Download Application of Field Emitter Arrays to Microwave Power Amplifiers

Application of Field Emitter Arrays to Microwave Power Amplifiers
D. R. Whaley, B. Gannon, C. R. Smith
Northrop Grumman Corporation, Defensive Systems Division
600 Hicks Rd., M/S H6402, Rolling Meadows, IL 60008-1098
email: david_whaley @ mail.northgrum.com Tel: (847) 259 9600 x6454
C. A. Spindt
SRI International
333 Ravenswood Ave., Menlo Park, CA 94025
email: capp_spindt @ sri.com Tel: (415) 859-5548
Field emitter arrays (FEAs) have potential for use as an electron source in a wide
variety of applications including microwave power amplifiers, flat panel displays,
electron microscopy, electron beam lithography, and space propulsion systems. Use of an
FEA as a cold cathode source for a microwave power amplifier such as the traveling wave
tube (TWT), presents significant technical challenges when one considers the high current
and high current density requirements coupled with the necessity for excellent beam
control. With the recent advances in FEA technology, FEA current densities as well as
total array currents have reached values meeting or exceeding many moderate power
TWT beam requirements and thus have become an attractive alternative to the thermionic
emitters currently used almost exclusively in such devices. One of the most attractive
characteristics of FEA emitters is that, by the nature of their emission process, modulation
of the electron beam at rf frequencies becomes possible. Interesting and previously
unattainable performance appears within reach with the advent of emission gating. Not
only does modulated operation offer exciting performance enhancements, many
advantages exist for unmodulated operation as well, an aspect of FEA cathodes
sometimes overlooked.
This work describes the operation of a field emitter array as the electron source of a
traveling wave tube amplifier. Issues of beam control and focus at high current density
and low magnetic field are addressed as well as issues relating to the inherent high
emittance of the FEA beam and cathode protection from ion bombardment. Large signal,
non-linear RF-modulated FEA-TWT interaction simulations show circuit efficiencies that
approach 50% even for minimal bunching of average-to-peak current ratios of 0.7 or
greater. RF modulation is predicted to significantly improve linearity in the high
efficiency regime as well as reduce harmonic power levels. An unmodulated C-Band
FEA-TWT was built to test the focusing approach as well as the robustness of these
emitters in an operating vacuum amplifier. The device uses a 1 mm diameter Spindt
emitter with a custom-designed electron gun and helix circuit. The FEA-TWT has
operated to-date with a maximum current of 91.4 mA and shows 99.5% transmission
under both drive and no-drive conditions. Output power of the device is 55.0 W at
4.5 GHz with a saturated gain of 23.4 dB and efficiency of 17%. During all operation, the
FEA emission appears extremely stable, with no temporal variations observed at any
time.
100
100
91.4 mA
80
beam current (mA)
90.9 mA
90
collector / helix current (mA))
90
70
60
50
40
30
20
10
80
collector
70
60
50
40
30
20
10
0
0
0
50
100
150
200
250
0
300
20
base-gate voltage (V)
40
60
80
100
total beam current (mA)
Figure 1. Measured FEA-TWT beam current as a
function of applied base-gate voltage.
Figure 2. Measured FEA-TWT helix and collector current
showing 99.5% transmission at highest current level.
60
0.25
55.0 W
design current
50
circuit efficiency
0.20
40
30
20
0.15
17.2 %
0.10
0.05
10
0.00
0
0
20
40
60
80
0
100
20
40
80
100
Figure 4. Measured FEA-TWT efficiency
variation with beam current.
Figure 3. Measured FEA-TWT output power
variation with beam current.
(
50
48
46
44
42
40
38
36
34
32
30
0
60
total beam current (mA)
total beam current (mA)
output power (dBm)
output power (W)
0.44 mA
helix
10
20
30
input power (dBm)
Figure 5. Measured FEA-TWT output power variation with input
drive power at Ibeam = 91.4 mA and 4.5 GHz.