Three-dipole kicker injection scheme for the Advanced Light Source upgrade accumulator ring

The Advanced Light Source Upgrade will implement on axis single-train swap-out injection employing an accumulator between the booster and storage rings. The accumulator ring (AR) design is a twelve period triple-bend achromat that will be installed along the inner circumference of the storage-ring tunnel. A nonconventional injection scheme will be utilized for top-off off axis injection from the booster into the AR meant to accommodate a large $\sim 300\mathrm{nm}$ emittance beam into a vacuum-chamber with a limiting horizontal aperture radius as small as 8 mm. The scheme incorporates three dipole kickers distributed over three sectors, with two kickers perturbing the stored beam and the third affecting both the stored and the injected beam trajectories. This paper describes this “3DK” injection scheme and how it fits the AR’s particular requirements. We describe the design and optimization process, and how we evaluated its fitness as a solution for booster-to-accumulator ring injection.

Figure 1

The Advanced Light Source Upgrade complex. The three dipole kickers and injection septum of the 3DK booster-to-accumulator injection scheme are highlighted. Every 1.4 s up to four bunches are generated and accelerated to 2 GeV through the existing linac and booster and then injected through the BTA into the new AR to fill or top-off bunches in a single 26- (or 25-) bunch train. Every $\sim 30\mathrm{s}$ this train is swapped with one of the eleven trains circulating in the Storage Ring (SR).

Figure 2

Lattice and magnet distribution of an arc in the ALS-U AR. The ring is composed of 12 such arcs. Shown are the beta and the dispersion functions (top), the aperture model and the distribution of magnets (bottom).

Figure 3

The layout diagram of the BTA thick and thin septa in sector 1. The separation between the injected beam and unkicked stored beam is 14 mm and the thickness of the thin septum blade is 1 mm.

Figure 4

Trajectories of the stored (green) and injected (red) beams during an injection cycle for the “balanced” Mode A kicker strength settings. Solid black profile indicates horizontal physical aperture. Dashed lines delineate $3\sigma$ of the beam size. Kickers are labeled and represented by orange bow-ties. The first prekicker is located at the start of sector 12. The second prekicker and injection septum are located in sector 1. The main injection kicker in sector 2 kicks both the stored and injected particles.

Figure 5

Injected and stored beam profiles out to $3\sigma$ at septum exit during an injection cycle for the three injection modes. The white region is the acceptance in $x–x^{\prime }$ space at the septum exit. The septum is drawn in black and is 1 mm thick with its inward edge 8 mm from the stored beam reference trajectory. The septum drawing was obtained by tracking. The parameters of the three modes are detailed in Table 2.

Figure 6

Twiss functions (a) and layout (b) of the BTA transfer line.

Figure 7

Top: particle losses in the injected (left) and stored (center and right) beam vs kickers’ angles for the ideal lattice. The prekickers’ kick (vertical axis) is reported relative to Mode A settings (red lines). Bottom: for each one of 100 lattice-error realizations, a DK scan is carried out on a grid to identify the setting yielding the highest injection efficiency. The ensemble of the best DK settings is reported in the right figure. The left figure is the injection efficiency cumulative density function (CDF).

Figure 8

Temporal and spatial profiles of the thin-septum leakage field as included in the injection efficiency studies. The left plot shows the integrated kick at various horizontal coordinates as a function of turns. Conversely, the right plot shows the integrated kick at various times as a function of a particle horizontal coordinate.

Figure 9

Sensitivity of the injection efficiency to the nonuniformity of the injection kickers’ pulse profile. The three data sets correspond to a 0%, 4%, and 10% sloping of the pulse profile. The simulation is conducted with a complete set of errors including shot-to-shot variation of the kick and septa strengths, as well as septum leakage fields.

Figure 10

Transverse short-range wakefields in the AR from individual components and their combined total. The wake potentials are calculated for a Gaussian beam with rms bunch length of 1 mm and serve as pseudo-Green functions in beam dynamics simulations. Left: horizontal plane, right: vertical plane.

Figure 11

Horizontal phase space of the stored (orange) and injected (blue) bunches after 2000 turns, without wakes (above) and with nominal wakes (below). The left figures show the results for Mode A, the right for Mode C.

Figure 12

Comparison of the evolution of the size (top) and centroid motion (bottom) of the injection bunch for the cases without wake fields (red) and with nominal wake fields (green). The left figures show results for the 3DK settings referred to as Mode A (“balanced”), and right figures show Mode C, which is tuned for better performance in the presence of wake fields. The corresponding injection efficiencies for the two cases are 83.6% and 93.1%.

Figure 13

Capture efficiency as a function of number of passes around the ring after injection, comparing Mode A, where the kicker settings are optimized without including wake field effects, to Mode C, which uses an alternative set of kicker strengths which is more tolerant to wake fields and also changes the injection beta function. Results are shown with and without wake fields, for enhanced wake fields, and for additional mitigations.

Figure 14

Per-mode growth rates of a 25 bunch train in the accumulator ring determined by multibunch macroparticle tracking with resistive wall wake fields. During steady-state, all RW modes are safely damped with a total growth rate of about $-2.5{\mathrm{ms}}^{-1}$. During the injection transient, the effectiveness of the TFB is significantly diminished by the necessity of masking the four buckets into which charge is injected, though all modes remain damped.

Figure 15

NLK-design concept. Top: main conductors’ transverse positions (the return conductors are not shown). Bottom: magnetic-field profile in the mid plane.

Figure 16

Injection efficiency for the conventional 4DK scheme evaluated over a population of 100 lattice-error realizations including orbit and optics corrections. Results are for the septum positioned at 8 mm (left) and 13 mm (right) from the vacuum chamber center. Tracking simulations were done with 10k particles over 10k machine turns.