Microwave Field Emission
E. Tanabe, A. McEuen*, M. Trail*, G. Meddaugh* and S. Bandy•ホAET Associates, Inc”
20370 Town Center Lane, Cupertino, California 95014* Varian Associates, Inc.
911 Hansen Way, Palo Alto, California 94303
ABSTRACTA demountable S-band single-cavity test fixture
designed to study the influence of surface materials
and processing techniques on microwave field-emission is described. Results for polycrystalline and sin­gle crystal copper (machined/polished and electrodischarge machined) as well as molybdenum samples
are presented.
INTRODUCTIONMicrowave field-emission is detrimental to highgradient accelerator operation, since it can absorb asignificant portion of the available rf power as well as
produce unwanted beam and x-ray radiation.
Moreover, it may induce voltage breakdown which
can interrupt operation and possibly damage thestructure. On the other hand, controlled microwave
field-emission from a metallic cold-cathode would
make possible a high-current density, low-emittanceelectron-beam source for application in high-power
tubes, accelerators and microwave switches. Interestin both these aspects has prompted efforts to under­
stand and develop techniques to control microwave
field-emission (REF: 1,2,3).
This paper describes a demountable, single cavity
field-emission tester which was developed and tested
to study the macroscopic (geometric) field-enhancement factor (3 and the dependence of microwave
field-emission on material, surface finishes, tempera­ture, pulse-width, repetition rate, pressure level, rf
processing, poisoning agents and cavity surface
treatment (including coatings of various materials).
The modified Fowler-Nordheim plots obtained arepresented and discussed. A limited number of results
are available at the present time. The program, how­ever, is continuing and further results will be reported
in future papers.
EXPERIM ENTAL SET-UP AND PROCEDURE
Measurements were made in a test fixture
consisting of one-half of a typical high-gradientaccelerator cavity, shown in Figure 1• Field emissionof electrons occurs from the outside radius of thenose surrounding the beam hole, where the highest

Cooling

Coupling

Matching

Figure 1 - Schematic of Field-Emission Test Cavity.

voltage gradients exist. Therefore, in this cavity thenose was made removable, so that the effects ofmaterial and surface finish could be determined. The
test cavity was clamped to a flat shorting plate atwhat would be the central plane of symmetry o f a fullaccelerator cavity. The distance from the nose tip to
the shorting plate is 1.85 cm. Both cavity andshorting plate were made of oxygen-free high-conductivity (OFHC) copper and were cooled bycirculating water. A gasket of annealed copper wasused as a vacuum and rf seal at their junction. The
field-emission current collector was placed behind a
0.2 cm diameter, radiused hole in the shorting plate
directly opposite the nose. RF power was introduced
into the cavity through a smoothly radiused couplingiris near the periphery, matched to standard WR284
waveguide by a two-step quarter-wave transformer.
The test system was isolated from the pressurized
input waveguide by a ceramic disk window and wasmaintained at a pressure level of
10 7 torr by ion pumps in the input waveguide andthrough the center hole of the cavity nose. For eachexperiment the test nose, a new gasket, the cavity,
and the surface of the shorting plate were carefully
degreased and rinsed with acetone and deionizedwater. The parts were then assembled and pumped
down to the 10_フtorr level, which was maintained
during a 12-hour bake-out at 200°C.

EXPERIM EN TA L RESULTS
Figure 3 shows the typical variation o f measured(time-averaged) field-emission current with the peakrf surface field (Eo) on a machined, diamond-pol­ished OFHC nose. The threshold level of Eo for initi­
ating field-emission in an S-band cavity is about
lOOMV/m, while the breakdown threshold level isabout 250MV/m. No hysteresis was observed as Eo
was increased and decreased.
Figure 4 shows the detected field-emission current
and rf field waveforms within the test cavity. The
field-emission current rises slowly to its maximum
value (1.5|iisec), but falls o ff very rapidly (0.6|isec).
Figure 2 - Field-Emission Tester RF System and Instrumenta­tion Schematic.

With each nose installed, the room-temperature
low-power cavity resonant frequency (〜2996 MHz),input VSWR (typically <1.1:1), and cold Q0 (9,50010,000) were measured. High-power tests wereperformed with the equipment set-up shown in Figure
2. Power was generated by a tunable magnetron
which was isolated from the cavity by a circulator.
Incident and reflected power were measured via cali­brated 60 dB directional couplers in the waveguidenear the window. Peak power was varied from 0.2 to
2.0 MW by varying the magnetic field and anodevoltage of the magnetron. RF pulse-width was 4.5
j^sec, and pulse repetition rate was varied between
100 and 300 pps, yielding an average power level of0.09 to 2.7 kW. The cooling water which circulated
through the cavity body and the shorting plate wasmaintained at 40±5°C. Thermocouples were attachedto the nose insert and to the body of the cavity formonitoring the temperature rise during high-power
test. The cavity pressure level was estimated bymeasuring the vac-ion pump current. After rf pro­cessing, data were taken at a fixed repetition rate as
the peak power was increased from a low level to justbelow the breakdown threshold. Tests were repeatedin a 100-200-300 pps repetition rate sequence until
stable results indicated that the surface was wellconditioned.
An automated instrumentation system, incorporat­
ing a Macintosh Ilfx and LabVIEW 2 software, wasused for rapid and accurate data collection and pre­sentation. The system, shown in Figure 2, automati­
cally collected the required data and recorded it on
the Macintosh via the IEEE-488 Bus and A/D card.
Vac-ion pressure, cavity temperature and nose tem­
perature were monitored through the A/D card.

EO(MV/m)Figure 3 - Typical Field-Emission vs. Electric-Field Curve fora machined, diamond-polished OFHC copper nose.

0 .0

1 .0

2 .0

3 .0

4 .0

jisec

5 .0

6 .0

7 .0

Figure 4 - Field-Emission and detected rf-field waveforms.

1/EoM
Figure 5 - Fowler-Nordheim Plots at initial testing and after
processing for a machined, diamond-polished OFHC copper
nose.

Figure 5 shows the modified Fowler-Nordheim
results for a machined, diamond-polished OFHC nose
as a function of rf processing. The |3 value dropped
from an initial value of 58 to 45 after approximately
18 hours of processing. Once the (3 reached a valueof 45 additional processing did not seem to produce
further improvement.
At higher field-emission current levels the Q of
the cavity decreased, due to an increase in the effec­
tive beam-loading of the cavity. This effect can be
determined by measuring the reflection coefficient at
higher power levels. The measured, unloaded Q
dropped from a low-power value of 9700 to 8500 fora peak surface field of 230 MV/m. The correspond­
ing power loss was 0.2 MW compared to the total
input power of 1.8 MW.

l/Eoiio-9]
Figure 6 - Typical Fowler-Nordheim Curves: Machined, dia­
mond-polished Cu, EDM-Etched Cu, Single-Crystal (EDMEtched), Molybdenum (machined). Data shown is for fully
processed noses.

Figure 6 shows the Fowler-Nordheim results forvarious materials and surface finishing techniques.
Chemical polishing can usually minimize, if not elim­
inate, mechanical defects such as burrs and scoring,which are typically left by machining and subsequentmechanical polishing. However, it usually enhances
the sharp edges at the grain boundaries due to the rel­
ative differences in etch rates o f the adjacent, ran­
domly-oriented crystallites. Electro-discharge ma­chining (EDM) can minimize these effects, as well as
the formation of dislocations caused by machining.
Therefore, the use of a single-crystal material in com­bination with EDM finishing should produce thesmoothest microscopic surface possible.
The results show that both field-emission current
level and (3 (〜50) are very similar for EDMed andmechanically-polished cooper. The single crystal
copper produced a slightly higher (3 than polycrystal­line copper but resulted in reduced field-emission
current (〜50% lower at high Eo)_ Molybdenum had ahigh p (〜83) and higher field-emission current at
lower electric-field levels. The scratched nose (ma­chined copper purposely scarred) resulted in a high p(〜77) and the highest recorded field-emission current.
CONCLUSION
A test system capable of evaluating (3 and fieldemission characteristics of various materials has beensuccessfully developed and tested. Test results ap­
pear to indicate that there is a lower limit to p. Im­
provements to the surface microstructure by polish­
ing, EDM machining and use of single-crystal copper
provide little improvement beyond the p threshold of〜50. Since the geometric field-enhancement can only
account for a p of approximately 10, the high valuesof (3 obtained must be due to other factors. Possible
explanations are surface contaminants, hot electronemission through a dielectric layer, and surface poly­merization by impinging electrons or x-rays. Future
tests are planned to explore some of these possibili­
ties.
REFERENCES[1]G.A. Loew and J.W. Wang “Field Emission andRF Breakdown in Copper Linac Structures”
SLAC-PUB-5059, Aug., 1989
[2]S. Takeda, M. Akemoto, H_ Hayano, H. Matsumoto, and T. Naito “High Gradient Experiment by theATF”，1991 IEEE Proceedings of Particle Accel­
erator Conference, pp2061-2063.
[3]J.W. Wang, “RF Properties of Periodic Accelerat­ing Structures for Linear Colliders”，SLAC-Report-339, July 1989, Ph.D. Dissertation.