1. Vapor Condensation Study for HIF Liquid Chambers by Patrick Calderoni UCLA – Fusion Engineering Sciences 15 th International Symposium on Heavy Ion Inertial Fusion Princeton Plasma Physics Laboratory Princeton, NJ, June 7-11, 2004

2. Experimental research work major accomplishments Developed an innovative and inexpensive scheme to generate flibe vapor in conditions relevant to fusion technology design studies involving a liquid protection scheme (HIF, IFE, Z-pinch) Measured flibe vapor clearing rates suggest that high repetition rates in HIF power plants are feasible provided that high purity of the molten salt is ensured Found that for flow conditions characterized by high kinetic energy flibe vapor condensation is partially inhibited on metal surfaces perpendicular to the main component of the vapor velocity

3. HYLIFE-II parameters relevant to vapor condensation studies (Moir, 1994) Total mass generated from x-ray absorption on liquid surfaces: 14 kg Total volume for vapor expansion: 280 m 3 Initial n: 0.9x10 18 #/cm 3 (0.5x10 18 #/cm 3 ) Recovered n: 3x10 13 #/cm 3 (2x10 15 #/cm 3 ) Energy from explosion coupled with x- ray and debris: 110 MJ Energy density: 7.85 kJ/g (7.5 kJ/g) Structural surface for condensation: 40 m 2 Surface from droplet injection: 1060 m 2 Ratio of surface area per unit mass of generated vapor: 785 cm 2 /g (4300 cm2/g for LiF and 10205 cm 2 /g for flibe) Droplet spray injection design: T = 843 K Spray flow rate = 2.4x103 kg/s (1.6% of main flow rate)

4. Experimental approach: staged design of vapor generation facility Characterization of superheated vapor source in comparison to other experiments using electro-thermal sources Diagnostic development Limited availability, cost and toxicity of materials: demonstrate efficiency, repeatability and reliability before using flibe Reduce residual non-condensable gases

5. Stage 1: Lexan with Argon background (1 Torr) time: 0 time: 820  s time: 1640  s Typical vapor parameters in the source : n = 10 19 - 10 20 # / cm 3 T = 1-3 eV Argon background is ionized (10-100 ns) forming initial plasma column Injected electrical power radiated to surface, ablates material of interest Pressure gradient drives injection, ablation balances axial mass loss Energy stored in cap banks maintains plasma at 1-3 eV for 100 micros

6. expansion chamber volume: 4000 cm 3 surface area: 1720 cm 2 Stage 2: Teflon and LiF in vacuum chamber time of flight view ports vaporsource current in current out

7. Cathode: ¼’’ D 10’’ long W rod Flibe liquid pool: 1.6 cm 3 volume Anode: Nickel crucible with embedded high density cartridge heater Stage 3: Flibe vapor generation SS witness plates for SEM and EDX analysis Pressure sensor, water cooled (Tmax = 260 C) Total length: 34 cm Expansion volume: 400 cm 3 Surface area: 420 cm 2 Insulation: high-vacuum ceramic breaks

8. High-speed camera frames sequence from flibe discharge 100  s 120  s 140  s 160  s 180  s 200  s 220  s 240  s 260  s 280  s 300  s 320  s

9. Pressure data and residual gas composition: flibe t 300 = 4.27 ms (4.22 ms at 1.44 kJ) t 500 = 6.58 ms H2H2H2H2 28,16 amu: hydrocarbons condensation is completed in 30 ms: no residual BeF 2 traces at 47 amu 44 amu: CO 2 300 C 500 C

10. p(T) → n(T) assumes thermodynamic equilibrium with a liquid surface Comparison with HYLIFE-II chamber clearing models n = n 0 x e -t/T n 0 = 0.9x10 18 #/cm 3 n end = 3x10 13 #/cm 3 Clearing period for HYLIFE-II = 68 ms

11. Measured composition of flibe vapors in equilibrium with a liquid surface Heating sequence (linear) from 460 C to 700 C (about 30 min) at 460 C H2H2H2H2 hydrocarbons CO 2 BeF 2

12. Additional data: post-analysis of side collecting plates Collecting plates parallel to the radial direction 300 C 460 C Film forms during first expansion phase (100  s) when vapor velocity is highly directional Drops condense in the chamber volume after the velocity has become uniform and deposit on the liquid film without merging At low T film is thinner and breaks due to quick cooling and solidification Surfaces are gold plated for SEM analysis

13. Additional data: post-analysis of front collecting plates Collecting plates perpendicular to the radial direction 300 C 460 C Film condensation is inhibited at 300 C At 460 C thin film starts at fixed r from plate side center (flow stagnation point) EDX analysis confirms qualitative results

14. Additional data: post-analysis of collecting plates Further evidence of volumetric condensation Evidence of liquid displacement by the pressure front generated during the discharge: large liquid drops are entrained in the flow and deposit around the crucible and the collecting plates 300 C 460 C Large drop is flibe sideplatefrontplate

15. Conclusion Condensation rates of flibe vapor in conditions relevant to IFE power plant studies have been measured experimentally - Vapor density decays exponentially with a time constant of 6.58 ms in the range between 5x10 17 cm -3 and 2x10 15 cm -3 Extending to HYLIFE-II expected density cycles the vapor clearing rate is 68 ms, compatible with the desired 6 Hz repetition rate Data suggest that for flow conditions characterized by high kinetic energy flibe vapor condensation is partially inhibited on surfaces normal to the main component of the vapor velocity Control of the impurities dissolved in the molten salt is a fundamental issue for applications that require recovery of vacuum conditions in the 10 13 #/cm 3 range

17. Discharge parameters Teflon I and V for 10 and 5 disk configuration LiF I for different energy experiments I for flibe experiments

18. HYLIFE-II concept Liquid protection for Inertial Fusion Energy power plant chambers Thick liquid “pockets” or thin liquid layers shield chamber structures: fluid mechanics questions replace materials questions Neutron damages and activation of flowing liquid accumulate only in the short residence period - no blanket replacement required, increases availability

19. The molten salt flibe: High Tritium Breeding Ratio Low electrical conductivity High neutronic thickness Chemical, radioactive and thermal stability Limited material availability Uncertain composition and purity level of available material Beryllium safety hazard Lack of data on physical and chemical properties For fusion system design: For small scale experiments:

20. Flibe composition is a compromise between melting temperature (structural costs) and viscosity (pumping cost) 2 LiF + 1 BeF2 in moles

21. In equilibrium with the liquid at 600 C, flibe vapor pressure is about 1 mTorr, corresponding to a density of 1.2 10 13 cm -3 Flibe low vapor pressure is a key property for efficient driver coupling with the fusion target at high repetition rates Reference data for flibe vapors are based on the assumption of thermodynamic equilibrium at the liquid interface and ideal gas equation

22. Weight loss measurements 18 experimental runs with 10 Teflon disks as sleeve material 1 with 5 disks 5.76 kJ 2.54 kJ 2.54 kJ Volume ratio between source and chamber: 1:25 Evidence of C soot deposition on source component

23. Pressure data: Lexan and Teflon Dynamic sensor mounted at center of back plate (high noise from vibration) Teflon = CF 2 chain Residual gases (20-70 Torr): 28 amu (C 2 H 2 - C 2 H 4 - C 2 H 6 ) 69 amu (CF 4 ) 20 amu (HF) about 5 times decay

24. Jet velocity optical measurement system Diode axis separation: 7.62 cm Peak time delay: 6 microseconds Estimated initial vapor velocity: 10110 m/s

25. Jet velocity optical measurement system Sensor closer to center of chamber sees a second peak in the light, about 2ms after triggering Peak due to first reflection of the vapor jet at the chamber bottom Vapor cools and expands in the chamber, emitting front does not reach the upper sensor Estimated average velocity of vapor in the chamber after first reflection: 320 m/s - 4 kV 210 m/s - 3 kV

26. Flibe experiments results Exponential decay fits data points good Decay time constant t 1 is a measure of the condensation rate t 1 = 4.22 ms

27. Flibe experiments results Exponential decay fits data points good Decay time constant t 1 is independent from initial particle density t 1 = 4.27 ms

28. Additional data: emission spectroscopy Strong lines from neutral and first ionization state of both lithium and beryllium atoms are present in the spectrum Be vapors diagnostic development: measure at different times the ratio of line intensity of Li I and Li II transitions steady-state emission calibration with langmuir probe will associate line ratio with thermodynamic vapor parameters (pressure and density)