Toward transparent injection with a multipole injection kicker in a storage ring

Achieving transparent Top-Up injection, characterized by stored beam position perturbations lower than 10% of the root mean square (rms) size of the stored beam, is very challenging for third-generation storage ring-based synchrotron light sources. The standard bump-based injection scheme used at SOLEIL 2.75-GeV storage ring for daily operation relies on two pulsed septum magnets and four pulsed dipole kickers installed in a 12-m-long straight section. Perfect tuning of the kickers to meet the perturbation tolerances is impossible since it would require at least four identical magnets, Ti-coated ceramic chambers, and electronics. On the way toward transparent injection, the four kickers can be replaced by a single pulsed magnet, benefiting from a field-free region in the stored beam path. This Multipole Injection Kicker (MIK), developed in the frame of a collaboration between SOLEIL and MAX IV, and currently installed in the MAX IV 3-GeV storage ring, has proven to meet the transparency specifications. A copy of the MIK has been installed in a short straight section of the SOLEIL storage ring in January 2021. Perturbations of less than 2% of the rms size of the stored beam were measured using the newly installed high-performance diagnostics. In this article, a start-to-end model of the MIK-based injection is presented as well as major challenges inherent in designing such an innovative magnet. Then, the results of the MIK commissioning are presented in depth along with the high-performance diagnostics, which are essential when such levels of beam stability are to be achieved.

Figure 1

Twiss functions of a quarter of the SOLEIL storage ring. Horizontal and vertical betatron functions (${\beta }_x$, ${\beta }_z$) are plotted in blue and red solid lines, respectively, while the horizontal dispersion ${\eta }_x$ corresponds to the green solid line. The electromagnetic elements of the lattice are drawn at the bottom of the figure: dipoles (yellow), focusing and defocusing quadrupoles (blue and red), sextupoles (pink and green), and rf cavities are in the first to medium straight sections around $s=20$ and 40 m (pink) while remaining short and medium straight sections are filled with insertion devices. The black dots and vertical bars symbolize BPMs and correctors, respectively.

Figure 2

Dynamic aperture of the bare lattice, including the permanent magnet–based 2.8 T superbend magnet (presented in Sec.2b). All the scrapers are fully extracted and the septum blade is inserted to $-19.5\mathrm{mm}$. The color code refers to the diffusion index of the tune for 1000 turns using frequency map analysis [16].

Figure 3

Standard bump-based injection scheme. The stored beam is bumped (blue dashed line) from the central orbit (black solid line) by the four kickers (blue squares), in the injection section. The booster beam (red dotted line) is brought parallel to the stored beam by the septum magnets (in purple). Eventually, the stored beam is brought back to the central orbit with the injected beam (red solid line), as the orbit bump falls.

Figure 4

MIK injection scheme. The orbit of the stored beam is left unperturbed (black solid line), while the booster beam (red dotted line) is brought close to the septum blade thanks to the septum magnets (in purple). The injected beam propagates freely (red solid line) until the amplitude of the horizontal oscillation is minimized by the MIK (blue square).

Figure 5

Simulated MIK injection: at the injection point, the booster beam (100 blue dots $3\sigma$ envelope) is injected close to the septum blade (shaded area) outer face and captured within the dynamic aperture (black dots) after the MIK kick. Because of its size, the injected beam (multicolor) exhibits the footprint of the MIK field during the first turns (six represented).

Figure 6

Front view of the MIK cross section with red and green copper rods numbered from 1 to 8, striped sapphire vacuum chamber (46.8 mm (H) $\times 7.8\mathrm{mm}$ (V) physical aperture), injected (blue), and stored (orange) beams. The red and green dots on the conductors represent the current direction.

Figure 7

MIK section. The MIK is installed in a short straight section of the storage ring, which starts and ends with a beam position monitor (blue squares). A pair of taper/absorber chambers, upstream and downstream of the MIK, adapt the physical apertures of the standard vacuum chamber and protect the MIK from the synchrotron radiation. Two beam loss monitors (red dots) surround the MIK for radiation safety purposes. The ultrahigh vacuum is guaranteed by a triplet of pumps.

Figure 8

Horizontal variations of the measured (red) and the ideal simulated (blue) horizontal kick ${\theta }_x$, at $z=0$ and $t=1.2\mathrm{\mu }\mathrm{s}$, scaled to match the operation at 10 kV. The measured field satisfactorily matches the simulated field in the field-free region and on the injected beam path at $x=10.3\mathrm{mm}$.

Figure 9

Variations of ${\theta }_x$ with respect to the horizontal position, under 10 kV at three different times of the pulse. The extrema of the quadrupolar strength are reached during the current rise, at $t=0.2\mathrm{\mu }\mathrm{s}$ (blue), and the current fall, at $t=2.4\mathrm{\mu }\mathrm{s}$ (red), while the field exhibits its nominal octupolar shape at peak current, $t=1.2\mathrm{\mu }\mathrm{s}$ (green).

Figure 10

Horizontal betatron oscillations of the injected beam. If the MIK is switched off, the beam is lost 70 m downstream of the injection point (red dashed curve). If the MIK is switched on, but is too strong ($+50\%$ here), then the injected beam oscillates with large amplitude (green dashed curve) and will be lost after a few turns. If the MIK is switched on, and its kick set up to cancel the horizontal deviation of the injected beam at the MIK location, denoted $x_{\mathrm{MIK}}^{\prime }$, then the oscillation amplitude of the injected beam is minimized passing from roughly 25 to 10 mm after the MIK (blue curve). The horizontal scrapers are inserted to their nominal position and the position of the vertical scrapers is represented by the black dotted line.

Figure 11

Residual horizontal betatron oscillations due to the four kickers (blue) and the MIK (red). Measurements with ${\beta }_x=13.3\mathrm{m}$.

Figure 12

Residual vertical betatron oscillations due to the four kickers (blue) and the MIK (red). Measurements with ${\beta }_z=8.6\mathrm{m}$.

Figure 13

Turn-by-turn variations of the vertical position of the stored beam. Measurements with ${\beta }_z=12.1\mathrm{m}$. The MIK perturbs the beam which exhibits large betatron oscillations without correction, for ${\nu }_z=0.334$ (blue). When the corrector is switched on, the amplitude of the betatron oscillations is divided by a factor of 6 (green). The correction is perfect for ${\nu }_z=0.374$ (red).

Figure 14

Typical turn-by-turn imaging of the stored beam with the KALYPSO camera. The vertical white dotted line identifies the trigger of the injection devices and the beginning of the residual oscillations. (a-i) and (a-ii) are related to the kickers in the horizontal and vertical planes, respectively. (b-i) and (b-ii) are related to the MIK in the horizontal and vertical planes, respectively. The active correction is off.

Figure 15

Variations of the horizontal position of the stored beam due to a single triggering of the MIK, when the active correction is off (blue) and on (red). The slow varying modulation corresponds to synchrotron oscillations. This figure illustrates two processed KALYPSO images.

Figure 16

Variations of the vertical position of the stored beam due to a single triggering of the MIK, when the active correction is off (blue) and on (red). This figure illustrates two processed KALYPSO images.

Figure 17

Variations of the rms horizontal stored beam size due to the MIK. The peak variation is 3.5% of the rms horizontal stored beam size.

Figure 18

Measured (blue) and simulated (red) horizontal first turn orbit. The horizontal physical aperture is plotted in black solid lines.

Figure 19

Turn-by-turn losses distribution in the case of the standard injection scheme. BLMs 1 to 25 monitor the beam losses from the injection to the MIK sections.

Figure 20

Turn-by-turn losses distribution in the case of the MIK injection scheme. BLMs 1 to 25 monitor the beam losses from the injection to the MIK sections.