1. (Session 23 - Cathodes II )
13:40 Thursday, 30 April 2009
High Current Density and Long Life Nanocomposite Scandate Dispenser Cathode Fabrication
Jinfeng Zhao, Larry Barnett, and Neville C. Luhmann Jr.
Department of Applied Science, University of California-Davis (UCD), CA 95616, USA
Na Li and Ji Li
Beijing Vacuum Electronics Research Institute, Beijing, China
10th International Vacuum Electronics Conference (IVEC2009)
April th 2009

2. Why W-Sc Nanopowder Cathodes ?
Millimeter/THz sources require reduced cathode area and low to moderate beam
compression →high current density fully space charge limited operation: A/cm2
Gyro-TWTs at W-Band and beyond require smooth surfaces for reduced velocity
spread (proportional to roughness(1/2)), uniform emission, and high current density:
40+ A/cm2
End of tube life often due to barium deposits leading to arcing as well as Ba depletion
→low temp. operation critical (lifetime improvement by x4 for each 50 °C reduction)
Long lived HPM cathodes for conditioning and rep rated operation: 100+ A/cm2

3. Gyro-TWT Performance Dependence on Beam Quality
Nonlinear large signal code predicts output power, efficiency and gain
For predicted velocity spread Dvz/vz = 5%
-Bandwidth Dw/w = 5%
- Pout= 110 kW
- h = 22%
- Large signal gain = 45 dB

4. Impact of Cathode Roughness on MIG* Beam Velocity Spread
W-Sc Nanopowder
Conventional Scandate
Cathode
New UCD MIG Design: 50 degree cathode with compression of only 12 (Bgun=3.0 kG).
EGUN gives 3.9% axial spread and 1.7% transverse spread.

5. BVERI-BJUT W-Sc Nanopowder Cathode Concept
Benefit of adding scandia into nano tungsten powder by chemical synthesis
Improve emission uniformity.
Increase emission capability.
Resist ion bombardment.
Nano sized scandia-doped tungsten powder
Combined by aqueous solution method (Liquid-solid or Liquid-liquid )
Spherical tungsten grains
Scandium oxide absorb on the surface of spherical tungsten grains.
Sc-W powders with more uniformly distributed Sc2O3 were obtained
Space charge limited
current densities of more
than 30 A/cm2 at 850 oCb achieved
500 hours achieved at 100 A/cm2 in tests at SLAC
Data from Beijing Vacuum Electronics Research Institute (BVERI) & Beijing University of Technology (BJUT)

6. 1. Nano Sc2O3-doped W powder Fabrication
Nano Composite Cathode Fabrication—UCD
Optimize Nano composite Sc2O3-doped W matrix
1. Nano Sc2O3-doped W powder Fabrication
Sol-Gel Process

7. Nano Sc2O3-added W Powder Fabrication—UCD
Sc2O3-added W powders
SEM Results

8. Nano Sc2O3-added W Powder Fabrication—UCD
Sc2O3-added W powders
SEM Results

9. Nano Sc2O3-added W Powder Fabrication—UCD
Sc2O3-added W powders
SEM Results

10. Nano Sc2O3-added W Powder Fabrication—UCD
Sc2O3-added W powders
SEM Results

11. Nano Sc2O3-added W Powder Fabrication—UCD
Sc2O3-added W powders
SEM Results

12. Nano Sc2O3-added W Powder Fabrication—UCD
Conclusions:
Uniform Nano Sc2O3-doped W powder has been made with different particle size, such as average particle size around 72 nm, 146 nm, 272 nm, and 614 nm, respectively.

13. Sc2O3-added W Matrix — UCD
Top Surface
By using 72 nm initial nano powder
After Regular Furnace Sinter
Grain size in matrix is nm
Pore size ~ 400 nm
Cross Section

14. Sc2O3-added W Matrix — UCD
Top Surface
By using 272 nm initial nano powder
After Regular Furnace Sinter
Grain size in matrix is nm
Pore size ~ 400 nm
Cross Section

15. Sc2O3-added W Matrix — UCD
Top Surface
By using 587 nm initial nano powder
After Regular Furnace Sinter
Grain size in matrix is 1 – 2 µm
Pore size ~ 0.5 µm
Cross Section

16. Sc2O3-added W Cathode — UCD
SEM images on the top surface of UCD cathode
After Furnace Sinter: Average grain size in cathode is 600 nm and very uniform

17. Sc2O3-added W Cathode — UCD
SEM images on the top surface of UCD cathode
After Furnace Sinter: Average grain size in cathode is 700 nm and very uniform

18. Sc2O3-added W Cathode — UCD
SEM images on the top surface of UCD cathode
After Furnace Sinter: Average grain size in cathode is 900 nm and very uniform

19. Cathode Testing at BVERI
Current Density vs Cathode Button Voltage
J = A/cm2 , 1000 °C
Using UC Davis Material

20. Cathode Testing at UC Davis
Multiple rapid cathode life test
facility
New High Perveance
Cathode Life Test Vehicle
Rapid button test
System operational
Cathode testing and life testing underway at UC Davis: eight vehicles operational with another
four nearing completion
G. Scheitrum and A. Hasse
Three 3.0 P CLTVs completed

21. Cathode Testing at UC Davis
Current Density versus Cathode Button Voltage
Comparison:
Spectra-mat 311X 20 A/cm2 at 1150 °C
UCD cathode: 80 A/cm2 fully space charge limited

22. Cathode Testing at UCD Current Density vs Cathode Button Voltage
UC Davis Pellet impregnated by Spectra-Mat

23. Interpretation of Cathode Testing Results at UC Davis
The 1150o C data increases to 40 A/cm2 and drops slightly from the SCL line (slope of 1.5),then continues to 80 A/cm2 at a slope of 1.5 (full space charge limited).
Interpreted as field emission in the high current range where the deviation from the ideal SCL (zero extraction voltage) is not a drop in emission current capability, but a drop in the actual space charge limited current due to the required extraction field.

24. Interpretation of Cathode Testing Results at UC Davis
Speculate that field emission points are formed:
The optimum point formation, density of points and/or sharpness of the points, is at ~1150o Cb
The initial activation is very fast at ~1150o Cb and minimal at < 1100o Cb
Assuming the point density is proportional to the number of grains leads to the conclusion that smaller grains will have more points and the cathode will have higher emission density up to the limit that it starts decreasing the field on each point and limiting total emission. Hence, there is a maximum grain-point density (under study)

25. Cathode Testing Summary Spark Plasma Sintering
Summary of cathode testing:
80 A/cm2 fully space charge limited current density has been obtained at 1150 oCb
Cathode life testing has been at 1150 oCb for 800 h following 768 h at 950 oCb
Cathode made by smaller particles had 50/cm2 emission at 1050 oCb
Future Plans for cathode testing:
Investigate performance of cathodes made by different initial Sc2O3-W nanopowder average particle sizes: 100, 300, 500 nm, etc.
Cathodes made with different porosities
Cathodes made with different Sc2O3 concentrations
Investigate robustness with respect to reactivation
Conduct life tests in new 3.0 P CLTV’s
Spark plasma sintering
Sumitomo SPS-2050
Spark Plasma Sintering

26. Thank You Work supported by:
AFOSR under Grant F (MURI04 “NanoPhysics of Electron Dynamics near Surfaces in High Power Microwave Devices and Systems”)
NSWC Crane
HiFIVE DARPA, Contract No. G8U543366, through a subcontract from Teledyne Scientific.

27. TE01 Gyro-TWT Dispersion Diagram
27/34
TE01 Gyro-TWT Dispersion Diagram
w = sWc + kzvz
s = 1
s = 2
kz(/m) 50
100
150
200
-4000
4000
w/2p (GHz)
TE11(1)
TE21(1)
TE01(1)
TE02(2)
operating point
(grazing intersection)
Potential Gyro-BWO
interaction
s=1
s=2
100 kV, a=1.0
Beam mode dispersion: w = sWc + kzvz
Wave mode dispersion: w2 = wc2 +c2kz2
Absolute instabilities must be stabilized
: TE11(1), TE21(1), TE02(2) ,TE01(1)