# Presentation: 498298

## Vapor Condensation Study 

**for HIF Liquid Chambers**

- by
- Patrick Calderoni
- UCLA – Fusion Engineering Sciences
- 15th International Symposium on Heavy Ion Inertial Fusion
- Princeton Plasma Physics Laboratory
- Princeton, NJ, June 7-11, 2004

## 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

## 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 m3
- Initial n: 0.9x1018 #/cm3 (0.5x1018 #/cm3)
- Recovered n: 3x1013 #/cm3 (2x1015 #/cm3)

- 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 m2
- Surface from droplet injection: 1060 m2
- Ratio of surface area per unit mass of generated vapor: 785 cm2/g (4300 cm2/g for LiF and 10205 cm2/g for flibe)

- Droplet spray injection design:
- T = 843 K
- Spray flow rate = 2.4x103 kg/s
- (1.6% of main flow rate)

## 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

## Stage 1: Lexan with

- Argon background (1 Torr)

- time: 0

- time: 820 s

- time: 1640 s

- Typical vapor parameters in the source :

- n = 1019 - 1020 # / cm3
- 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

## Stage 2: Teflon and LiF in vacuum chamber

- expansion chamber
- volume: 4000 cm3
- surface area: 1720 cm2

- time of flight view ports

- vapor
- source

- current in

- current out

## Stage 3: Flibe vapor generation

- Cathode: 1⁄4’’ D 10’’ long W rod

- Flibe liquid pool: 1.6 cm3 volume

- Anode: Nickel crucible with embedded high density cartridge heater

- SS witness plates for SEM and EDX analysis

- Pressure sensor, water cooled (Tmax = 260 C)

- Total length: 34 cm

- Expansion volume: 400 cm3
- Surface area: 420 cm2

- Insulation: high-vacuum ceramic breaks

## High-speed camera frames sequence from flibe discharge 

- 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

## 300 C

- Pressure data and residual gas composition: flibe

- t300 = 4.27 ms (4.22 ms at 1.44 kJ)

- t500 = 6.58 ms

- H2

- 28,16 amu:
- hydrocarbons

- condensation is completed in 30 ms: no residual BeF2 traces at 47 amu

- 44 amu:
- CO2

**300 C**
**500 C**

## Comparison with HYLIFE-II chamber clearing models

- p(T) → n(T) assumes thermodynamic equilibrium with a liquid surface

- n = n0 x e-t/T
- n0 = 0.9x1018 #/cm3
- nend = 3x1013 #/cm3
- Clearing period for HYLIFE-II = 68 ms

## Measured composition of flibe vapors 

- 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

- H2

- hydrocarbons

- CO2

- BeF2

## 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

## 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

## 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

- 300 C

- 460 C

- Large drop is flibe

- side
- plate

- front
- plate

## 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 5x1017 cm-3 and 2x1015 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 1013 #/cm3 range

## Extra slides

## Discharge parameters

- Teflon I and V for 10 and 5 disk configuration

- LiF I for different energy experiments

- I for flibe experiments

## 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

## The molten salt flibe:

- 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:

## Flibe composition is a compromise between melting temperature (structural costs) and viscosity (pumping cost)

- 2 LiF + 1 BeF2
- in moles

## Flibe low vapor pressure is a key property for efficient driver coupling with the fusion target at high repetition rates

- In equilibrium with the liquid at 600 C, flibe vapor pressure is about 1 mTorr, corresponding to a density of 1.2 1013 cm-3

- Reference data for flibe vapors are based on the assumption of thermodynamic equilibrium at the liquid interface and ideal gas equation

## Weight loss measurements 

- Weight loss measurements

- 18 experimental runs with 10 Teflon disks as sleeve material
- 1 with 5 disks

- 5.76 kJ

-  2.54 kJ

- Volume ratio between source and chamber: 1:25

- Evidence of C soot deposition on source component

## Pressure data: Lexan and Teflon

- Dynamic sensor mounted at center of back plate (high noise from vibration)

- Teflon = CF2 chain
- Residual gases (20-70 Torr):
- 28 amu (C2H2 - C2H4 - C2H6)
- 69 amu (CF4)
- 20 amu (HF)

- about 5 times decay

## Jet velocity optical measurement system

- Diode axis separation:
- 7.62 cm
- Peak time delay:
- 6 microseconds
- Estimated initial vapor velocity:
- 10110 m/s

## 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

## Flibe experiments results

- Exponential decay fits data points good

- Decay time constant t1 is a measure of the condensation rate

- t1 = 4.22 ms

## Flibe experiments results

- Exponential decay fits data points good

- Decay time constant t1 is independent from initial particle density

- t1 = 4.27 ms

## 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)