# Presentation: 170347

## GRAPHITE SUBLIMATION TESTS withInert Gas Mitigation

**C. C. Tsai, T. A. Gabriel, J. R. Haines, and D. A. Rasmussen**
**Oak Ridge National Laboratory**
** **
**2****nd**** International High Power Targetry Workshop**
**October 10 , 2005**

## ACKNOWLEDGEMENT

**Thanks to :**
**T.J. McManamy, R. H. Goulding, and A. Fadnek for valuable advice on design and construction of test apparatus**
**D.O. Sparks for implementing the electronic and power supply for the test stand**
**S.C. Forrester for technical support to vacuum system and test set up**
**D. E. Schechter for advice on cover gas technique **

## ORNL Proposed Passively Cooled Graphite Target for Neutrino Factories

- Graphite Rod Target
- (15 mm diameter, 800 mm long)

- Water-cooled  stainless steel support tube

- Graphite support spikes

## Motivation for Graphite Sublimation Tests

**A radiatively cooled graphite target was proposed as a candidate for a neutrino factory target **
- Radiation cooled design is very robust mechanically, but loss of material by sublimation will limit the power
**We proposed the use of Helium Cover Gas to greatly reduce the net erosion rate **
- Net erosion of graphite is limited by near surface interactions between sublimated carbon and He (mean-free-path ~ 1 μm), which leads to redeposition of sublimated graphite
**Graphite sublimation tests are being conducted in an attempt to better establish limits for a vacuum environment and validate the use of helium to suppress sublimation**

## Surface Temperatures Exceed 2000K for Radiatively Cooled Graphite Targets

## Test data and theoretical predictions of graphite coupon sublimation  

## Target Lifetime of One Month Is Possible at Power Levels of 1.2 MW

-   This lifetime is based on data from previous tests under vacuum conditions
-   Hopefully, net erosion rate can be reduced/lifetime extended with He cover gas

## Status of graphite sublimation tests

**Vacuum tests on small coupons successfully conducted, but we experienced arcing damage with helium gas**
**New tests have been conducted with target rod prototype**
**Joule heating at power densities equivalent to beam deposition**
**Requires high current (~ 1 kA)**
**Use existing facility in ORNL Fusion Energy Division**
**Test specimen**
**15 mm diameter, 300 mm long**
**Clamped at both ends for current feed**
**Includes water-cooled panel coil shroud and extensive heat shields**

- Small coupons used in previous tests

- Rod specimen that will be used for new tests

## Slide 9

## Water cooled

## Slide 11

## Slide 12

## Heat loss through Cu current feedlines is acceptable - Sublimation rate > 70% of theoretical

- 320 MW/m3 (corresponds to 1 MW beam with factor of 2 peak-to-average)
- 15 mm diameter
- 300 mm long
- 0.8 kA

- Expected mass loss = 650 mg/day

## New sublimation Test Apparatus Was Commissioned in Dec ‘04

## TEST PROCEDURE

**Heat up a new graphite rod (GR) under vacuum**
**Measure Vh / Ih (or resistance) and temperature of GR and use the recorded data to guide subsequent tests **
**Measure initial GR mass in an electronic balance after removing the GR from 1-bar He test chamber**
**Bake GR at 2000 K for 3 hours under vacuum**
**Conduct GR sublimation test at desired temperature (e.g. 2350 K) for a long period of time (5 to 20 hours) under VACUUM or 1-bar HELIUM (Flowing or Static)**
**Wait overnight for GR to cool down and fill the test chamber with 1-bar helium**
**Remove and measure GR mass to estimate sublimation mass loss rate     **

## Significant Test Results

- The high temperature oven can be operated continuously under vacuum or 1-bar inert cover gas such as helium, Argon, etc.
- GR Resistance character is similar in vacuum or in cover gas.
- The heating power in 1-bar inert cover gas is slightly higher than that under vacuum
- GR mass loss rate (mass loss in mg / test duration in hour) in 1-BAR HELIUM cover gas is lower than that under VACCUM conditions
- Argon cover gas works as well as helium cover gas for mitigating the GR sublimation erosion; measured mass loss rate is 1.7 mg/h for Ar and 2.56 mg/h for He.
- The following observed features could be due to gas impurity
- The mass loss rate in the static helium cover gas lower than that in the flowing helium cover gas.
- The mass loss rate decreases with the increase of the test duration under the static helium cover gas.

## Theoretical Curve of Graphite Rod with Test Data

## Graphite Rod Sublimation Test Parameters

## Static Helium Cover Gas Mitigates Sublimation of Graphite Rod To 1/30 Of That Under Vacuum

**Average mass loss rate of 2.56 mg /h for a test duration of 5 hours**
**Average mass loss rate of 0.78 mg/h for a test duration of 20 hours**
**If the test conditions are identical for the above 2 cases,  the average mass loss rate for the last 15 hours of the 2nd case is 0.19 mg/h**
**Ratio of 0.19 mg/h to 5.82 mg/h (mass loss rate under vacuum) is 1/30**** **

## Effective Sublimation Erosion Rate Measured Could be 0.025 g/h/m2

**Graphite Rod (GR): 305 mm Long & 15.88 mm in Diameter**
**38 mm long at each end of the GR was mounted to GR terminal.**
** ****~38 mm long at each end of GR yields insignificant sublimation.**
**Effective length of the GR center section contributing to sublimation erosion is ~153 mm.**
**Effective surface area of the GR is 76.3 cm**2**.**
**Effective sublimation erosion rate of the GR, equivalent to a mass loss rate of 5.82 mg/h under vacuum conditions, is 0.77 g/h/m**2**.**
**If the test conditions for the 2 static helium cases are identical, the mass loss rate during the last 15 hours could be 0.19 mg/h or the effective sublimation erosion rate to be 0.025 g/h/m**2**. **

## Concluding Remarks

**Graphite sublimation tests have demonstrated and proven that 1-bar inert cover gas could substantially mitigate graphite sublimation erosion rate**
**Gas impurity increased the average sublimation erosion loss rate **
**Additional tests with pure inert cover gas should yield better data for designing high  power and long life targets**