1. JLab Work on Low Temperature Near-Surface Diffusion aka “Infusion”
High Q, High Gradient
SRF cavity Nb Ti N2
Pashupati Dhakal
Wednesday, August 07, 2019

2. Let’s Re(Search), Before 2012/13
Low temperature baking (LTB) was the final step before rf test primarily to cure high field Q-slope.
It results in increase in residual resistance while reducing BCS surface resistance, no significant benefit at medium gradient.
No unanimous model is available but its linked to “oxygen diffusion”, “near surface hydrogen reduction”, “nano-hydride precipitations”, “vortices” and etc………
R. Geng

3. Let’s Re(Search), After 2013
Increase in Q0 in medium field range (<25 MV/m) with increasing RF field, due to near surface material diffusion (Ti and N2).
Limited theoretical work to explain the Q-rise phenomenon, but the available data and models suggest that the effect is due to decrease in BCS surface resistance with RF field.
The idea of increasing Q0 at higher field reported by FNAL and reproduced in some labs with low temperature baking in nitrogen (without nitrogen
With some “Recipes” of LTB in nitrogen, the Q-rise phenomenon were observed in medium field and quench field is extended to higher gradient.

4. Earlier Results “LTB with N2”
1.5 GHz single-cell fine-grain Nb cavity (RRR>300)
N-infusion: 800 °C/3h, N2 at ~25 mTorr at 290°C, cooling to hold temperature maintained for 48 h
P. Dhakal et al., Phys. Rev. Accel. Beams. 21, (2018)

5. Able to get some high Q with high Eacc
Earlier Results “LTB with N2” LG LG FG
Temperature measured at inside wall of furnace
The higher gas injection temperature
Cavity surface temperature unknown
No active RGA monitoring during the gas injection
Able to get some high Q with high Eacc

6. Furnace Upgrade Ari Palczewski Thermocouple attach to cavity surface
RGA monitoring during gas injection

7. In the mean time…..
75C magic bake reported during last TTC at Reiken from FNAL
Baseline-EP
+75C/4hrs+120C/48hrs
Typical baked cavity !

8. In the mean time…..
75C magic bake reported during last TTC at Reiken from FNAL
Baseline-EP
+75C/4hrs+120C/110hrs
Typical baked cavity !

9. Back to “LTB with N2”

10. Back to “LTB with N2” Previously with same recipe
Next: Health evaluation of furnace and components

11. Back to “LTB with N2”
No degradation in Eacc and small Q degradation above 25 MV/m could be due to some MP/FE activities

12. Summary
Earlier “Infusion” were reproducible when gas injected at higher temperature.
~75C hold for 4 hours during 120 C baking didn’t appear to be beneficial over the conventional 120C bake (statistics of 2 rf test).
Furnace itself appeared to be clean after the recent upgrade evident from the rf (single cavity) test.
First infusion run after furnace upgrade wasn’t as expected, probably due leak/contamination to gas injection line.
Work in progress to prepare some baseline cavities and infusion run once the furnace issue resolved.