The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Electron Beam Instability and Stabilization by Dilute Plasma in theDielectric-Loaded Slab-Symmetric Waveguide
Naotoshi KIRIHARA, Toshimitsu NISHINO, Hiroaki ITO, Noboru YUGAMI, Yasushi NISHIDA
Department of Energy and Environmental Science, Graduate School of Engineering,Utsunomiya University. 7-1-2 Yoto Utsunomiya,Tochigi, 321-8585, JapanAbstract
The Vp x B acceleration scheme with use of a transverse electromagnetic wave (TE-wave) is demonstrated experimentally. However, in order to couple particles with the transverse wave, at present have to use
a slow-wave structure because of low electron beam
velocity(~ c/2). When the electron beam propagatedthrough the slow-wave structure composed of dielectricloaded slab-symmetric waveguide in this present paper,an orbit of the beam became unstable. We have obeserved the electron beam the instability to investigate
its mechanism. As a result, an electron beam in the
dielectric-loaded waveguide is deflected to the opposite
direction from the material on the waveguide wall. By
adding the beam-generated plasma with the slow-wavestructure are have observed the electron beam to become
stable.

j'Diameter: 10 mm
Length : 200 mm
Xx-hFig. 1 Experimental apparatus. The inset shows thecross sectional view of the dielectric loaded
waveguide.
Table 1Dimensions of the dielectric-loaded waveguide1 Introduction
An electron linear accelerator was proposed, inwhich a TE-mode wave is used to drive charged particles using Vp X B acceleration acceleration [1-4], andthe experimental demonstration of this acceleration phenomena has been reported elsewhere[5,6]. However, inorder to couple particles with the transverse wave, atpresent have to use a slow-wave structure composed ofdielectric-loaded slab-symmetric waveguide, because theelectron beam velocity is low ( ~ c/2). When the electron beam propagats through the slow-wave structure,
an orbit of the beam become unstable. In the present
experiment, we used the beam-generated plasma in theaccelerator for stabilizing the beam orbit. The experimental demonstaration of Vp x B acceleration schemewith TE-wave assisted by dilute plasma for beam stabilization has been reported[7].
In this paper, we have observed the electron beam
orbit precisely in the slow-wave staructure in vacuum
and gases respectively, and investigated the mechanismof beam instability and its stabilization by assiting thedilute-plasma.
2 Experimental setup
A schematic view of the experimental setup is shownin Fig.l, and the cross section of the dielectric-loaded
waveguide employed here is shown in the inset of Fig. I.
The used dielectric materials are Macorl and Folsteright.
The dielectric constants of these materials are 5.68 (f= 8.6 GHz) and 7.0 (f = 1 MHz), respectively. The
dimensions of the dielectric-loaded waveguide are listed
in Table I.

Waveguide length
Thickness b.dWidth 2hSeparation 2d
Heightl ( = 2d + 2b.d)

48 em
50 mm
50 mm
13-33 mm
113-133 mm

The injected electron beam is initially accelerated
by a high-voltage DC power supply with a maximum
energy of up to 100 keV and current of 1 rnA. Here weused a Pierce-type gun with a hair-pin-type cathode.
The vacuum system employes the differential pumping system with one turbomolecular pump. The basepressure in the area of the electron gun, and thedielectric-loaded waveguide is below 1.5 x I0- 6 , and
1.5 x 10- 4 Torr, respectively. These area is separated
each other by a steel pipe with a diameter of 10 mm anda length of 200 mm. There is an oriffice with a diameter of 2 mm at the inlet of the waveguide, because oflimitting the electron beam size.
3 Beam envelope
Before observing the electron beam instability, wemeasured the electron beam current, beam radius, normalized emittance and beam envelope in the waveguidewithout dielectrics. In order to observe the beam current
h, we used a Faraday cap with an apartures of 10 mm
indiameter and a length of 20 mm. observing the beam~415~

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

radius rb and normalized emittance £n, we used a movable phosphor plate of 14 mm by 14 mm, which moves
along the axial direction (z direction). A beain shapeon the phosphor plate is recorded photographically after
it reflecting the shape by the mirror. The experimental
results with the injected electron beam energy of 60 keV,
beam current of 0.1 mA and 0.5 mA are listed in TableII.

4 Observation of electron beam instability inthe dielectric-loaded waveguide
When the electron beam propagates through thedielectric-loaded slab-symmetric waveguide, an orbit of
the beam become unstable[?). In order to investigate
the beam instability, we observed the beam orbit in thewaveguide under the several conditions shown below :Table 2
Experimental results of beam parameter• without dielectrics (only metal waveguide)• loaded only upper-side

Injected current (rnA] I 0.1
Ib f-lA
0.002
en [mmrad) .
1.67
ru [mm]2.6

0.5
0.35.30• loaded only lower-side• slab-symmetrically loaded.

3.4

Next, we. observed an electron beam envelope forthe investigation of beam characteristics. In order toobserve the beam envelope, we used the same method
mentioned above. Typical experimental results for thebeam envelope are shown in Fig.2. Here an example ofthe electron beam figure on the phosphor is shown in theinset.
In this figure, the solid circle is the experimental
result with the beam energy of 60 keV and current of
0.1 rnA. The oolid line denoted "Cal." i~ obtained fromthe enevelope equation,10 )~=0r''-~((,8;) 3 h r5'An electron beam orbit in the waveguide without
dielectrics is also shown in the inset of Fig.2. The centerof the beam propagates through the waveguide in stable,
without the deflection to the transverse direction.
Next, we observed an orbit the case of loading thedielectrics in either one side of the ·waveguide with anincident beam energy of 60 keV, and current of 0.1 rnA.
The distance between the beam center at the center tothe waveguide and the surfase of the dielectric materials
is 16.6 mm ( beam radius : 2.6 mm ).

0'DCD(1)

0•
•
•

-5-~~
Q; E -10--

where ,B = vjc, ; = {1- (vb/c) 2}- 112 is Lorentz factor of the beam energy, Io is space charge limited current, 17 kA, and r·0 is beam radius at the entrance of
the waveguide. h is substituted by above experimen~
tal result "0.002 11A ". This implies that the expantionof electron beam in the radial direction i~ cau~ed by thenormali:r.ed emitance, not by the space charge in electronbeam.

.c:E
E:!1 -15£ifll'llie~~ln}ecled
BeamCenlrokf•·20-25~L-~--~~--~~

0100

200

300

400500Distance from the entrance of
the waveguide [mm)Fig. 3 Observation result of electron beam orbit m
the upper-loaded material
6'E

5Exp.

.sen

4:0
(II3•

::1E•

•

25r-------------~~%~ 200:::(II•-E£ .§. 15CaL2qCD

IlliE.:::~i E~i0 L..--1---L.-~-...J....---10100 200 300 400 500

lrjecled

f10'··--·····-·······-· +BeamCentroid•
5:•0~~--~~--~~

0Distance from the entrance of
the waveguide [mm]100200300

400500Conduclor

Distance from the entrance of

the waveguide [mm)Fig. 2 Electron beam envelope with the incident
beam energy 60 keV and current 0.1 rnA. Inset
shows the beam shape observed at each location.

Fig. 4 Observation result of electron beam orbit m
the lower-loaded material
A typical experimental result of the beam orbit in
the case of upper-loaded and lower-loaded is shown in-416-

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Fig.3 and Fig.4, respectively. In these figures, the crosssectional view of the. waveguide employed here is shownin the inset, and the beam figures are taken by camera,is showing in the inset. The closed circle in each figure
shows a beam centroid on the photograph. From theseexperiments, we can see that the electron beam is shiftedto the opposite direction against the dielectric material,
and the beam shape changes to be "flat" along with thepropagation from the inlet of the waveguide.
Finally, we observed an orbit in the dielectric-loadedslab-symmetric waveguide with the incident beam energy of 60 keV, and current of 0.1 rnA. The distance
between the beam center at the inlet of the waveguideand the surfase of the dielectric materials is the same
condition with the above experiment.
A typical experimental result of the beam orbit in
the case of the slab-symmetric waveguide is shown in
Fig.5.
The beam centroid is static near the inlet, but it
becomes unstable to be defrected up and down. Thisfeature looks like ''rebound" as it approaches to the dielectric material in this waveguide.
30..----------.

Injected

1··--···--·····-1-<... BeamCentroid-30 I..---'---'----'-.....J....-..10

100

200

300

400

500

Conductor

Distance fromthe entrance ofthe waveguide [mm]Fig. 5 Observed result of electron beam orbit in theslab-symmetric dielectric-loaded waveguide.

5 Stabilization of the electron beam assisted bydilute plasma

In order to stabilize an electron beam in the slabsymmetric dielectric-loaded waveguide, a dilute plasmais introduced in the waveguide. The used neutral gas isAr.
It is reported in Ref.[7] that the beam-generated
plasma makes the electron beam stable in the slabsymmetric dielectric-loaded waveguide. Here, we observed that the change of location of the beam centroid
at certain z posision as a function of gas pressure. A
typical example of the experimantal results is shown inFig.6.
From this result, it is seen that the centroid of thebeam returned back to the initial beam position which isobserved at the inlet of the wavuguide as a gas pressureincreases gradually.
This result implies also that the beam-generated
plasma made the electron beam stable.

10"t3"'·c"' E--="'EE'"
0 0o!::"E2l 2l~ E-~ ~8642..••
••
••

•

O.o••

00.1

10
Gas prassure [mTorr]

Fig. 6 Change of beam centroid on certain position asa function of the gas prresure
We have descrived that the electron beam instability in the slab-symmetric dielectric-loaded waveguide iscaused by the induced space charges by beam current on
the mateial surfase in ref. [7].
However, we found that the factor which caused thebeam instability was not a space charge inducing by thebeam curret. Because the beam current is too small to excite the beam instability, and further-more thebeam orbit deflected to the opposite direction from thedielectric material, on which the opposite polarity spacecharge should be induced. The main factors for deflecting the beam are electric and magnetic field. We discussthat these fields will be electromagnetic field radiated byan electron beam in the waveguide.
Tremaine et al. reported that the beam instability
occurs by the "wake field" excited by the pulsed electron
beam in the slab-symmetric dielectric-loaded waveguige[8]. But, since we use the "coasting DC beam", wedon't find whether the wakefield is generated in our experimental setup or not.
In the future, we will perform the observation of the"wave field" in the waveguide.
Acknowledgement

One of the authors (N. K) appreciate greatly thestimulating discussions with Professor K. Takayama(KEK).
References[1] S. Takeuchi et al., Phys. Lett. A 122 (1987)257[2] S. Takeuchi et al., IEEE Trans. Plasma Sci. PS-15(1987)251[3] K. Otsuka et al., Jpn. J. Appl. Phys. 33 (1994)5933[4] Y. Nishida et al., Proc. Int. Conf. on Plasma Physics,
New Delhi, India, 1989, pp. 605-608[5] N. Yugami et al., Phys. Rev. Lett. 76 (1996)1635[6] N. Kirihara et al., Proc. Int. Conf. on Plasma Physics, Nagoya, Japan, 1996, pp.414-417[7] N. Kirihara et al., Nucl. Instr. and Meth. A 410(1998)373[8] A. Tremaine et al., Phys. Rev. E 56 (1997) 7204

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