Measurement of the Photoelectron Yield from the Synchrotron Radiation for the NEG-coated Tubes

NEG-coated chambers have been adopted as the beam ducts for large particle accelerators and synchrotron light sources for the sake of the lower yields of the photon stimulated desorption (PSD) and the photoelectrons (PE) from the NEG films in addition to their pumping performance. Measurement of the photoelectron yield (PEY) was performed at the BL19B (PSD) beamline of the 1.5 GeV Taiwan Light Source (TLS) which simultaneously measures the PSD-yield. An aluminium cathode was inserted in the tubes and a positive bias of voltage for extraction of the photoelectrons applied. The PEY was obtained by dividing the photoelectron current by the photon flux of the synchrotron radiation. Measurements of the PEY include various types of NEG-coated stainless steel tubes and the bare tubes of titanium and aluminium alloys for the comparison. The experimental system and the results will be described in this presentation.


Introduction
For large particle accelerators, extraction of the photoelectrons generated by the synchrotron radiation from the beam duct results in an e-cloud problem.For synchrotron light sources, significant outgassing from the surface of beam duct induced by the photon stimulated desorption (PSD) causes a beam instability problem.Investigations to find the lowest yield of PSD (PSD) and photoelectron PEY (e) from beam ducts with various surface treatments or interior coatings become important for designing a new machine.Nowadays, NEG-coated chambers adopted for large accelerators have become popular and investigations of the PSD or e for NEG-coated tubes by synchrotron radiation have been studied in some light source facilities [1,2].It should be interesting to know the relationship between PSD and e and if they can be simultaneously measured.The BL19B, a white-light beamline of the 1.5 GeV Taiwan Light Source (TLS), has been dedicated to measuring the PSD (PSD) of sample tubes or sheets at a critical energy of 2.14 keV [3].It is able to simultaneously measure the e, by inserting a metallic cathode into the sample tubes closed to the photon-exposure area, during the PSD-measurement.The NEG-coated tubes to be measured were prepared by ASTeC with different compositions of NEG film for various purposes.The measurement of the PSD and e for the NEG-tubes at BL19B beamline is described.

Beamline BL19B for the measurements
The beamline BL19B at TLS extracts a confined photon span angle (ϕ) of 2.53 mrad of synchrotron radiation (SR) from a bending magnet to the sample tube for measuring the PSD-yield (PSD), as shown in figure 1 [3].The total photon flux ( ̇), in units of photons/s, is obtained from equation (1) [4].Figure 2 shows the schematic layout of PEY-measurement in which an aluminium stick, 2 mm thick and 400 mm long, as an "Al-cathode" to measure the PEY (e), was inserted into the sample tube from the end.This cathode, had a positive bias voltage (+V) applied, the photoelectrons from the SRirradiated surface of the tube during the photon exposure are extracted and measured by a picoammeter (A) shown as the electric loop in figure 2. By transforming the value of the current (-Ie) to the photoelectron flux ( ̇ ), in units of electrons/s, in equation ( 2), the PEY (e) defined by  ̇/ ̇ in equation ( 3), in unit of electrons/photon, is obtained.In these equations, E (beam energy) = 1.5 GeV, I (beam current) ~ 362 mA (top-up), ϕ (photon span) = 2.53×10 -3 rad, Ie (photoelectron current) is in units of Amperes.

Sample Tubes for the Experiment
Table 1 lists the sample tubes, of inner diameter 38 mm and length 0.5 m, to be measured.Those tubes made of 316LN stainless steel (SS-) were coated with different compositions of the Non-Evaporable Getter (NEG) films provided by the vacuum science group of ASTeC, STFC, UK [5].To compare with bare tubes without NEG-coating, an aluminium (Al) tube provided by NSRRC and a titanium (Ti) tube provided by ASTeC were measured for PSD and e as well.The experiment for each NEG-tube was carried out by the following procedure:  Install the tube at beamline and bake (Ba-), at 80°C for NEG-tube and 120°C for ambient chambers, to UHV.  SR-exposure to a beam dose < 1 Ah for measuring the PSD and e of the NEG (Baked without activation). Activate the NEG (Ac-) for 24 hours by heating the NEG-tube at 180°C and keeping ambient chambers at room temperature (~ 24°C). Continue the SR-exposure to higher beam dose for measuring PSD and e of the NEG after activation.The non-coated tubes, AL and Ti, were baked at 150°C, 24 h to UHV prior to the experiment.

Measurement of PSD-yield
Each sample tube on the beamline was cooled with cooling jackets to keep the temperature rise < 0.7 ºC on the tube throughout the SR-exposure.The average pressure of the entire vacuum system after baking or activation was < 2.610 -8 Pa prior to the experiment.The curves of PSD for Al, Ti, and the three SS (NEG) tubes vs. the Beam Dose (D) in unit of ampere-hour (Ah) are shown in figure 3, where 1 Ah equals to 7.96310 19 photons/m of dosage [3].For the three SS(NEG) tubes, the left parts of curves (after baking) at D < 1 Ah are similar to each other and declined from an initial ~ 210 -3 of PSD to 110 -4 molecules/photon at 1 Ah of D. The right parts of three curves (after NEG-activation), at D > 0.7 Ah almost overlap and decrease linearly with a slope () ~ -0.55 and approach a PSD ~ 110 -6 molecules/photon at 10 Ah of D; "" is defined as equation ( 4) [3].The two curves of the Al and Ti tubes (figure 3) are close to or slightly higher than those of the SS(NEG) tubes (baked), and decreased linearly at slope () ~ -1.The PSD of 210 -5 and 110 -5 molecules/photon for Al and Ti tubes, respectively, at 10 Ah of D are more than 10 times higher than those for SS(NEG) tubes (activated).Several spike-like peaks appeared on the curves in figure 3 representing bursts of pressure associated with gas desorption stimulated by the photoelectrons (ESD) hitting on the Al-cathode when supplied with a positive bias.
Figure 4 depicts a typical mass spectra of the residual gas from PSD, e.g.SS-2 tube, at 1 Ah (Ba-) after baking and 10 Ah (Ac-) after NEG-activation, respectively.The dominant PSD-outgas components are H2, CH4, CO, CO2, CxHy (hydrocarbons), and a small amount of Kr.It clearly shows a significant decrease in the peaks of CO and CO2 between the two spectra in figure 4 that possesses the relatively lower yields of PSD from the NEG film after activation.

Measurement of PEY
During the SR-exposure for each sample tube, the photoelectron current (PE-current, Ie) was measured by applying a positive bias voltage, 0 ~ 1000 V, to the Al-cathode.Figure 5 depicts the curves of the Ie of tubes, after baking (Ba-) or after NEG-activation (Ac-), versus cathode bias voltage.The Ie increases with increased bias voltage and reaches to approximate maximum value at 1000 V.It is assumed that more than 90% of the photoelectrons are extracted by the Al-cathode at +1000 V bias and contribute to the measurement of Ie.By calculation from equation (3), 1000 μA of Ie equals to 0.035 electrons/photon of PEY (e) at a beam current (I) of 362 mA.The values of Ie for all the sample tubes (in figure 5) at +1000 V bias cover from 550 to 1300 μA, that equates to the e from 0.019 to 0.046 electrons/photon.Table 2 lists the values of PE-current and e for the sample tubes at various beam dose.Figure 6 depicts the values and curves of e versus beam dose for each tube in the cases of either baked (Ba) or NEG-activated (Ac).The e for SS(NEG) tubes "Baked", 0.024 ~ 0.036, are 1.2 ~ 1.6 times higher than "NEG activated", 0.019 ~ 0.028.While e of Al (Ba-) at D ~ 1 Ah is about 1.4 times higher than that of SS-3_Ba.It reveals that the NEG-coating (baked) samples possess lower e than the bare aluminium surface.The e of NEG-coating after activation is further reduced compared to those that are only baked.However, the e of Ti tube, 0.021 ~ 0.025, is as low as those of SS(NEG) tubes (activated).It is not clear why the Ti alloy possesses lower electron emission that is competitive with the activated SS(NEG) tubes.In figure 3, the curves of PSD for the Ti tube and baked SS-1(NEG_c) tube overlap, which implies the possibility of a similar mechanism of surface bonding and the consequent control of ESD by the e from the surface oxide layers.The change of e (in figure 6) for those tubes subject to even more beam dose of photon exposure is not much different.

Conclusion
The e for SS(NEG) tubes with different NEG-coatings and bare Al and Ti tubes was measured at beamline BL19B of the 1.5 GeV TLS.An Al-cathode inserted in the tube with a positive bias extracted the photoelectrons from the surface during SR-exposure; meanwhile the PSD was measured.The SS(NEG) tubes after NEG-activation possessed both lower e and PSD than those tubes after baking.The lower e results in a lower PSD, however, a longer exposure of SR beam dose decreases the PSD significantly with little reduction for the e.The SS-2(NEG_t) possesses the lowest e compared to the other tubes.The e of a bare Ti tube is as low as those of the SS(NEG) tubes after NEG-activation.More experiments on the investigation of PSD and e for NEG-coated tubes or other samples with various surface treatments or coatings can be performed at this beamline.

Figure 4 .
Figure 4. Typical RGA mass spectra of the PSD for the NEG-coated tubes at beam dose of 1 Ah (Ba-) black-line and 10 Ah (Ac-) red-line.

Figure 5 .
Figure 5. PE-current of sample tubes vs. the bias voltage.

Table 1 .
Sample Tubes for the Experiment.

Table 2 .
PE-current and PEY of Tubes vs. the Dose.