High Current Density and Long Life Nanocomposite Scandate Dispenser Cathode

Fabrication

High Current Density and Long Life Nanocomposite Scandate Dispenser Cathode

Fabrication

10th International Vacuum Electronics Conference (IVEC2009)

April 28 - 30th 2009

10th International Vacuum Electronics Conference (IVEC2009)

April 28 - 30th 2009

(Session 23 - Cathodes II )

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 LiBeijing Vacuum Electronics Research Institute, Beijing, China

13:40 Thursday, 30 April 2009

Why W-Sc Nanopowder Cathodes ?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:30 -100+ 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

Gyro-TWT Performance Dependence on Beam Quality

For predicted velocity spread vz/vz = 5%

-Bandwidth /= 5%

- Pout= 110 kW

- = 22%

- Large signal gain = 45 dB

• Nonlinear large signal code predicts output power, efficiency and gain

Impact of Cathode Roughness on MIG* Beam Velocity Spread

•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.

W-Sc Nanopowder

Conventional Scandate Cathode

• 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

BVERI-BJUT W-Sc Nanopowder Cathode ConceptBVERI-BJUT W-Sc Nanopowder Cathode Concept

500 hours achieved at 100 A/cm2 in tests at SLAC

Data from Beijing Vacuum

Electronics Research

Institute (BVERI) & Beijing

University of Technology

(BJUT)

Nano Composite Cathode Fabrication—UCD

1. Nano Sc2O3-doped W powder Fabrication

Optimize Nano composite Sc2O3-doped W matrix

Sol-Gel Process

Nano Sc2O3-added W Powder Fabrication—UCD

40 50 60 70 80 90 1000

5

10

15

20

25

Particle Size (nm)

Fre

qu

ency

Co

un

t (

%)

nm 7.106.71

SEM Results

Sc2O3-added W powders

Nano Sc2O3-added W Powder Fabrication—UCD

SEM Results

Sc2O3-added W powders

Nano Sc2O3-added W Powder Fabrication—UCD

SEM Results

Sc2O3-added W powders

400 500 600 700 8000

5

10

15

20

25

Fre

qu

ency

Co

un

t (

%)

Particle Size (nm)

nm 27.8468.586

Nano Sc2O3-added W Powder Fabrication—UCD

SEM Results

Sc2O3-added W powders

Nano Sc2O3-added W Powder Fabrication—UCD

SEM Results

Sc2O3-added W powders

Nano Sc2O3-added W Powder Fabrication—UCD

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.

Conclusions:

Sc2O3-added W Matrix — UCD

By using 72 nm initial nano powder

After Regular Furnace Sinter

Grain size in matrix is 400-500 nmPore size ~ 400 nm

Cross Section

Top Surface

Sc2O3-added W Matrix — UCD

By using 272 nm initial nano powder

After Regular Furnace Sinter

Grain size in matrix is 500-600 nmPore size ~ 400 nmCross Section

Top Surface

Sc2O3-added W Matrix — UCD

By using 587 nm initial nano powder

After Regular Furnace Sinter

Grain size in matrix is 1 – 2 µmPore size ~ 0.5 µm

Cross Section

Top Surface

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

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

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

Cathode Testing at BVERI Current Density vs Cathode Button Voltage

J = 73.56 A/cm2 , 1000 °C

Using UC Davis Material

Cathode Testing at UC DavisCathode Testing at UC Davis

Cathode Testing

Rapid button testMultiple rapid cathode life test facility

New High PerveanceCathode Life Test Vehicle

System operational

Three 3.0 P CLTVs completed

Cathode testing and life testing underway at UC Davis: eight vehicles operational with another four nearing completion

G. Scheitrum and A. Hasse

Cathode Testing at UC DavisCathode Testing at UC Davis

Current Density versus Cathode Button Voltage

Comparison: Spectra-mat 311X 20 A/cm2 at 1150 °CUCD cathode: 80 A/cm2 fully space charge limited

Cathode Testing at UCD

Current Density vs Cathode Button Voltage

UC Davis Pellet impregnated by Spectra-Mat

100 1000

1

10

100C

urr

en

t D

en

sit

y (A

/cm

2 )

Voltage (V)

UCD-#1 1050 1.33 56.5 UCD-#1 1000 1.33 17.5 UCD-#1 950 1.33 5.40

Type T (oC) Slope J(A/cm2)

Interpretation of Cathode Testing Results at UC Davis

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.

Interpretation of Cathode Testing Results at UC Davis

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)

Cathode Testing SummaryCathode Testing SummarySummary 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 Current:-3 kA-Pressure:--3 kN- 50 kNAtmosphere:--Vacuum

Current:-3 kA-Pressure:--3 kN- 50 kNAtmosphere:--VacuumSpark Plasma SinteringSumitomo SPS-2050

Thank You Thank You

Work supported by:Work supported by:

•AFOSR under Grant F9550-99-04-1-0353 (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.

TE01 Gyro-TWT Dispersion Diagram

• Beam mode dispersion: sc + kzvz

Wave mode dispersion: c+c2kz

2

• Absolute instabilities must be stabilized : TE11

(1), TE21(1), TE02

(2) ,TE01(1)

s c +

k zv z

s c +

k zv z

s = 1

s = 2

kz(/m)

50

100

150

200

0-4000 4000

/

2(

GH

z)

TETE1111(1)(1)

TE21(1)

TE01(1)

TE02(2)

operating point(grazing intersection)

Potential Gyro-BWO interaction

s=1

s=2

100 kV, =1.0

27/34