Proceedings of LINAC 2004, Lübeck, Germany

DEVELOPMENT OF FIELD-EMISSION ELECTRON GUN FROM CARBON
NANOTUBES
Y. Hozumi1,A), S.OhsawaB), T. SugimuraB), M. IkedaB)
The graduate university for advanced studies, B)KEK, 1-1 Oho, Tsukuba, JAPANA)

50Ω resistance connected to the pulser in series has arole of mitigating damage of the cathode from discharge,
and reflected pulses due to impedance miss match. The
emitted electrons are accelerated by DC high voltage of100 kV between the anode and grid electrodes, and thenemerge from the e-gun through an anode hole. Then,electron beams are adjusted by a magnetic lens to focus
on the beam catcher (BC) in solenoidal fields. The valuesof beam currents are measured by monitors such as a wall
current monitor (WCM), a core monitor (CM) and BC,
which are set along the beam line. Furthermore, afluorescence screen is available, if necessary, by insertingin the beam line instead of BC at the same place.

Abstract
Aiming to develop a high intensity electron beam withnarrow energy-spread for injector guns, we have beentested field emission cathodes of carbon nanotubes(CNTs), which have some features of easy handling andlow cost. Experiments for these three years brought us
important suggestions and a few rules of thumb. Now atlast, anode current of 3.0 A/cm2 was achieved with 8 kV
acceleration voltage by applying short grid pulses
between cathode and grid electrodes[1]. In order to proof
utility, 100-kV gun system had been designed andconstructed since last year. Then the value of 100 mA wasobtained based on 10-5~10-6 Pa back ground pressures.
With some improvements it would be expected that anodecurrents of Ampere order will be obtained from the CNTs
cathode gun.

INTRODUCTION
The heater-less electron guns (e-guns) are desired
because of its excellent emission properties, i.e., lowemittance, high brightness and low cost driving. Field
emission (FE) e-guns are under investigation for injector
linacs. This time, we have been fabricated and tested a
new type of FE-gun using the promising material emitter
called “carbon nanotubes (CNTs)”. The CNTs havestrong potentials as a field emitter; actually, 1×109 A/cm2
of current density has recently obtained in Japan. In thelast year, it was reported that anode current of 3.0 A/cm2
was achieved from CNT-FE e-gun by applying short grid
pulses between cathode and grid electrodes [1]. The CNTs
cathode which had enough current density and long lifetime could be developed by some companies’
cooperation[2]. The report results were obtained with as
low acceleration voltage as 8 kV, and this time, we haveconstructed a 100 kV practical test stand and started testof performance of the e-gun which were also designed tosuited EIMAC-Y796 standard. This paper describes thebasic design and the characteristics of the CNTs FE-gun.

Figure 1: The main components of the e-gun system

Structures of the E-Gun

COMPOSISIONS OF THE E-GUN
SYSTEM
Figure 2: The e-gun view

Measurement System and Electrical Circuits
At the gap distance between the grid and cathode (GC) electrodes, short pulses of 50 ~ 100 ns width and 0.1 ~-3.4 kV are applied through a BNC connector with arepetition rate of 7～10 Hz. 50 ns is the lower limit of thepulse generator PVX-4140 produced by DIRECTED
ENERGY, INC. Figure 1 shows this system outline. TheFigure 2 shows e-gun structure viewed from the gridside. A standard vacuum flange ICF88 is adopted for thee-gun in order that it is compatible with the often usedcathode EIMAC-Y796. The distance between the G-C isvery important, because it determines the field gradient
between the G-C with grid pulses, namely beam currents.
Therefore we designed the structure of the G-C assembly___________________________________________

1)

hozumiy@post.kek.jp

1Proceedings of LINAC 2004, Lübeck, Germany

cathode of which diameter are 6 mm could be extracted
satisfactorily through the anode hole.

so that the distance is given mechanically from thecomponents in an accuracy of 10 µm by means ofcompressing with a screw from BNC connector side. Bychanging height of a cathode base the distance will beeasily changed. Furthermore it is easy to assemble the GC mechanically it is suitable to make a test for manypieces of cathodes by changing one to another.
Figure 3 shows the Wehnelt electrode with a gridcathode assembly at the center. The slope angle of 22.5
degrees was chosen to get a parallel beam following
Pierce's theory. In the picture there is space between theWehnelt electrode and outside of the G-C assembly. Its
space is actually filled with a cover of SUS in case ofmeasurements. Figure 4 shows an anode electrode which
was made into monotonous structure, and attached theroundness of R2 to the beam hole, which was an entranceof the beam (10 mm diameter), and the anode surfaces
were similarly processed like a mirror to Wehnelt
electrode to avoid discharge.

C urrent D ensity (m A /cm ^2)

BEAM TEST420

B eforeD ischarge
A fter

370
320
270
220
170
4

4.2

4.4

4.6

4.8

55.2

Electric Field (M V /m )Figure 6: Reduced current density from anode current-23.7

Figure 3: Wehnelt

Figure 4: anode-23.8-23.9
ln (I/Eg-c^2)

Beam Trajectory Simulation-24-24.1-24.2-24.3

Before, β=7502
After, β=8465

-24.4
1.92

2.12.22.3
100000/Eg-c

2.4

2.5

Figure 7: The Fowler-Nordheim plot of Fig.6

The beam tests were performed at the e-gun test stand,where acceleration is capable up to 120 kV. Itsconstruction started from October last year, andcompleted it in February this year.
In this system, beams from the cathode are mainly
controlled by pulses applied between the G-C, namely
beam values such as intensity and pulse width controlled
by the pulses. On the other hand, beams are accelerated
by DC high voltage of 100 kV impressed between thegrid and anode electrodes. Beam characteristics have been

Figure 5: Simulation results image

Trajectories were simulated by the code EGUN withan assumption that electrons were uniformly emitted fromthe cathode surface at room temperature (defined as 300K) with a cathode current of 3 A at an acceleration
voltage of 100 kV. It was conformed that beams from a2

Proceedings of LINAC 2004, Lübeck, Germany

investigated versus field strength near the cathode bychanging voltage of grid pulses. Vacuum pressuresachieved were in the order of 10-6 Pa. Figure 6 shows
anode current dependence measured at BC point, versus
electric fields between the G-C. Figure 7 presents FowlerNordheim plot of the same data, where the symbol of Eg-cmeans that applied field strength between the G-C and β’s
are field enhancement factors. Large values of β’s gotten
from the plot indicate that field emission is the source ofthe beams as well as linearly changing dependence in theplot. The beam pulse shapes are shown in Fig. 8, which
are measured by three kinds of monitors. The beam tests
were performed with pulses of 65 ns width at a fixed
repetition rate of 7 Hz. In Figures 6 and 7, the datameasured before discharge occurred are shown withsquare marks, and ones with triangles are the datameasured after discharge occurred three times serially.
Although 400 mA/cm2 was obtained before discharge
happened, the discharge between the G-C occurredoccasionally, and whenever it happened, reduction inemission current took place. Estimating from the data in
Fig. 6, the rate of reduction due to a discharge was about
4 ~ 5 % in intensity.

cathode seems to be affected by discharges. It could besaid that the beam shapes observed on the fluorescencescreen reflects directly the state of cathode as they are.

CNTS CATHODE SURFACE STUDY
In order to investigate directions for CNTs cathode
optimization, the FE-SEM observation was performed.
Figure 10 shows a FE-SEM image of a cathode sample
used in this experiment, of which CNTs thin films
produced on a SUS substrate. The FE-SEM observation
was done in the secondary electron detection mode, at aworking distance of 14.8 mm, with a probe current of
10.6 µA at 10 kV accelerating voltage. CNTs can be seen
near the center of the photograph. The EDS investigation
was also done to check the materials of the concavoconvex parts around CNTs substances, which revealed thematerials were mainly of spherical particles of carbon-like,and partly of un-grown up CNTs. In the investigatedcathode, the quite large variations were observed in CNT
density and height.

Figure 10: FE-SEM image of a CNTs cathodeFigure 8: Monitor of beam current (CH1:WCM, CH2: CM,CH3:BC)

SUMMARY
Anode currents were obtained as high as 100 mAfrom CNTs cathodes, which were equivalent to thedensity about 400 mA/cm2. It became promising thatAmpere order beams would come true with some
improvement in vacuum pressure and CNTs cathodestructure.

ACKNOLEDGEMENTS
The FE-SEM work was conducted in AIST NanoProcessing Facility, supported by Nanotechnology
Support Project of the Ministry of Education, Culture,Sports, Science and Technology (MEXT), Japan.

Figure 9: Beam shapes at BC before (a: left) and after discharge(b: right)

It is considered that CNTs sites emitting electrons
might be broken by discharges, and number of site
became fewer and fewer gradually. Then, in order toobserve change in the beam shape the fluorescence screen
was inserted instead of BC at the same place (see Fig. 9).
The beam of Fig. 9a shows rather uniform shape whichindicates uniform emission from the cathode. On the otherhand, in Fig. 9b there is obviously a change in brightness:a portion of the beam disappeared due to discharges. TheREFERENCES[1] S. Ohsawa et al., Proceedings of the 28th LINAC
meetings in Japan, pp. 120-122 (In Japanese).
[2] A. Yamamoto et al., Proceedings of the particle
accelerator conference in America, pp. 3326-3328(2003).

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