Study on the vacuum properties of laser-etched oxygen-free copper

The performance of operating particle accelerators has been seriously affected by the electron cloud (e-cloud) effect. The secondary electron emission (SEE) and the e-cloud can be effectively suppressed through laser-etching the inner surface of the vacuum chamber. Oxygen-free copper (OFC) has become the first choice for the vacuum chambers of modern accelerators due to its high electric and thermal conductivity and effective radiation shielding property. It is necessary to study the vacuum properties of the laser-etched OFC for the application in the particle accelerators. In this paper, the photon stimulated desorption (PSD) yield and the outgassing rate of the laser-etched OFC were measured. The results show that the laser-etched OFC presents lower PSD yield compared to the untreated OFC, while the outgassing rates of the laser-etched and unetched samples are similar.


Introduction
Electron cloud (e-cloud) effect has been recognized as a major threat to modern particle accelerators, particularly in positron-, ion-and proton-accelerators, due to its adverse effects, such as dynamic pressure rise, beam instability or loss, emittance growth, and additional heat load [1].Several methods have been proposed to mitigate the e-cloud and maintain the steady operation of the accelerators.Laseretching technology is known for its simplicity, reliability and stability [2].The relationships between the laser-etching parameters (average power, scanning speed, pitch space, and scanning pattern) and the secondary electron yields (SEY) of the laser-etched samples have been studied in detail [3,4].The results show a slight increase in the roughness and the surface resistance of the laser-etched sample as the SEY decreases [5].It is important to figure out the effect of laser-etching on vacuum properties, due to the laser-etched surface might cause other problems [6].Some measurements have been conducted, such as the electron stimulated desorption (ESD) [7] and the thermal desorption of cryopumped gases of laseretched copper [8].
In this paper, laser-etching was applied to the surfaces of oxygen-free copper (OFC) samples.The photon stimulated desorption (PSD) yield and the outgassing rate of the laser-etched OFC were investigated.The same measurements were performed on the untreated OFC for comparison.

Laser-etching system and sample preparation
The laser-etching system in National Synchrotron Radiation Laboratory (NSRL) is composed of a laser generator, a focusing system, a three-dimensional mobile platform, and a computer controller system.The laser irradiated to the sample surfaces with a wave-length of 355 nm, an average power of 3.7 W, a spot size of 15 μm, and beam quality of M 2 < 1.1 (The closer M² is to 1, the better the quality of the beam and the smaller the corresponding divergence angle).The scanning pattern is determined as parallel lines to improve efficiency.The other scanning parameters include a fixed pitch spacing of 20 μm with scanning speeds of 30 mm/s for PSD measurement and 10 mm/s for outgassing measurement.All OFC samples are cleaned by NSRL's standard cleaning process [9].

PSD measurement
The facility for PSD measurement of Hefei Light Source Ⅱ (HLS Ⅱ) is exhibited in Fig. 1, which is comprised of the test chamber (between the sample holder and the orifice), the pumping chamber (between the orifice and the metal gate valve), the aperture, and the pumping system.The synchrotron radiation extracted from a bending magnet irradiated the sample at normal incidence.The aperture is set to limit the photon flux and the orifice is set to calculate the gas volume.Two Bayard-Albert gauges and quadrupole mass spectrometer (residual gas analyser, RGA) are installed on either side of the orifice to measure the total and partial pressure changes in the test and pumping chamber.The beam energy and current of HLS Ⅱ are 800 MeV and 400 mA respectively.The PSD yield (η) is defined as the ratio of the number of desorbed gas molecules (Nm) to the number of incident photons (Nph) Eq. ( 1), which can be calculated by the following formula: where G is the conversion coefficient, C is the conductance of the orifice, P2 and P1 are the pressure of the test chamber and pumping chamber, respectively, and the Pt and P0 are the dynamic pressure and the background pressure.

Outgassing Rate measurement
The outgassing rate was measured by switching between two pumping paths method (SPP), which schematic is illustrated in Fig. 2. The SPP consists of four steps: • Putting the sample into sample chamber with the orifice 1 opened and the orifice 2 closed; • Closing the orifice 1 and opening the orifice 2; • Taking out the sample, then opening the orifice 1 and closing the orifice 2. where C is the conductance of the orifice, A is the superficial area of the sample, P1 is the pressure displayed of EG1, the superscript 1, 2, 3, and 4 represent four steps respectively.

Figure 2．Schematic
illustration of outgassing rate measurement.SIP is the sputter ion pump, TMP is the turbo molecular pump, DP is the dry pump, V is the valve, MV is the metal valve, SV is the solenoid valve, EG is the extractor gauge, DG is the diaphragm gauge, PG is the penning gauge, and QMS is the quadrupole mass spectrometer.

PSD yield
According to the analysis of RGA, the main desorption gases are hydrogen, carbon monoxide, carbon dioxide, methane, and water.The PSD yields of different gases as a function of photon dose for the laser-etched and unetched OFC samples are shown in Figs. 3 and 4, respectively.It can be seen that the trends of curves for the laser-etched and unetched OFC are different within the test range, where the laser-etched OFC shows a continuous decline except for water, while the unetched OFC first declines, then rises slowly followed by a rapid decrease.The PSD yield curve of water has a completely different trend, which rises rapidly to a peak and then goes down.At the beginning of the irradiation, for both types of OFC, H2 has the highest PSD yield, followed by CO, CO2, CH4, and H2O.The results are different from the ESD test [10], because the differences in testing devices and cleaning processes.Initially, the gas desorption of the laser-etched OFC was slightly higher than that of the un-treated OFC.With the photon dose increase, the desorption of the laser-etched OFC decreases rapidly, with all yields of gases in the order of 10 -5 at photon dose up to 0.1 Ah.For the unetched OFC, the yields do not decrease to the order of 10 -5 until photon dose accumulates to 10 Ah.
The comparison indicates that the laser-etched OFC will not release other residual gas affecting the ultra-high vacuum environment, and the gas desorption for the laser-etched OFC is lower than that of the unetched OFC.Such results can be ascribed to the changes of the surface morphology and chemistry.A GeminiSEM 500 Schottky field scanning electron microscope (SEM) was utilized to characterize the morphologies of the surface and an X-ray photoelectron spectroscopy (XPS) was carried out to obtain element composition before and after laser-etching, which results are shown in Fig. 5 and Table 1.After laser-etching, the surface of OFC become rough and irregular, leading to a reduction in photoelectron yield (PEY), while a high PEY will promote the gas desorption.The adsorption of gas is related to the chemical composition of the surface.The Cu content and O content on the surface increase, while the C content decreases after laser-etching, resulting in the difference of the desorbed gas content.

Outgassing Rate
The outgassing rate measurement consists of three processes: • The first test conducts at 48 h after pumping at room temperature (RT).
• Then baking the sample at 150 ℃ for 24 h and cooling down to the RT.
• The second test conducts at 48 h after baking.The results are listed in Table 2.It can be seen that the outgassing rates of the laser-etched and unetched OFC are similar, the tiny difference is further reduced after baking.The outgassing rate after baking is an order of magnitude lower than before baking.The main desorbed gas of OFC in vacuum, displayed by QMS, are CO, H2, H2O, CO2, and a little O2.

Conclusion
The surface morphology and chemical composition of OFC was modified by the laser-etching technique in this paper.The PSD yield and the outgassing rate of the laser-etched OFC were measured.The highly regular topography of the laser-etched OFC is the reason for the decrease of the PEY, which will further affect the gas desorption.Thus, the PSD yield of the OFC after laser-etching is much lower than before.Moreover, the laser-etching technique does not have a significant effect on outgassing in vacuum, which can be inferred from the similar outgassing rate of the laser-etched and unetched OFC.
According to the results of the current measurement, the use of laser-etching OFC in the accelerator vacuum chamber has no effect on the PSD yield and outgassing rate.However, further study is necessary due to the measurements in this paper is still imperfect.PSD measurement under oblique incident angle for sample and pipe and outgassing rate measurement after higher baking temperature will be performed later.

Figure
Figure 1．PSD measurement facility.The PSD yield (η) is defined as the ratio of the number of desorbed gas molecules (Nm) to the number of incident photons (Nph) Eq. (1), which can be calculated by the following formula:

Figure
Figure 3．PSD yield of the laser-etched OFC.Figure 4．PSD yield of the unetched OFC.

Figure 5．The morphology images
Figure 5．The morphology images of OFC before (a) and after (b) laser-etching.

Table 1．The
surface chemistry of OFC before and after laser-etching (at.%).
Table 2．Outgassing rates of OFC samples.