Christian Hammill, Wayne State University Outgassing of Stainless Steel Vacuum Chambers and The Vacuum Pumping Speed and Capacity Evaluation of a Titanium Sublimation Pump Christian Hammill, Wayne State University Dr. Yulin Li & Dr. Xianghong Liu, Cornell University

My Project at a glance First part; Second part; extremely low => Extreme high vacuum Second part; Very high pumping => Able to handle very large gas loads November 11, 2018 REU 2007, LEPP Cornell

Contents Outgassing of stainless steel chambers Introduction and purpose What is the goal of this project? Methodology What methods are used to determine ? What do these methods involve? What is the difference in these methods? Results Conclusions Vacuum characteristics of the TiSP Introduction and purpose What is a TiSP? What do we want to determine from the TiSP? Methodology How do we determine Q(t) and STi(t)? Results Conclusions November 11, 2018 REU 2007, LEPP Cornell

SST Outgassing Introduction Ultra-High Vacuum: 10-9~10-12 torr Extreme-High Vacuum: <10-12 torr XHV must be achieved in parts of ERL (i.e. photo-chamber) Stainless Steel outgassing hinders this When put under vacuum 400°C bakeout reduces outgassing Does this last forever?! My mission: do stainless steel chambers that have been stored properly in N2 for ~8 months keep low outgassing property? November 11, 2018 REU 2007, LEPP Cornell

SST Outgassing Methodology I Vacuum system comprised of two parts: Testing Chamber Enclosed by oven Heating gun/ air blower Water cooling coils/ heating tape SRG/ sample chamber Sensor Chamber Valve to Testing Chamber RGA CCG Ion Pump & Turbo Pump November 11, 2018 REU 2007, LEPP Cornell

SST Outgassing Methodology II Two methods of determining Rate of Rise method Where V is the volume of the chamber (28L) and As is inner surface area of the sample chamber (7500 cm2) Throughput method Where is Sip the pumping speed of the ion pump and As is inner surface area of the sample chamber ∆P (pressure increase due to outgassing) is explained later November 11, 2018 REU 2007, LEPP Cornell

SST Outgassing Methodology III Both methods follow a similar procedure: Set up and leak check All vacuum flanges are connected and properly tightened Then, entire vacuum system is pumped down and checked for gas leaks Bakeout The adsorbed water molecules (from air exposure) are eliminated via temperatures >120°C Measurement is determined using either ROR method or throughput Repeat Baking temperature is increased each time (150°C to 200°C to 250°C) November 11, 2018 REU 2007, LEPP Cornell

The Rate of Rise Method In RoR measurements, the sample chamber is closed off from any pumping This allows gas to accumulate in the closed system With a constant outgassing rate, , a linear pressure rise is expected The rate of rise in the pressure, , is measured to calculate such that: November 11, 2018 REU 2007, LEPP Cornell

The Spinning Rotor Gauge SRG consists of a magnetized rotor ball in a gimble tube, and a removable head that contains sets of coils The Rotor is magnetically levitated and spinning Molecules in the chamber collide with the rotor, ever slightly slowing down the rotor, the “molecular drag” The SRG measures the pressure by measuring the slowing down rate of the rotor due to the molecular drag Rotor Removable SRG coil head Gimble SRG do not Alter the vacuum – Ideal for RoR November 11, 2018 REU 2007, LEPP Cornell

Throughput method Used as crosscheck for RoR method The pressure at the CCG is recorded while the sample chamber is still closed off from the sensor chamber => baseline pressure The chamber is then opened and the pressure is left to settle then recorded The difference between the settled pressure and the baseline pressure is known as the change in pressure, ∆P Then, using the pumping speed of the ion pump, Sip (~9L/s), we calculate : November 11, 2018 REU 2007, LEPP Cornell

A Typical RoR Result Pressure measured by SRG clearly show a linear rise in time. The fitted slope: dP/dt~ 2x10-12 Torr/s. Very stable temperature control ΔT ~ 0.17 °C November 11, 2018 REU 2007, LEPP Cornell

A Typical Throughput Result Sip is determined via a pump down of sample chamber: Fitting (blue) curve, P(t): Sip=V/τ1 Where, Gas composition at point “A”: In this case, ∆P≈1.5x10-11 Torr, Sip ≈9 L/s therefore, =1.9x10-14 Torr•L•s-1•cm-2 November 11, 2018 REU 2007, LEPP Cornell

Outgassing Results – Summary Current project data Tbake(°C) (10-15 Torr•L•s-1•cm-2) Rate of rise Throughput 120 2.6 30 6.0 19 7.5 n/a 150 25 18 15 200 6.7 28 250 9.3 35 38 Data from ~8 months ago TBake (°C) (10-15 Torr•L•s1•cm-2) 150 19.0 200 16.0 250 14.0 November 11, 2018 REU 2007, LEPP Cornell

SST Outgassing Conclusions The outgassing rates measured in this study are comparable with the results from 8 months ago. This indicates that the extremely low outgassing property can be maintained. The comparison between the rate of rise method and the throughput results is very good, when one considers at least two factors: The measurements were done at very different pressure ranges (~10-6 torr for rate of rise, and ~10-10 torr for the throughput). They both use very different gauges (SRG vs. CCG). Therefore, in certain places in the ERL (i.e. the photo-cathode) one can vent out the vacuum, properly store the stainless steel components in N2, and not damage the outgassing properties of these components November 11, 2018 REU 2007, LEPP Cornell

TiSP Performance Introduction After measuring the properties of the electron beam, it must be safely terminated at the beam dump in the Cornell Proto-type Photo-cathode Injector Very large gas loads of H2 gas are generated at the beam dump when this happens A large TiSP will be used together with two large ion pumps to control the H2 gas load Will this do the trick? My mission: evaluate the pumping performance (particularly the pumping speed and the pumping capacity) of the TiSP to determine if it is suitable for this application Beam Dump TiSP November 11, 2018 REU 2007, LEPP Cornell

TiSP Performance methodology I Experimental Setup The test system was leak checked and baked at 170°C Ultra-high purity hydrogen is used to measure the TiSP pumping performance The flow-rate of H2 was experimentally determined before any Ti-sublimation, After Ti-sublimation, H2 flows through the TiSP chamber, and the pressures are monitored by two cold cathode gauges November 11, 2018 REU 2007, LEPP Cornell

TiSP performance methodology II Ti Sublimation There are 3 Ti cartidges on the end of the chamber Each filament is heated via voltage power source This heating causes the Ti to sublimate in the chamber This process is known as “flashing” The Ti is flashed in the chamber for a certain period of time at a certain power Ti cartrige with 3 filaments November 11, 2018 REU 2007, LEPP Cornell

Determining STi(t) After flashing, we open the H2 valve again and let the gas flow Because the Ti layer inside the chamber is so reactive, it will capture much of the gas as it enters the chamber This causes the pressure to decrease at the CCG At the CCG there are now 2 pumping speeds The effective speed of the turbo, , and the pumping speed of the chamber, STi(t). Knowing this, we derive the formula for the pumping speed of the chamber over time  this follows the Equation, =SP November 11, 2018 REU 2007, LEPP Cornell

[STi(tj)• ]∆tj Q(t)= Determination of Q(t) We want to determine the total amount of H2 gas pumped away by the TiSP chamber, Q(t). As the Ti film pumps more and more H2 is pumped away it becomes saturated with H2 As it becomes more saturated, its pumping speed decreases The accumulated Q(t) is directly related to STi(t) such that: [STi(tj)• ]∆tj Q(t)= November 11, 2018 REU 2007, LEPP Cornell

TiSP Performance Results I STi(t) was measured at 2 flashing settings (A) 3 minutes at 170W (B) 5 minutes at 195W Higher Ti-flashing power & longer duration => much thicker Ti layer The STi(t)’s are plotted against the Q(t)’s Limited Ti flashing and H2 saturation cycles were done Thick, H2 rich, Ti films are known to be “flaky” November 11, 2018 REU 2007, LEPP Cornell

TiSP Performance Results II Q(t) is arbitrary and depends on its application. Here, we chose Q(t) to be the point where STi(t) drops to ~100L/s (i.e. ~10% of its initial pumping speed). With this definition, one sees from that: QA≈ 4 Torr•L QB≈ 25 Torr•L With maximum pumping speeds of: STi, A(t) ≈900L/s STi, B(t) ≈1200L/s As a side note, it took ~80 hours for B to saturate from the beginning to 100L/s Hydrogen Pumping by Ti Step 1 – Dissociative adsorption H2  2Hads Step 2 – Bulk diffusion Hads  HBulk Step 1 depends on surface reactivity of Ti film Step 2 depends on Ti film thickness and solubility November 11, 2018 REU 2007, LEPP Cornell

TiSP Performance Conclusions It can be seen that the max STi ~ 1200 L/s & that it is capable of pumping out ≤25 Torr•L of H2 gas. The estimated gas load at the beam dump (A5 section of the Cornell Proto-type Photo-cathode Injector) can be as high as 1.8x10-3 Torr•L/s. Considering that two other ion pumps will be sharing half of the load, the TiSP will have to handle a gas load of 9.0x10-4 Torr•L/s. With a little math, we can see that the TiSP can go ~8 hours before needing to go through a flashing cycle again. This is considered to be sufficient for the injector operations Therefore the TiSP is suitable for the ERL prototype project November 11, 2018 REU 2007, LEPP Cornell

Acknowledgements Thanks are due to : My mentors; Dr. Yulin Li and Dr. Xianghong Liu who have assisted me in the writing of this paper, the assembling of the experiments, and the overall guidance though this project. The lab technical staff; Tim Giles, Tobey Moore, and Brent Johnson. Dr. Rich Galik and Dr. Claude Pruneau who are the assemblers of this REU project and made it able for me to attend the summer here. The National Science Foundation who every year make it possible for students much like myself to have experiences like this one. November 11, 2018 REU 2007, LEPP Cornell