ideal cryogenic permanent magnet undulator built with the best material available PrFeB. After a summary on the design and main parameters of the device, we present here the characterization in terms of spectral properties and the long term operation of the SCU15 in the ANKA storage ring.

Figure 1

SCU15 installed in the ANKA storage ring.

Figure 2

On the left, a scheme of the SCU15 indicating the beam vacuum chamber (liner), the magnets embedded in the insulation vacuum, and the motors used to change the vacuum gap, is shown. On the right side, a scheme of the separate cooling circuits cooling the magnets (cooling circuit A) and the liner (cooling circuit B) allowing a good thermal separation between magnets and coils, as well as the difference between magnetic and vacuum gap, is presented. The dimensions shown are all indicated in mm.

Figure 3

Cooldown time of the SCU15. The temperature of the superconducting coils are plotted as well as the temperature of the liner as a function of time. In the inset the lowest temperature reached by the SCU15 coils is shown.

Figure 4

Warm up time of the SCU15. The temperature of the superconducting coils are plotted as well as the temperature of the liner as a function of time.

Figure 5

Upper plot: current of the power supply feeding the main superconducting coils at the maximum operating current as a function of time; the inset shows the stability during the first 30 min after ramping. Lower plot: temperature of the superconducting coils reported as a function of time.

Figure 6

Current of the power supply while ramping up and down the main superconducting coils (left axis) and temperature of the top and bottom coils (right axis) as a function of time.

Figure 7

Temperature of the SCU15 coils and of the liner as a function of time with gap closed and open.

Figure 8

Sketch of the superconducting coils with the fiducials, the points used on the first and last poles, as well as with the magnetic plane and axis.

Figure 9

Top. Current of the main coils (magenta line) of the SCU15 and the temperature of the top (blue line) and bottom (red line) coils, as well as the temperature of the liner (green line), as a function of time. Bottom. Beam parameters as a function of time: current (blue line), energy (red line), lifetime (green line).

Figure 10

Top. Lifetime and pressure measured in the vacuum chambers at room temperature placed upstream and downstream with respect to the SCU15, as a function of time when the gap is closed. Bottom. Beam parameters as a function of time: current (blue line), energy (red line), lifetime (green line).

Figure 11

Reported are the beam current (blue), the beam energy (red), and the beam lifetime (green) as a function of time before and after installation of the SCU15 in the ANKA storage ring. The SCU15 is here with gap closed and powered to 150 A.

Figure 12

Pressure in the vacuum chambers upstream and downstream with respect to the SCU15, beam current, beam energy, and beam lifetime as a function of time after installation of the SCU15 in the ANKA storage ring.

Figure 13

The beam current, the current and voltage of the main coils of the SCU15 during a quench (lower plot), the temperatures of the SCU15 main coils (middle plot), and the temperatures of the SCU15 liner (upper plot) are plotted as a function of time.

Figure 14

Temperature of the top and bottom coils before and after a quench as a function of time.

Figure 15

Top. Current of the one of the quadrupole families (violet line) and the temperature of the top (blue line) and bottom (red line) coils as well as of the liner (green line) as a function of time. As the current of the quadrupole grows the beam gets shorter. Bottom. Beam parameters as a function of time: current (blue line), energy (red line), and current in the main coils (magenta line) of the SCU15. The SCU15 gap is open in the second run.

Figure 16

Setup used at the ANKA storage ring for the spectral characterization of the SCU15.

Figure 17

The counts of the second ionization chamber normalized to the counts of the first ionization chamber, showing the Ta absorption edges from the L shell, are plotted as a function of the photon energy, after calibration.

Figure 18

Flux generated by the SCU15 at the maximum operating current 150 A in the main coils, at 2.5 GeV and normalized to 100 mA, through a pinhole of $50\mu \mathrm{m}$ diameter at 14.9 m, measured with a channel cut Si 111 monochromator and the first ionization chamber, as described in the previous subsection (black line). The measured flux is compared with the ones using as input a magnetic field generated with FEMM (Finite Element Method Magnetics) adopting a geometry that takes into account the mechanical variations of the half period length and pole height measured at room temperature along each coil (RT tol.) with an offset of 7 mT (red line), with a tapering of $0.375\mathrm{mm}/\mathrm{coil}$ (olive line), and with a bending of $0.250\mathrm{mm}/\mathrm{coil}$ (magenta line). The spectrum of an ideal field with 0.73 T peak field on axis is also reported (blue line).

Figure 19

Flux generated by the SCU15 at the maximum operating current 150 A in the main coils, at 1.3 GeV and normalized to 100 mA, through a pinhole of $50\mu \mathrm{m}$ diameter at 14.9 m, measured with a channel cut Si 111 monochromator and the first ionization chamber, as described in the previous subsection.

Figure 20

The measured flux (black line) is compared with the ones using as input a magnetic field generated with FEMM adopting a geometry that considers the mechanical variations of the half period length and pole height measured at room temperature along each coil (RT tol.) and an offset of 7 mT (red line), as well as by the field obtained including a tapering of $0.375\mathrm{mm}/\mathrm{coil}$ (cyan line), and by including a bending of $0.250\mathrm{mm}/\mathrm{coil}$ (blue line).

Figure 21

A sketch of the geometry of the taper and parabola long range deviations used as input in FEMM to simulate the magnetic field for the spectral simulations presented in Figs. 18 and 20.

Figure 22

Spatial distribution of the flux density generated by the SCU15 at the maximum operating current 150 A in the main coils, at 2.5 GeV measured with a CCD camera at 26 m from the source (SCU15), as described in the previous subsection (first column) at 4 different photon energies on the falling flank of the third harmonic. The measured flux is compared with the ones simulated using as input a magnetic field generated with FEMM adopting a geometry that considers the mechanical variations of the half period length and pole height measured at room temperature along each coil (RT tol.) and an offset of 7 mT (second column), as well as by the field obtained including a tapering of $0.05\mathrm{mm}/\mathrm{coil}$ (third column), and by including a bending of $0.05\mathrm{mm}/\mathrm{coil}$ (fourth column).

Figure 23

Horizontal profiles from the measured (squares) and simulated (lines) flux density distribution at 3 fixed energies on the falling flank of the third harmonic: 7800 eV (black), 7826 eV (red), and 7843 eV (blue). The measured horizontal profiles are extracted at $y=3.7\mathrm{mm}$ from the first column and the simulated ones at $y=0\mathrm{mm}$ from the third column of Fig. 22.

Figure 24

The measured flux (black line) is compared with the ones simulated considering an ideal case with 0.73 T peak field (blue line), as well as with the ones using as input a magnetic field generated with FEMM adopting a geometry that considers the mechanical variations of the half period length and pole height measured at room temperature along each coil (RT tol.) and a bend of $0.05\mathrm{mm}/\mathrm{coil}$ with (red line) and without an offset of 7 mT (cyan line).

Figure 25

Top. Transmission of the 0.5 mm thick Be window as a function of energy [23]. Middle. Ionization chamber conversion factor $f_C$ as a function of energy [17]. Bottom. Darwin width $\omega$ of the Si111 crystal as a function of energy [18].