Reduction of SEY was achieved by surface engineering through laser ablation with a laser operating at • = 355 nm. It was shown that the SEY can be reduced to near or below 1 on copper, aluminium and 316LN stainless steel. The laser treated surfaces show an increased surface resistance, with a wide variation in resistance found de-pending on the exact treatment details. However, a treated copper surface with similar surface resistance to aluminium was produced

L. Gurran (7172342)

Michael Cropper (1250793)

N. Sykes (3744725)

O.B. Malyshev (7170422)

P. Goudket (7172345)

Reza Valizadeh (2815552)

S. Wang (673797)

T. Sian (7170428)

English

Loughborough University Institutional Repository

LOW SECONDARY ELECTRON YIELD OF LASER TREATED SURFACES OF COPPER, ALUMINIUM AND STAINLESS STEEL  R. Valizadeh, O.B. Malyshev, S. Wang1, T. Sian2, L. Gurran3, P. Goudket,  ASTeC, STFC Daresbury Laboratory, Warrington, UK M.D. Cropper, Department of Physics, Loughborough University, Loughborough, UK N. Sykes, Micronanics Ltd., Didcot, Oxon, UK 1also at Department of Physics, Loughborough University, Loughborough, UK 2also at The Photon Science Institute, The University of Manchester, Manchester, UK 3also at Lancaster University, Lancaster, UK  Abstract Reduction of SEY was achieved by surface engineering through laser ablation with a laser operating at  = 355 nm. It was shown that the SEY can be reduced to near or below 1 on copper, aluminium and 316LN stainless steel. The laser treated surfaces show an increased surface re-sistance, with a wide variation in resistance found de-pending on the exact treatment details. However, a treated copper surface with similar surface resistance to alumini-um was produced.  INTRODUCTION The reduction of secondary electron yield (SEY) is a very effective way of mitigation of electron cloud (e-cloud) and electron multipacting in accelerator beam chambers and RF wave guides.  Our recent discovery demonstrated that SEY can be efficiently reduced by surface engineering through laser ablation [1,2]. The major limitation of our original treatment was high sur-face resistance, therefore the further work aimed at pro-ducing low SEY surfaces with a smaller increase in sur-face resistance (RS).  SAMPLE PREPARATION The samples were made of oxygen free copper (Cu), aluminium (Al) and 316LN stainless steel (SS) plates, cleaned in acetone and isopropyl alcohol in an ultrasonic bath followed by a rinse with de-ionised water. The sam-ples were treated using a Coherent Aviva NX laser operat-ing at  = 355 nm, f = 40 kHz, pulse duration of 75 ns and 3W average power. The 1/e2 diameter of the focused laser spot on a sample was set at 15 µm. The laser was rastered in one direction and the hatch distance was 10 µm. The applied parameters were the same for all samples except the laser scanning speed, which is shown in Table 1.  The 15 mm  15 mm samples were used for SEY measurements. However, these samples are too small for the surface resistance measurements, and so the 100 mm  100 mm large samples Cu-1L, Cu-2L and Cu-3L were produced with exactly the same laser treatment parame-ters as the samples Cu-1, Cu-2 and Cu-3, respectively.       Table 1: List of Samples Sample Scan speed  Cu-1, Cu-1L 180 mm/s Cu-2, Cu-2L 120 mm/s Cu-3, Cu-3L 90 mm/s Cu-4 60 mm/s Cu-5 30 mm/s Al-1 30 mm/s Al-2 60 mm/s SS 60 mm/s METHOD AND SET-UPS SEY Measurements The SEY measurements were performed after installing and overnight pumping in the SEY study facility de-scribed in Ref. [1], where the primary electrons were provided by a Kimble electron gun (ELG-2/EGPS-2). The FWHM electron beam size was set to be less than 2 mm diameter with a current of a 10 nA. The primary energy was varied between 60 and 1000 eV. The sample was biased to -18 V with respect to the Faraday cup which was kept at ground potential. The total dose was limited to    10-6 Cmm-2 to minimise the effect of beam induced scrubbing during the data collection.  Surface Resistance Measurements Two types of cavities were used to measure the surface resistance of the samples; a 7.8 GHz circular choked pill-box cavity detailed in [3] was used for anisotropic meas-urements, while a half-pill-box cavity provides a strong directional preference for the current paths on the sample surface. Cavity 1 for non-directional measurements. The 3-choke cavity operates in a TM010-like mode. It therefore has a circular H-field distribution on the sample plate, which induces radial currents, as can be seen in Fig. 1.  Cavity 2 for directional measurements. A half-pillbox cavity was used to take some measurements of the surface resistance with a well-defined current direction. The sample piece is placed on the symmetry plane paral-lel to the cavity axis. This causes the fundamental mode Proceedings of IPAC2016, Busan, Korea TUOCB0207 Accelerator TechnologyT14 Vacuum TechnologyISBN 978-3-95450-147-21089 Copyright©2016CC-BY-3.0andbytherespectiveauthorsof the cavity to be the TM110 mode, which has a strong transverse H-field (often used in deflecting or crab cavi-ties). In our case, it also causes a strong electric current to flow in the axial direction on the surface of the sample plate (see Fig. 2).    Figure 1: The 3-choke cavity and the current path on the sample plate.  Figure 2: A half-pillbox cavity and the current path on the sample plate. RESULTS  Copper Samples Figure 3 shows low and high resolution planar scanning electron micrographs (SEM) of treated Cu surfaces as described in Table 1. The laser treatment resulted in a topography that consists of a microstructure of grooves superimposed with submicron and nanometre sized struc-tures. In comparison to the surfaces reported in Ref. [1], where the grooves were as deep as 100-200 µm, the sur-faces reported here have much shallower grooves ranging from 5 to 60 µm.    The results of SEY measurement of Cu as a function of primary electron energy, Ep, are shown in Fig. 4. Sample Cu-3, treated at 90 mm/s, has the lowest or equal lowest SEY over the measured range of primary electron energy.  A lower scanning speed of 60 mm/s (sample Cu-4) re-sults in an increase of SEY at low primary electron ener-gy. Conversely, a higher scanning speed of 120 mm/s (sample Cu-2) results in the increase of SEY at high pri-mary electron energy.  The further reduction of scanning speed to 30 mm/s (sample Cu-5) and increase to 180 mm/s (sample Cu-1) both result in further increases in SEY over the entire measured range of primary electron energy. The results of the surface resistance measurements are shown in Table 2. The samples Cu-1L, Cu-2L and Cu-3L were first measured using the contactless Cavity 1 to give the resistance averaged over all directions relative to the laser patterning. Then the samples Cu-1L and Cu-3L were measured using Cavity 2, which provides directional surface resistance measurements with 0 referring to when the current induced by the cavity is flowing along the laser scan path. Since Cavity 2 operates at 3.9 GHz, the RS was normalised to 7.8 GHz to make it easy to compare results from both cavities. For comparison, the results for untreated Cu, Al and SS are also shown in Table 2.   Figure 3: Low (on the left) and high (on the right) resolu-tion planar SEM micrographs of 1 mm thick Cu samples.   Figure 4: SEY measurement of Cu samples as a function of primary electron energy. The results show that the surface resistance reduces with laser scan speed, which is directly associated with the depth of the grooves. The shallowest grooves on Sam-ple Cu-1 produced RS comparable to untreated alumini-um, while all other tested samples have RS lower than untreated stainless steel. Another result is that there is no strong directionality in the surface resistance of the sam-ples Cu-1L and Cu-3L; the difference varies within 15-25%.  TUOCB02 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-21090Copyright©2016CC-BY-3.0andbytherespectiveauthors07 Accelerator TechnologyT14 Vacuum TechnologyTable 2: Surface Resistance at 7.8 GHz Sample Cavity Measurement RS []  Cu-1L 1 average 7.8×10-2 2 0 0.11 2 45 0.11 2 90 9.5×10-2 Cu-2L 1 average 0.13 Cu-3L 1 average 0.14 2 0 0.15 2 45 0.19 2 90 0.2 Cu untreated 1 average 3.3×10-2Al untreated 1 average 7.2×10-2 SS untreated 1 average 0.17 Aluminium Samples The results of SEY measurement of Al as a function of primary electron energy are shown in Fig. 5. The higher scan speed used for sample Al-2 gave the lowest overall SEY with 1 in all the primary electron energy range. The sample Al-1 has  < 1 at Ep ≤ 200 eV, but  > 1 at Ep > 200 eV reaching  = 1.65 at Ep = 1 keV.    Figure 5: SEY measurement of Al samples as a function of primary electron energy.  Figure 6: Low resolution planar SEM micrographs of SS sample. Stainless Steel Samples Figure 6 shows the planar SEM of, and Figure 7 the SEY measurement of, the treated 316LN stainless steel (SS) sample surface. This sample has  < 1 at Ep < 250 eV and   ≤ 1.1 at Ep ≥ 250 eV.  Figure 7: SEY measurement of SS sample as a function of primary electron energy. DISCUSSION The results of this and our previous study [1] show that laser ablation surface engineering has a large window of laser treatment parameters to produce different types of surface topography which reduces SEY below 1. For all three materials studied (Cu, Al and SS), laser ablation can produce surface topographies which can reduce the SEY to below or near 1. In all cases, the processing applied resulted in a grooved topography, on which is superim-posed submicron and nanometre sized structures. The depth of the grooves was engineered by varying the laser scan speed which controlled the number of pulses per area. Higher scan speeds will result in a lower number of pulses and consequently reduce the groove depth. How-ever, the submicron and nanometre sized structures re-mained similar in all samples. Both grooves and submi-cron and nanometre sized structures have an impact on SEY reduction which provides an optimisation between SEY and surface resistance. In the case where the lowest RS was achieved (sample Cu-1) the SEY shows the high-est value, but with  < 1 for energies up to Ep = 500 eV which is the critical energy range for e-cloud generation in modern particle accelerators and  < 1.1 for higher energies. CONCLUSION Surfaces with  < 1 on copper, aluminium and stainless steel can be produced through controlled laser ablation with a laser operating at  = 355 nm. It was demonstrated that  < 1 can be achieved with tolerable level of surface resistance. ACKNOWLEDGEMENT This work was conducted under the aegis of the Sci-ence & Technology Facility Council (STFC). The authors would also like to acknowledge the support provided by EuroCirCol collaboration (European Union’s H2020 Framework Programme under grant agreement no. 654305). Proceedings of IPAC2016, Busan, Korea TUOCB0207 Accelerator TechnologyT14 Vacuum TechnologyISBN 978-3-95450-147-21091 Copyright©2016CC-BY-3.0andbytherespectiveauthors REFERENCES [1] R. Valizadeh, O.B. Malyshev, S. Wang, et al. Low second-ary electron yield engineered surface for electron cloud mitigation. App. Phys. Lett. 105, 231605 (2014). [2] R. Valizadeh, O. Malyshev. Apparatus and methods relating to reduced photoelectron yield and/or secondary electron yield. Patent publication number WO2015189645 A1. 17th Dec 2015. [3]  P. Goudket, L. Gurran, G. Burt, et al. Surface Resistance RF Measurements of Materials Used for Accelerator Vacu-um Chambers. In Proc. of IPAC’15, p. 3235.  TUOCB02 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-21092Copyright©2016CC-BY-3.0andbytherespectiveauthors07 Accelerator TechnologyT14 Vacuum Technology

Low secondary electron yield of laser treated surfaces of copper, aluminium and stainless steel

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Low secondary electron yield of laser treated surfaces of copper, aluminium and stainless steel

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