J. Plasma Fusion Res. SERIES, Vol.2 (1999) 385-388

Some Aspects of Strong Langmuir Turbulence Driven by anlntense Relativistic Electron Beam
MASUZAKI Masaru, ANDO Ritoku, YOSHIDA Hiroshi, OOYAMA Shintarou, TANIGUCHI Shingo,NITTA Mituhiro. NATSUME Ken-ichi. KAMADA Keiichi and KONDO Tomohiro
Department of Physics, Faculty of Science, Kanazawa University, Kanazawa 920-1192, Japan(Received: 9 December 1998 / Accepted: 23 March 1999)

Abstract
Caviton fields driven by an intense relativistic electron beam (IREB) were probed with a weak lowenergy electron beam. From spectra of high-power microwave radiation from beam electrons interactingwith caviton fields it was found that the injected beam was modulated and the bunch length was about 5mm. Relation among the turbulence, the radiation, the beam modulation, and the ratio of the beam
density to the plasma density was studied.

Keywords:
strong Langmuir turbulence, intense relativistic electron beam, electron beam probing, high-power
broadband microwave radiation, beam modulation

1. lntroduction
Recently the strong Langmuir turbulence has
attracted much attention theoretically, computationallyand experimentally. In this state formation, collapse andFrom electric fields obtained from both methods, thestrong field regions were found to occupy a few percentof the beam volume. The final scale of cavitons wasestimated to be - 20 AD,
being the Debye length. The

burnout of cavitons, spatially localized lower-density

plasma remained in the ^Dturbulence state about 30times

volumes with large amplitude electrostatic waves
trapped, are repeated by the nucleation process. Thisof the IREB duration (- 900ns) or longer after the IREB
passed through the plasma.

turbulence state can be driven by an intense relativistic
electron beam (IREB) [1].
We have been carrying out experiments on thebeam-driven strong Langmuir turbulence by injecting an

We also observed energy spread and perpendicularvelocity scattering of beam electrons due to interaction

with caviton fields [31. The measured results wereanalyzed using the theory of multidimensional transit-IREB into an unmagnetized plasma. Previously wetime interaction given by P.A. Robinson and D.L.

measured strong high frequency turbulent electrostatic
fields using two optical diagnostic techniques; the Stark

Newman [4] and it was confirmed again that the plasma
was in a strong Langmuir turbulence state.
High-power broadband microwave radiation wasobserved, too. The radiation was found to be from beamelectrons interacting with the caviton fields [5,6]. Thestrong Langmuir turbulence state was a necessary

shift measurement and the plasma satellite method [2].
The former showed that strong fields of 50kV/cm or
more with Gaussian distribution existed in the plasma,
and that the dimensionless electrostatic energy density
W - l.l. From this result it was concluded that theplasma was in a strong Langmuir turbulence state. Fromthe latter mean electric field in the plasma was obtained.

condition, but not the sufficient one [7]. The radiation
was emitted when the ratio of the beam density to theC orre sponding author' s e -mail : masuzaki @ plasma. s.kanazawa-u. ac.j p385

@1999 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchMasuzaki M. et al., Some Aspects of Strong Langmuir Turbulence Driven by an Intense Relativistic Electron Beam

plasma density, n,o/n, was between about 0.001 and 0.1

diode. Figure 2 shows np at z = 17.5cm as a function ofr, which is the delay time from the beginning of the gun

and it was strongest at about 0.01 [8].

This paper reports an attempt to measure strong
turbulence electric fields using a weak probe electrondischarge. An IREB, which was generated with a source

beam, relation between high-power broadband

into a matched load, was injected through an anode of
20 pm thick titanium foil into the plasma at a selected r.
The injected IREB current was about 10kA and theenergy of the beam electrons was l.4MeV. The plasma
diameter was typically Scm and that of the IREB wastypically 3cm. The chamber had two observation port atz = 77.5cm, and its inner wall was covered with anelectromagnetic-wave absorber, Eccosorb AN.

capable of producing a pulse of 1.5MV, 30kA, and 30ns

microwave radiation and beam modulation, and relation

among the strong Langmuir turbulence, microwave
radiation, the IREB modulation, and ratio of the beam
density to the plasma density, n,o/no.

2. Plasma and IREB
Figure 1 shows the experimental setup (for theobservation of the microwave radiation). An

3. Weak Beam Probing into Caviton Fields

unmagnetized plasma was produced in a stainless steel
chamber of l6cm in diameter by discharging a pair ofrail-type plasma guns set opposite to each other at z =In order to investigate further the strong Langmuir
turbulence state after the IREB passed through theplasma, we began an attempt to study strong turbulence
electric fields by measuring scattering of a weak lowenergy electron beam (probe beam) injected across theplasma. Rough estimation is done on the scattering
angle of the probe beam due to caviton fields. It is basedon the data obtained in a previous experiment l2l thatthe strong field regions occupy a few percent of theplasma-beam volume and that the typical field strengthis 50kV/cm. The deflection angle is estimated to befrom -3.3 to 3.3 deg. for probe beam energy of 50keV.
Figure 3 shows the experimental setup. A probe
beam (50kV, - 50mA, 150ns) produced using anelectron gun was injected across the plasma at 2 =30.5cm. Beam pattern on an observation fluorescentscreen was photographed. Figure 4 shows the beam
patterns observed when no - 1.0 x 10-10cm-3 at 4 =30.5cm. The figure in each pattern is the delay time ofthe probe beam injection from the injection time of the8.5cm, z being the distance from the anode of the IREB

Horn antenna
for K

Pulserad 110A

Rogowskicoils

ifoil anodeO nait-type guns

+
0.1 m

Horn ant.

Vacuum pumpforEorF
Fig. 1 Experimental set up {for radiation measurement).

(r)E()'6co

E(UE1014101310121011o(U(L 1010

0 1020304050 60 70
r [prs]Fig.2 The plasma density at z = 17.5cm as a functionof r.

Fig. 3 The experimental set up for weak beam probing.
386

Masuzaki M. et al., Some Aspects of Strong Langmuir Turbulence Driven by an Intense Relativistic Electron Beama),toTable 1 Center Frequency and band width of each*,channel.
BandFreqency range [GHz]

Center frequency [GHz]Ktv26.5')) )<Ka

26.5 - 40

33.25

U40-6050

E-168 - 72.270.r

E-2

72.2 - 76.s

74.35

E-3

76.5 - 81

78.75

E-4

80.7 - 85.7

83.2

E-5

85.5 - 90

87.75

F-190 - 117 (140)

103.5

F-2

Lt1 - L40

128.5

:

-256 ns

340 nsd)"c)

&.
#'.f1
ns,o10'0.25

i tou

Fig.4 Photographs of probe beam patterns on the(a)oo 10'

observation fluorescent screen.

IH ro"
E to-'

2rc"

IREB. Scattering is seen even about 600ns after theIREB passed through the plasma. This experimental

go.2

o0.15

o0.05

(b)0.10-n -10 0

Frequency [GHz]result supports the previously obtained conclusion thatthe plasma remained in a strong Langmuir turbulence
state after the IREB passed through the plasma [2]. In
this state the background incoherent waves may beenergy reservoir for caviton formation. Further study isf1020

Length lmml

Fig. 5 (a) Typical radiation spectrum with a Gaussian fit
curve taking account of a single particle spectrum.

::iJj"

spatial beam distribution from the fitted

needed to clarify this point.

relativistic factor, c is light speed, and D is the caviton
size [9]. For high-power radiation the IREB should bemodulated [0]. As shown in the Fig. 5 (a), we fit to the

4. Microwave Radiation and Beam
Modulation
We observed radiation spectra radially (Fig.l)
using a 3-ch. filter-bank spectrometer, a 5-ch.

observed spectrum a curve calculated using the radiation

spectrum of a single beam electron interacting withcavitons having parallel dipole moment and assuming
Gaussian spectral function for the beam modulation.
Here we assumed that the phase velocities of waves

heterodyne spectrometer, and a 3-ch. filter-waveguidecombination spectrometer. These spectrometers covered
frequency range of l8-140GH2. Frequency range andcenter frequency of each channel are given in Table L
Typical spectrum is shown in Fig. 5. It is nearly flat orslightly increasing with frequency up to about 40GHz
and declines steeply at the higher frequency side. The
radiation was relativistically beamed with an angle ofequal the beam velocity. From this spectral function thesymmetric spatial beam distribution of Gaussian shapeis obtained as shown in Fig. 5 (b). The bunch length isestimated to be 5mm. This value coincides roughly withthe wavelength of the beam-plasma instability which isbelieved to be the origin of the turbulence.

about 10' to the chamber axis (the detail will be
published elsewhere). This angle agrees roughly withthe angle of 7o for the radiation from a single beamelectron interacting with the caviton field, calculated

5. Turbulence, radiation, IREB modulation,and density ratio
We measured, at z = 30.5cm and varying r (nr), theperpendicular velocity scattering as a measure of theturbulence [3], the sum of the radiation power in K, Kaand U bands, and the excited power in a waveguide
pickup due to a small portion of the beam traversing

under the assumption that the caviton field is of dipole

type and the dipole moment is parallel to the beam
direction [9]. The radiation spectrum of a single beamelectron interacting with caviton field extends to the

angular frequency a = 2y2c/D. where 7 is the387

Masuzaki M. et al., Some Aspects of Strong Langmuir Turbulence Driven by an Intense Relativistic Electron Beam0(a)

f*l

QOt

a''toI
0.1
0.01

0F

050T lllsl

100tE0rrR o.orenergy analyzer.

a,!...-....._.......r_.-.|a-.-----.4---"References

T lllsltll P.A. Robinson, Rev. Mod. Phys. 69, 507 (1997).
t2l M. Yoshikawa, M. Masuzaki and R. Ando, J. Phys.
Soc. Jpn. 63, 3303 (1993).

IaB,!

.l

lr. "-:.1 .t-"-"'."---

0 .oo1(c)

Eto

0.001

100

energy tail should appear. Now we are preparing tomeasure this high energy tail after the IREB passes
through the plasma by using a differential electrostatic(b)looa...-..-....-i..-....-...9...-....-...

! ']'r€ o.ort3l H. Koguchi, M. Masuzaki, R. Ando and K.

aKamada, J. Phys. Soc. Jpn. 67,1273 (1998).

0.001

050

T |l/slt00[4] P.A. Robinson and D.L. Newman, Phys. Fluids B

o.0 0012,3r2O (1990).

T l/lslt5l M. Masuzaki, R. Ando, A. Yoshimoto, M.

Fig.6 (a) The weighted mean of sine of theIshikawa, M. Yoshikawa, K. Kitawada, H. Moritaand K. Kamada, Proc. of the 8'n Int. Conf. on High-perpendicular scattering angle of IREB electrons,(b) total power of the radiation, {c) power radiatedPower Particle Beams, Novosibirsk: World

into the waveguide pickup by a small portion ofthe beam traversing it as a measure of the beam

Scientific, 1991, Vol. 2, p.683.
M.Masuzaki,
R. Ando, M. Yoshikawa, H. Morita,
t6l
J. Yasuoka and K. Kamada, Proc. of the 9th Int.
Conf. on High-Power Particle Beams, Washington,
DC: NTIS, 1992,Yol.2, p. 1227.
t7l M. Yoshikawa, M. Masuzaki, R. Ando and K.
Kamada, J. Phys. Soc. Jpn.65,2081 (1996).
t8l R. Ando, M. Masuzaki, H. Morita, K. Kobayashi,
M. Yoshikawa, H. Koguchi and K, Kamada, J.
Phys. Soc. Jpn.65, 2518 (1996).

modulation, and (d) nuln".

across the waveguide (WR-42) as a measure of thebeam modulation I l]. The density ratio n,olno was alsomeasured. Figure 6 shows the result. For IREB electrons
to emit radiation it is necessary that the plasma is in astrong Langmuir turbulence state, and that the beam ismodulated. The radiation was maximized at n6/n,

around 0.02. One of problems to be solved ismechanism of the beam modulation includine its
relation to the strong Langmuir turbulence.

t9l J.C. Weatherall, Phys. Rev. Lett.60, 1302 (1988).
[0] G. Benford and J.C. Weatherall, Phys. Fluids B 4,4ttt (1992\.

6. Goncluding Remarkllll H. Yoshida, M. Masuzaki, S. Ooyama, R. Ando,

When cavitons burn out the field energies areand K. Kamada, to be published.

transferred to plasma electrons and nonthermal high388