1. Cryogenic Heater Controls in C100 Cryomodules
T. Powers
(With data stolen from, Ed Daly, and other)

2. Why Do We Need Heater Compensation?
The CTF has large motors, expanders, valves, etc. In most of these devices the speed or position can not be changed rapidly.
The gain on the cryomodule JT-Valve loop controls are set such the loops are slightly underdamped, in order to reduce the steady state error, and they are operated in a slow manner in order to reduce overshoot. (i.e. dramatic changes in heat load may cause liquid helium to spill out into the return line or for the level to go below the set point and trip the zone off.
Thus the cryogenic system is best operated with a steady state load.
We do this by adjusting the resistive heat in each cryomodule to compensate for the cryogenic losses in the cavity.
Resistive_Heat = Requested_Heat – E2 / Q0(r/Q)/L)
Where
Requested_Heat = Expected_RF_Heat + Swing_Heat
Where Swing_Heat is a overall compensation heat that is determined by the cryogenics operations group.

3. What is Heat Riser Choke
As heat is transferred through the pipe the temperature difference between the input and outlet will increase. For superfluid helium when the entrance temperature reaches Tλ (2.18K) the helium will no longer be superfluid and it will boil. The expanding gas will cause a pressure transient at both ends of the pipe.
The pressure transient is sufficient to detune the offending cavity and potentially other cavities within the cryomodule to the point where there is insufficient RF power to operate it in generator driven resonator (GDR) mode. The control system generally indicates such a trip as a quench.
Another byproduct of this phenomena is that the liquid level will appear to oscillate a few percent for a few minutes after this occurs. I suspect the oscillation is a “feature” of the sensor electronics.
Details regarding this are described in chapter 5 of Helium Cryogenics by S.W. Van Sciver.
I first observed this in 2003 when we were commissioning SL21 which was the first prototype Cryomodule.

4. SL21 Piping Helium riser pipe. 7 cm long, 4.2 cm dia for SL21
7cm long x 8.2cm for C100
(C100 length should be verified)

5. Generalized Limiting heat flux
Note the strong dependence on temperature as one approaches Tλ. Also note that this is the best case scenario.
Van Sciver p145

6. Example of heater choke test demonstrating the liquid level indication of a heat riser choke test in FEL3.

7. SL21/FEL3 results
Basically at the current operating pressure FEL3 is limited to about 14W per cavity.
We operated that zone in the FEL about 0.2 MV/m below the threshold where it would choke for about 2 years.
Minor pressure oscillations (1 mBar over 30 min) that usually occurred in the summer would trip us off on a regular basis.
𝑇ℎ𝑒𝑜𝑟𝑖𝑡𝑖𝑐𝑎𝑙 𝑃𝑜𝑤𝑒𝑟 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦= 𝑞 𝐿 1 𝑋 𝐴 𝐿 1 𝑋 = 𝑞 𝐿 1 𝑋 𝜋 𝑟 2 𝐿 1 𝑥

9. Some C100 Results
Ed Daly results –
With the refrigerator at normal CEBAF pressures, bar (2.07K) at the T, The RT valve was opened beyond the normal CEBAF set point.  Liquid level at 92%.
Saw boiling onset at ~290 watts equally powered between 8 cavities.  
Reduced to 270 watts for extended run, 33.8 watts per cavity.   
We then explored individual limits on cavities as we lowered the total power. 
With 21.6 watt on 7 cavities the limit was around 48 watts on the 8th cavity, total of 199 watts. 
With 31.6 watt on 7 cavities the limit was around 41 watts on the 8th cavity, total of 262 watts.  
This also seemed to be a function of the cavity position, so cavity 1 had a lower individual limit than cavity 8.  Possibly within measurement error.

10. Some C100 Results
When we lowered the pressure at the “T“ to bar (2.05K)
Boiling onset around 315 watts equally powered between 8 cavities
Did an extended run with a total of 300 watts (37.5 w per cavity) vs 270 above.
Increase in pressure in the end cans was ~ ATM from 0 to 300 watts 
(more Ed Daly words)
Clyde Mounts recently ran the R100 heaters equally powered between 8 cavities and found that there was boiling at 240 W which is 30W per helium vessel.

11. R100 Results (From Monday)
Operating all cavities at 17 MV/m (about 20W per cavity) except 7 which was at 10 MV/m.
Heaters were on at 70W which added about 9W per cavity.
When cavity 7 was turned up much above 10 MV/m the zone tripped off due to the helium boiling.
The helium pressure gauge was replaced with a new in the box unit and was found to be reading 42.7 mBar or 2.11K.

12. Individual Heater Controls
When SL21 was installed in the FEL we needed to get all of the energy out of the cryomodule as possible.
One of the problems that we found when we had 8 heaters powered off of one supply is that if one or more cavities was below the maximum gradient, the heaters on all of the cavities were turned on to compensate. This meant that we would have the RF heat (which was pushing the limits) plus the heater heat which pushed us over the threshold and we had boiling.
The fix for FEL3.
Implemented individual heater controls for each cavity
Operated with expected RF Heat set about 20% below the drive high RF heat.
Disabled swing heater function for that zone.

13. Considerations for C100/R100
Install individual heaters for each helium vessel.
Adjust the heater algorithm in a manner similar to FEL3.
Train operators to watch for helium level oscillations due to helium boiling events when they start getting anomalous “quenches”.
Develop a software watch dog tool that is set up to recognize cavities that trip off due to helium boiling.
Consider a test plan to understand the improved gradients that can be achieved by going to a lower pressure.
Implement the changes to the C100 RT valves to allow greater flow.
Weigh the tradeoffs of operating costs, wear and tear on equipment and overall CHL capacity limitations as a function of pressure to see if it is possible to gain in energy by lowering the helium pressure.