First accelerator test of vacuum components with laser-engineered surfaces for electron-cloud mitigation

Electron cloud mitigation is an essential requirement for high-intensity proton circular accelerators. Among other solutions, laser engineered surface structures (LESS) present the advantages of having potentially a very low secondary electron yield (SEY) and allowing simple scalability for mass production. Two copper liners with LESS have been manufactured and successfully tested by monitoring the electron cloud current in a dipole magnet in the SPS accelerator at CERN during the 2016 run. In this paper we report on these results as well as the detailed experiments carried out on samples—such as the SEY and topography studies—which led to an optimized treatment in view of the SPS test and future possible use in the HL-LHC.

Figure 1

(a) Schematics of the ECM system. (b) Assembly of the C-magnet in the SPS tunnel.

Figure 2

(a) Schematics of the laser treated region on the top half of the liner; the bottom half is treated symmetrically. (b) One of the finished liners assembled with the stripline monitor before installation in the SPS magnet.

Figure 3

(a) SEM top view of a Cu sample treated with CH pattern and (b) LH pattern at the University of Dundee. (c) Optical microscope image of a mechanically cut and polished cross section of (a) where the average depth of the grooves is estimated at $53\pm 5\mu \mathrm{m}$ and (d) cross section of (b), where the average depth of the grooves is estimated at $35\pm 6\mu \mathrm{m}$. (e) SEM cross-section image illustrating the larger ablated depth at crossing patterns.

Figure 4

Comprehensive view of a FIB cross section of a LH sample analogous to Fig. 3, at progressive magnifications clockwise from top left. The platinum top layer, of lighter color in the images, is added as a part of the FIB etching process, to improve image quality minimizing artifacts. The colored rectangles indicate the different regions mentioned in the text.

Figure 5

SEM images of test samples treated with LH pattern by ASTeC at $120\mathrm{mm}/\mathrm{s}$ (a) and $90\mathrm{mm}/\mathrm{s}$ (b) showing an hierarchy of surface structures at different magnifications on top views (three top rows) and cross section views (bottom row).

Figure 6

Perforated side of the liners treated (a) at the University of Dundee, and (b) by ASTeC. In (b), the two regions treated with a different set of laser parameters are clearly visible, the darker area having been treated at $90\mathrm{mm}/\mathrm{s}$.

Figure 7

SEY comparison of CH (green circles) vs LH (red squares) patterns for samples from the University of Dundee.

Figure 8

Secondary electron yield of a University of Dundee LH sample as received (red squares) and after 12 months of aging (purple circles).

Figure 9

SEY of samples from ASTeC treated at $90\mathrm{mm}/\mathrm{s}$ (dark blue circles) and $120\mathrm{mm}/\mathrm{s}$ (light blue squares) with LH pattern.

Figure 10

SEY curves for: (top) LH samples produced at the University of Dundee before (red squares) and after (grey circles) the CERN standard cleaning procedure; (bottom) LH $120\mathrm{mm}/\mathrm{s}$ samples from ASTeC before (light blue squares) and after (black circles) the same cleaning procedure.

Figure 11

SEM images at different magnifications of a CH reference sample produced at the University of Dundee (top row) and of a twin sample after cleaning with ultrasonic agitation for 5 minutes (bottom row).

Figure 12

Integrated signals from the ECMs during the total duration of the experiment in the SPS. (top) Reference copper liner; (bottom) liners with e-cloud mitigation: blue squares—treated by ASTeC, brown triangles—treated at the University of Dundee and green lozenges—a-C coating. Note the difference in vertical scales.

Figure 13

Progressive e-cloud development in the four batches of 72 proton bunches. Clockwise from top left: liner treated at the University of Dundee, liner treated by ASTeC, a-C coated liner and Cu liner. Note the difference in vertical scale for the latter.