Commissioning of an experiment for thermodynamic and spectroscopic studies of hydrogen isotopologues at cryogenic conditions

TL;DR

The paper reports the commissioning of the T2ApIR experiment at the Tritium Laboratory Karlsruhe to investigate the thermodynamics and dynamic phase-space behavior of the six hydrogen isotopologues in cryogenic conditions, using IR and Raman spectroscopy. A Tritium-compatible cryostat with an optical cell, ortho/para converter, and windows is integrated into TLK's closed-loop tritium infrastructure, achieving temperatures as low as $12$ K and pressures up to $2.5$ bar a to enable solidification of $T_2$ and other isotopologues. Inactive commissioning results include cryogenic performance, calibration with inert gases, and initial optical validations showing the system can monitor ortho/para states and perform in-situ calibration, while identifying issues with the cryogenic converter that are under investigation. The work demonstrates feasibility of safe, high-density $Q_2$ spectroscopic studies and lays the groundwork for active tritium commissioning in 2026 to explore radiolytic effects and calibration of optical methods for tritium-bearing mixtures.

Abstract

To study thermodynamic properties and dynamic phase space behavior of hydrogen isotopologues (Q$_2$) at cryogenic temperatures and at high density, the Tritium Absorption InfraRed Spectroscopy 2 (T$_2$ApIR) experiment has been set up and commissioned at Tritium Laboratory Karlsruhe (TLK). In the frame of the experiment, Q$_2$ behavior in different phases, ortho/para states, temperatures (10 K - 300 K) and pressures (up to 2.5 bar a) will be investigated with optical methods, infrared and Raman spectroscopy. The facility consists of a fully tritium compatible cryostat, which includes an optical cell, ortho/para converter and windows for optical and spectroscopic studies. The cryostat can be cooled below the H$_2$ triple point by a two-stage cryocooler and contains openings in the cryogenic shielding for the optical access. The challenge of combining these scientific requirements in a design with high amounts of tritium (14 g), in a limited space, all while maintaining the TLK safety philosophy was solved by the presented design. The experiment is ready to be fully integrated into the TLK closed loop tritium infrastructure. This contribution reports a comprehensive overview of the commissioning phase of the experimental facility and the results of the first commissioning experiments, including cryogenic performance tests, commissioning experiments with non-radioactive gases, and tests of the analytical instruments.

Figures (9)

• Figure 1: Experimental idea including IR-system, throughpass laser Raman-system and photography in a measurement cell. The sample cell is included in the figure multiple times to illustrate the photography and Raman beam path. Only one cell at a fixed position is present in the setup.
• Figure 2: Simplified flow diagram of the primary system (optical system depicted in \ref{['fig:optical_sketch']} not shown).
• Figure 3: Cryostat design changes. The top half shows the old design, the bottom half the new design. ZrO$_2$ rods and copper ribbons were removed (red) and replaced by a solid copper block (yellow). Heat is conducted to a cold head (not shown) on the right. TVO sensor and Pt1000 of the cryogenic measurement cell (grey) are shown in blue.
• Figure 4: Cooldown curve of the cryostat.
• Figure 5: Calibration of the cell temperature sensor using the vapor pressure derived temperature of the measurement cell contents. The dashed line represents the ideal exact correspondence between cell temperature and sensor reading and is shown to guide the eye.