JAERI-Conf 95-021

PA39
Experimental Demonstration of Yp x B Acceleration Schemewith Use of Transverse Electromagnetic Waves
Syuichi SANJYO, Naotoshi KIRIHARA, Noboru YUGAMI, and Yasushi NISHIDA
Department of Electrical and Electronic Engineering,
Utsunomiya University,
2753 Ishii-machi, Utsunomiya, Tochigi 321, Japan.

Abstract
Electron linear acceleration, using a slow transverse
electromagnetic wave (TE wave) supplemented with acrossed static magnetic field, has been demonstrated.
An energy gain of 3.5 keV is measured for electrons
with a 64 keV incident energy, in a 0.5 m long accelerator, when an external magnetic field of 0.67 G
is applied. The results show the feasibility of a highgradient, compact accelerator using TE mode intenselaser or short wave length microwave without slow
wave structure or mode converter.
I. Introduction

An accelerator is proposed in which a TE-mode
wave is used to drive charged particles in contrast
with the usual linear accelerators based on the Vp x B
acceleration mechanism in which longitudinal electricfields or TM-mode waves are supposed to be used. 1"" 6 )Using TE mode as a driving wave allows higher gradient and more compact accelerators, because the slow
wave structure, or mode converter, is not necessary
for high energy accelerators. In this paper, we report results of proof of principle experiments on theVp x B acceleration scheme using the TE wave in vacuum. In order to couple particles with the transverse
wave we use a slow wave structure created waveguide composed of parallel dielectric materials and ofconductors. 2 >II. Theory
Suppose that the transverse electromagnetic wave
which has a maximum electric field E., and a maximum magnetic field By in the x andy direction, respectively, propagates with a phase velocity Vp in thez direction (see Fig. 1) . When an external static
magnetic field Bo, with an amplitude smaller than
By is applied in the y direction, there exist two magnetic neutral points (A and B) where the electric field
is non-zero. Around point B, the Lorentz force actson the particles with its velocity, v R:l vp, which areaccelerated by the electric field in the x direction, inthe z direction as restoring force. Therefore the particles bunch there and are accelerated by the wave
electric field in the :z; direction, continuously.
The motion of a charged particle with the rest
mass, m, and charge, q, in the above-mentioned system is de&cribed by the equation of motion. 2 ) The
trapping condition is obtained by the balance of theequation of motion in the wave propagation direction(z direction) such as,(1)

where /p is the Lorentz factor measured with thewave phase velocity, vp. Inequality represents theunlimited acceleration condition. In other words, if
this condition is satisfied, electrons can be accelerated continuously without detrapping from the wave
trough.
y{
By+ Bo

Ex =Exo sin(kz - cot)
By =8yo sin(kz - c:ot)

Fig.l. Schematic diagram of the tip X Bo acceleration using atransverse electromagnetic wave. An electromagnetic wave
with a transverse electric field component propagates in the z
direction with the phase velocity tip. A static magnetic field
Bo is applied in the y direction. A charged particle isaccelerated in the x direction.

III. Experimental set-up and results
A schematic view of the experimental set-up is
shown in Fig. 2(a). The injected electrons with beam-136-JAERI-Conf 95-021

Table I. Dimensions of the dielectric wave guide.
Dielectric materials
Accelerator length
thickness D.dwidth 2h
separation 2dhight P.( = 2d + 2D.d)

Macorl and Folsteright
48 em
50 mm
50 mm
13-33 mm
113-133 mm

current: 1 rnA are initially accelerated, by a highvoltage DC power supply, up to a maximum energyof 100 keV. Here, we use a Pierce-type gun with ahair-pin-type cathode which is made of a tungsten
filament originally made for an electron microscope.
The acceleration section has a slow-wave structure ofTE mode 2 ) and a schematic view of this structureis shown in Fig. 2(b ). This structure is designed sothat the phase velocity is 0.46c (corresponding energyof 65 ke V) by adjusting the seperation to a value 74
mm. The dimensions of the slow wave structure arelisted in Table I. This structure consists of 20 dielectric blocks and 48 em long in the wave propagation
direction. The blocks are installed upper and lower
side of the wave guide with the separation for theelectron path.

The electrons accelerated through the accelerator
are analyzed by a magnetic field bending type energy analyzer which has a uniform magnetic field of
200 G. The electrons are detected by a micro channel plate (MCP) electron multiplier with a slit of 2mm in front of it. The MCP is scanned spatiallyin the analyzer and the position at which the signal is detected from the MCP corresponds to the energy and the signal intensities to the electron fluxes.
The maximum resolution of the energy analyzer isless than 0.1 ke V. This instrument is calibrated withthe above-mentioned electron beam source. The calibration is carried out by changing the acceleration
voltages without RF power before and after the main
experiments. In the experiments, the orbit of the electrons is bent by the applied magnetic field. When theelectron energy was measured, the energy analyzer isadjusted such that the electrons are injected nomal
to its slit inlet.
A pulsed electromagnetic wave, with a 10 kW maximum power, is generated by a magnetron which istriggered by an external timer with a typical pulse
width of 5 ps with repetition of 10 Hz. The generated microwave is absorbed by a nonreflection dummyload after going through the slow-wave structure.
The static, vertical magnetic field for the vp x B 0acceleration is generated by a pair of saddle-shaped
external coils. Uniformity less than 3% covers lengthof 32 em ( 40 em for 5 %) in the z direction and 5 emin the y direction. A maximum field strength of 10
G is measured at the center of the accelerator. Thisvalue is high enough to demonstrate the Vp x Bo principle with TE mode under the present experimental
parameters.

Ex =40 kV/mEx= 30 kV/m
Ex= 20 kV/m(b)

4~~~~~~~~~~

55

Fig.2. Experimental apparatus and the crossectional view ofthe wave guide loaded dielectric material. (a) Experimental
apparatus for the proof of the principle of the electron
accelerator using TE wave. (b) Schematic drawing of thedielectric wave guide.

6560e0 (keV)70

Fig.3. Energy increment, .O.e, as a function of the incident
elcetron energy, eo, without external magnetic field. Incident
microwave power: P = 10 kW. The solid lines indicate thecalculated maximum energy increment, at the maximum
electric field value of E:z: = 20, 30 and 40 kV /m.

Figure 3 shows the electron energy increment as afunction of incident electron energy co with no externally applied magnetic field. The maximum electron- 137-

JAERI-Conf 95-021

energy increment, ~c = 1.3 ke V, is observed when theincident electron energy co is 64 ke V. This implies
that this slow wave structure is resonated with thefrequency of 2.45 GHz, and the phase velocity is corresponded to the energy of the order of 64 keV. The
vertical error bars indicate the variation in electronenergy due to the instability of beam transparency
arised from the induced space charge on the dielectricsurface through the wave guide. The resonance at theelectron energy of 64 ke V is slightly smaller than thedesigned value of 65 ke V. The results show that an
efficient acceleration is occured when the wave phasevelocity is slightly larger than the beamvelocity. A
strong deceleration is observed for electrons with anenergy co = 60 keV. In Fig. 3 the calculated resultsof the electron energy increment are also displayed bysolid lines when the wave electric field is Ex = 20, 30
and 40 kV /m (shown in the next section).

the distribution of an incident microwave power inthe wave guide. The Runge-Kutta method is used tothese calculations with 100 test particles uniformly
distributed initially in the phase space. Although theprofile of the electric field amplitude in the wa~e guidedepends on the position, we select the value on theaxis of the waveguide, because almost all electrons
propagate through the center of the wave guide in
the present experiments. In the present experiment,the trapping condition inequality (Eq. (1)) is violated, because it takes more than 20 ns to reach thesteady state. When Ex = 30 kV /m, it is good agreement with the experimental deta in Fig. 3. Therefore the assumption on the intensity of the electricfield is reasonable to energy increment the electron
motion in the wave guide. In Fig. 4 , the observed
energy increment is larger than the calculated value.
This discrepancy is not clarified yet. The conditionof the transient state, however, should be taken into
consideration for more precise explanation of the experimental results.
V. Conclusion
Electron acceleration using a slow transverse electromagnetic wave (TE wave) supplemented with astatic magnetic filed has been demonstrated. An energy gain of 3.5 keV for electrons is observed froman incident energy of 64 ke V in a 0.5 m accelerator,
when an external magnetic field of 0.67 G is applied.
The results show the feasibility of high gradient compact accelerator using an intense laser or short wave
length microwave without slow wave structure.

8 0 (Gauss)
Fig.4. Energy increment, ~e, as a fuction of the static
magnetic field Bo with P = 10 kW. The solid lines show thecalculated values when Eo = 20, 30 and 40 kV /m. Symbols
0 and .C:. stand for eo 64 keV and 66 keV, respectively.

=Figure 4 shows the electron energy increment as afunction of an external applied magnetic field. Symbols 0 and b. stand for values of the incident electronenergy of co = 64 and 66 keV, respectively, and solidlines are the calculated results with an electric field
amplitude Ex= 20, 30 and 40 kV jm. In Fig. 4, as thestatic magnetic field is increased, the electron energy
increases until Bo = 0.67 G at co = 64 keV. A maximum energy increment of 3.5 keV is observed at theincident energy of 64 ke V with an external magnetic
field of 0.67 G.
IV. Discussion
The energy increment is computed by solving theequation of motion of an electron for the parametersof the present experiment, assuming vp = 0.461c, Ex= 20, 30 and 40 kV /m and r = 3.2 ns, where r is thetime that the electrons travel throughout the wave
guide. The electric field strengths are estimated from

Acknowledgment
Vivid discussions with Prof. S. Takeuchi weregreatly appreciated. Part of the present work wassupported by the Grant-in-Aid for Scientific Researchfrom the Ministry of Education, Science and Cultureof Japan. A part of the work and the constructionof the machine were supported by IDX Co., Tokyo,
Japan.
References[1) A.Loeb and L. Friedland, Phys. Rev. A 33, 1828
(1986).
[2)
S.Takeuchi,K.Sakai,
M.Matsumoto,and R.Sugihara, Institute of Plasma Physics, Nagoya
University, Research Report,N o. IPPJ-7 40(1985).
[3) S. Takeuchi, K. Sakai, M. Matsumoto and R. Sugihara, Phys. Lett. A 122, 257 (1987); S. Takeuchi, K.
Sakai, M. Matsumoto and R. Sugihara, IEEE Trans.
on Plasma Sci. PS-15, 251 (1987).
[4] S. Kawata, T .Maruyama, H .Watanabe, and I .
Takahashi, Phy. Rev. Lett. 66, 2072 (1991).
[5) R.Sugihara, Jpn. J. Appl. Phys., 30, 76 (1991).
[6] Y.Nishida, and K.Otsuka, Proceedings of I.C.P.P.,
New Delhi, India, 1989, pp.605-608.

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