1. Wolfgang Anders, BESSY, Berlin
CW Tests of TESLA cavities and RF-sources at the HoBiCaT Test facility at BESSY
Wolfgang Anders, BESSY, Berlin
BESSY FEL Layout
HoBiCaT test facility
CW Modules
HOM problems
Microphonics
CW coupler tests
CW transmitter

2. BESSY FEL Layout Full Energy 2.3 GeV Duty Factor CW
BC1
(200 A)
RF Photoinjector (65 A)
BC2
(2 kA) L2
753 MeV
EXT1 L3
1020 MeV
FEL1
Arc (240 A)
2300 MeV L4
FEL2
FEL3
EXT2
Collimators
L1 + Booster (L0)
3rd harmonic cavity
219 MeV
Full Energy
2.3 GeV
Duty Factor CW
Number of cavities per module 8
Number of modules 18
Accelerating field
16 MV/m (< 20 MV/m)
Operating temperature
1.8 K
Cavity quality (Q0)
> 1.3 x 1010 or higher
Dynamic losses at 1.8 K per cavity
< 30 W
Dynamic losses at 1.8 K per module
< 151 W
Total dynamic losses at 1.8 K
< 3 kW

3. HoBiCaT Cavity Test Facility
The HoBiCaT Test facility include all things you need for measurements of cw-related topics of TESLA type cavities:
Cryogenics K
Cryostat for two cavities
Cavities
Tuner
CW Transmitter
Low level electronics

4. Cw Bottelnecks in LHe tank
Bottelnecks in the LHe tank
ca. 80 cm
DT(0.5 W/cm2) = 8.8 mK
T = K, Tsat = K
0.5 W/cm2
DT = 2 mK
1.8 K, mbar
Want to avoid boiling in He-II, otherwise danger of microphonics
Rule of thumb: Heat flux in 1.8 K He-II must be less than 1 W/cm2 to avoid boiling
To be verified experimentally in HoBiCaT

5. Microphonics vs. Heater power
Microphonic detuning (sigma Hz) versus heater power of a foil heater on the Cavity vessel
O. Kugeler

6. Cryogenics versus heater power

7. TESLA  CW Modul TESLA: 10 modules (string) form one string
All supply and return lines span the entire string
Supply by one JT valve
Total dissipated power per string = 150 W ~ 1 CW Module
 Should have one supply per CW module
TESLA: One conn. between 2-phase line and GRT / module (15 W)
 Flow in 2-phase line more „relaxed“ than in CW Module
 2-phase line should be as big as possible
 Add connections
TESLA: GRT handles gas from 16 strings = 2.4 kW
 Size of GRT should be OK for CW operation

8. BESSY CW-Module Layout
Enlarged chimney to 90 mm ID
Reservoir with heater and level meter.
Heater likely to be distributed along module
Enlarged 2-phase supply line to 100 mm ID
No connection between modules
Additional connection
GRP unchanged

9. Pressure Drop
Helium plant in the middle of the linac  pressure drop on GRP low and vapor velocity well below 4 m/s
Section 4
Section 3
Section 2
Section 1
EXT1
Arc+BC2
BC1
Helium plant VB DB
Modules 1-8
9-18

10. HOM damper problems  We are working on the problem !!!
Slight heating of HOM
Q-switches seen MV/m
Test with saphire feedthrough from DESY/JLab
Measurements extremely preliminary  show no data
Next steps:
Instrumentation
Improve cooling
Determine source of heat
Without cable
cold intercept in cable
More ideas ???
 We are working on the problem !!!

11. Microphonics measurements at HoBiCaT
Microphonics compensation for 9-cell superconducting TESLA cavities using a Piezo-tuning system:
PLL based measurement setup
Piezo-tuning system included into the TTF-Saclay-I tuner
design
O. Kugeler, J. Knobloch, A. Neumann

12. Microphonics measurements at HoBiCaT
Microphonics spectrum: LHe pressure fluctuations, mechanical resonances
Mech. resonance
<1Hz due to LHe pressure
Example of integrated microphonics measured at 1.8 K
f1/2=21.6 Hz
Microphonics spectrum at 1.8 K without compensation
O. Kugeler, J. Knobloch, A. Neumann

13. Microphonics compensation
Feedforward and feedback compensation tested:
W. Anders, O. Kugeler, J. Knobloch, A. Neumann

14. Warm coupler mounted on HoBiCaT flange
CW TTF III coupler test
Inner conductor bellows
Test the coupler at room temperature on test stand while measuring the temperature of ceramic windows (IR sensor) and inner conductor (PT 1000)
Critical component is the inner-conductor bellow
test under cryogenic (standard operating) conditions
Mount coupler test stand in HoBiCaT Test Facility and operate at LN2 temperature
Standing wave and travelling wave tests
Test with and without (normal) air cooling of inner conductor
Example of the temperature rise as a function of time as RF power is applied.
Warm coupler mounted on HoBiCaT flange

15. CW coupler test Thermal time constant is very long (50 mins)
Results and Conclusions
Temperature rise per kW of applied power at the PT1000 sensor in the inner conductor
All tests in HoBiCaT except the one marked „on the test stand.“
Thermal time constant is very long (50 mins)
Inner conductor bellows region becomes very hot. We imposed a limit of 300 C, but outgasing limits to about 180 C.
Interlock trips due to vacuum and reflected power limited the tests (but these are not a fundamental limit).
Extrapolation: 10 kW TW and 5 kW SW operation with existing coupler should be possible.
No significant difference seen between warm and cold tests  Cold window ceramic is not the limiting item for heat conduction
Tests up to 10 kW (klystron limited) were also done with the inner conductor air cooled. If the results are extrapolated, 25 kW SW operation is feasible

16. Klystron IOT
The Efficiency of a klystron and IOT at peak power is comparable, but in the typical operating range the IOT has much better efficiency due to the class B operation mode.

17. Installed RF Transmitters at HoBiCaT
CW Transmitter Development for the BESSY FEL
Development steps for 1.3 GHz tranmitter
IOT Prototype
Power supply
10 kW Klystron
10 kW klystron transmitter
10/2004
Stability tests of power supply with IOT protoype at reduced power level
04/2005
Optimized IOT transmitter at
full power
12/2006
Price optimized prototype for series production
8/2007
High precision low level electronics using FPGA micro processor technique
in test phase
Installed RF Transmitters at HoBiCaT

18. Conclusion
HoBiCaT is running producing many results on cw-operation mode related issues of TESLA cavities.
Measurements are ongoing. We are open for questions on cw-related issues. Suggestions are welcome!
Please contact our team: