Sample Exposure Probe
The Sample Exposure Probe figure~\ref{iso_SEP}, also referred to as STS-2 (Sample Transfer System – 2) in some design documents or the LTX-$\beta$ SEP, is effectively a vacuum suitcase. The science goal of the probe is to get XPS and Depth Profiling measurements for Lithium coated stainless steel samples using PHI. PHI is an XPS system in T-260, mounted with an X-Ray source and an Ion Gun for sputter cleaning. LTX base pressures were 1E-8 Torr, water partial pressures are estimated to be an order of magnitude lower. At these pressures the Li over-layer on the SS PFC is expected to change over a period of hours \cite{lucia,bob-lucia}. Therefore, it is conceivable to get ex-situ, in-vacuo XPS characterization of LTX-Beta PFC, provided, LTX-Beta samples can be transferred to an XPS system with atleast LTX like and preferably better vacuum conditions. Therefore the science goals of the Sample Exposure Probe can be summarized as follows.
- Get XPS Characterization of LTX-Beta PFC using PHI, the XPS system in T-260.
- Enable in-vacuo sample transfer between LTX-Beta and PHI under LTX like or better vacuum conditions. \end{itemize}
Hardware design
The SEP, figure~\ref{iso_SEP}, is mounted in a bellows drive bolted to a wheeled support structure with telescopic hydraulic legs to facilitate mounting on LTX-$\beta$ and a surface analysis chamber. The surface analysis chamber is a stainless steel ultrahigh vacuum (UHV) chamber (modified PHI 5300 ESCA System) in the Surface Science and Technology Laboratory (SS\&TL) near LTX-$\beta$ at PPPL. The modification involved moving the X-Ray source such that the orientation of the SEP sample head doesn't have to change from LTX-$\beta$ to the analysis station. The chamber is equipped for XPS, TPD, Ion Scattering Spectroscopy (ISS) and sputter depth profiling. The chamber has a base pressure of 2 $\times$ $10^{-10}$ Torr; pumping is provided by a 120 L/sec ion pump with 1000 L/sec titanium sublimator (Perkin-Elmer), a 170 L/sec turbomolecular pump (Pfeiffer Balzers, TPU 170 C), and a 33 L/sec turbomolecular pump (Leybold, TurboVac 50) for differentially pumping the ion source. The SEP is equipped with a battery powered 67 L/sec turbomolecular pump (Pfeiffer Vacuum, Hi Cube) and a 100 L/sec non-evaporable getter pump (SAES Getters, CapaciTorr D-400) and has a base pressure of $1.6 \times 10^{-9}$ Torr. The base pressure of LTX-$\beta$ after lithium evaporation is $6 \times 10^{-8}$ Torr. Therefore, analysis can be performed for LTX-$\beta$ PFCs by moving samples in-vacuo, under better vacuum conditions to a high resolution system that is nearby. In doing so, design constraints that are imposed on the analysis station, for example by the test cell of the tokamak are eliminated, enabling analysis with systems that posses higher resolution and better signal to noise ratios. Better vacuum conditions during transfer and analysis reduce the rate of sample contamination and therefore, provide adequate time required for physical transfer of the probe, its analysis and return to the tokamak. XPS is conducted with a dual-anode X-ray source (PHI model 04-548) and a spherical capacitor analyzer (SCA; PHI model 10-360). All XPS spectra are taken with Mg K$\alpha$ X-rays generated at 15 kV and 300 W. ISS and ion sputtering are conducted with a differentially pumped ion source (PHI model 04-303).
The probe head, figure~\ref{head} is made of SS-304, to be the same as the plasma facing side of the shells in LTX-$\beta$. An Ultra High Vacuum (UHV) button heater is placed in the back of the probe-head. The insulator is electrically and thermaly insulated from the support sturcture with a MACOR washer. The support struction is connected to a SS-304 rod that makes with another MACOR bushing, secured inside a 1 inch ID tube with retaining rings. The MACOR bushing provides an additional barrier between the bellows and the probe head. The SS-304 1 in tube slides through the bellows supported by three zirconia roller bearings (at 120 degrees from one another) mounted on a collar on the outlet of the bellows. The back of the 1 in tube is threaded into a double faced flange tapped with a 32 thread per inch tap and die set.
Instrumentation and commissioning
The instrumentation on the SEP consists of a PID controller, that takes the thermocouple as an input and can be programmed to give a step or a ramp input to a DC power supply to a temperature setpoint. The probe head is floating with respect to the probe body and therefore, can be biased to measure ion current when being bombarded by an ion beam. The probe ion gauge can be digitized using a raspberry pi module that communicates with an RS-485 cable. The turbopump and backing pump controller have their own power supply. Everything is powered by a 650 VA UPS that should provide 40 minutes of backup at full load. The physical distance between LTX-$\beta$ and the surface analysis chamber is roughly 180 feet, and travel time with SEP, is about 3-5 minutes. On both sides, while mounting the SEP to the surface analysis chamber and LTX, a transfer volume between the machines has to be pumped down, after multiple trial and error it was decided to use a standard 2.75 in conflat tee for this connection figure~\ref{mate}.
