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

Construction of Plasma Micro-Undulator by Laser Interference and Resonant Photoionization
Noriyasu KOBAYASHI, Yasuo SUZUKI, Kiyoshi HASHIMOTO, Nobutada AOKI,Hironobu KIMURA, Chikara KONAGAI, and Satoshi NAKAGAWA
Toshiba Corporation
8 Shinsugita-cho, Isogo-ku, Yokohama 235-8523

Abstract
An electrostatic plasma micro-undulator was formedexperimentally using a combination of laser interferenceand resonant photoionization techniques. Plasma microundulator of this type are expected to be applied as compact
and short-wavelength insertion devices. Stable microscopic
interference fringes were obtained using a prism as an
optical device for interference, and atomic vapor wasresonantly ionized by this interfered laser beam. As a result,an emission from plasma corresponding to interference
fringes, with periodic length of about 300 J.1 m, wasobserved. Thus, formation of a plasma micro-undulator wasdemonstrated by these techniques.

For example, as a relativistic electron beam, withenergy of about 50 MeV, crosses a plasma-ripple, withperiodic length of about 20 J.1 m, it radiates a soft X-ray withwavelength of about I nm.

Periodic electrostatic field

Plasma-ripple-Relativistic electron beam-~

Synchrotron radiation

1 Introduction

Figure I. Electrostatic plasma micro-undulator

A plasma micro-undulators are a focus of attention,
because they are capable of radiating short-wavelength
synchrotron radiation without increasing electron beamenergy. An electrostatic plasma micro-undulator is ofparticular interest in view of its ability to form an undulator
with microscopic periodic length of I 0 to I 00 J.1 m. In aplasma micro-undulator of this type, an interaction between
a relativistic electron beam and a periodic ion-ripple leads
to synchrotron radiation. One of the technical problems is
how to form a microscopic ion-ripple, and some methods
have been proposed.[ I], [2]Ikehata, Suzuki et al. proposed a combination of laserinterference and resonant photoionization techniques.[3]Since neutral gas is ionized by a interfered laser beam, apattern of an plasma-ripple formed by these techniquescorresponds to that of interference fringes. Features of thesetechniques are that it is easy to control periodic length anddensity of the plasma-ripple, and it is possible to form anplasma-ripple with great regularity. [3] However, formationof a plasma micro-undulator by these techniques has notbeen demonstrated previously.
The present report describes the experimental
apparatus and results respecting the construction of theplasma micro-undulator.

2 Techniques for Formation of Plasma Undulator[3]A schematic of an electrostatic plasma microundulator is shown in Figure I. As a relativistic electron
beam crosses a periodic plasma-ripple obliquely, relativistic
electrons remove electrons in a plasma-ripple from an
orbital path of electron beam, because a relativistic electronis heavier than an electron in a plasma. As a result, an ionripple remains on a beam orbit. Then, a periodic
electrostatic field acts on an electron beam as an undulator
force.

Figure 2 shows the combination of laser interference
and resonant photoionization techniques for formation of aplasma micro-undulator.
Neutral atomic vapor is generated by heating, a laser
ablation, etc. Interference fringes are composed of two laser
beams with whose wavelength the neutral atoms areresonant, and they are irradiated neutral atomic vapor. Then,neutral atoms are ionized at a position such that the twolaser beams are made stronger within an interference region
and a plasma-ripple is formed.

Laser beamInterference region
Neutral atomic vapor

Figure 2. Combination of laser interference and resonant
photoionization techniques-110-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

3 Experimental Apparatus
3.1 Vapor Source[3]It is necessary that the shape of an ion-ripple be kept
during the interaction between a relativistic electron beam
and an ion-ripple. Because A velocity of a heavy ion be low,neodymium (Nd) was selected as a vapor source. Theionization scheme for Nd has been developed at thewavelength of 441.96 nm.
A laser ablation capable of generating pulsed highdensity vapor was used as a method of vaporization.
3.2 Optical Devices for lnteiference
Periodic length of laser interference fringes is affectedby vibration of optics devices. In order to avoid thisinfluence, a prism was used as an optical device forinterference. A laser beam is divided into two beams by aprism, and they form interference fringes as shown inFigure 3.
Two prisms, with the prism angle of 3.2 and 0.54
degrees, were prepared for interference fringes, withperiodic length of 17 and I 00 f1 m, by wavelength of 441.96
nm. Figure 4 shows the observed interference fringes
formed by two prisms using a continuous wave He-Ne laser
with wavelength of 632.8 nm. The periodic length of theinterference fringes was stable, and about 25 and 150 f1 m,respectively. These results mean that periodic length of thefringes is about 17 and 100 f1 m in the case of laser
wavelength of 441.96 nm. Accordingly, it was confirmedthat a prism was able to form stable interference fringes
with any periodic length.
Prism angle··::::"rSide viewarranged for each incident beam. A Nd target was set up
horizontally in the vacuum chamber and irradiated by thelaser for an ablation at an incident angle of about 50 degrees.
Then, Nd atomic vapor was generated in the verticaldirection of the target and irradiated by the laser for anionization. A SHG-YAG laser and a Ti-sapphire laser wereused for an ablation and an ionization, respectively. The
pulsed energy of each laser was approximately 100 mJ andImJ.
Image intensifier

Ti-sapphire laser for ionization

Laser for ablation ~Sensor'·,

Laser for ionization

vapor·,. '.

Figure 5. Experimental apparatus

The experimental procedure was as follows. The
shape of an emission from Nd particles generated by anablation was observed, and an irradiated position and timefor an ionization and periodic length of a plasma microundulator were determined. The number of Nd particles wasmeasured with sensors set up at several points above atarget to measure deposition thickness. An emission from aplasma micro-undulator was observed. An image intensifier
with a high-speed gate was used to observe an emission.

Figure 3. Interference fringes by prism

4 Experimental Results(a) 3.2 degrees(b) 0.54 degrees
Figure 4. Observed interference fringes

3.3 Experimental Apparatus
The experimental apparatus is shown in Figure 5. The
apparatus mainly consisted of the vacuum chamber, thelasers for an ablation and a resonant photoionization, andthe prism. On the side of the vacuum chamber, a port was

4.1 Observation of Emission from Nd Particles by Ablation
Figure 6 shows the observed emission from Ndparticles at I or 4 11 s after the target is irradiated. Fromthese . results, the Nd particles moved in the verticaldirection of the target as they extended and the velocity of
the center was approximately 5000 m/s. The intensity ofemission remarkably decreased at 4 11 s after the irradiation.
In order to observe an emission from a plasma microundulator, it is desirable that intensity of emission by anablation be lower. Therefore, the irradiated position andtime for an ionization were determined to be 25 mm above
the target and 4 f1 s after the irradiation for the ablation.
From the Nd particles velocity, the periodic length of theplasma micro-undulator was decided to be about 300 and500 f1 m.

-111-

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

30mm

''',

~20mm~~

IOmm~'.;(;~''"' ,._ ~~'","""~/•(a)1J1S(b)4J1S
Figure 6. Observed emission from Nd particles by ablation

4.2 Measurement of the Number of Nd Particles by Ablation
Figure 7 shows the measured distribution of Nd
particles generated by the ablation per laser pulse. This
distribution WaS approximately proportional to COS 10 B, andit was confirmed that the number of Nd particles was about
6 X 10 14 n/pulse from the estimation by an integration of thisdistribution.
~....c 1.5E+l5"""~~ 1.0E+l5Sensor
tl

"f"

Q,

~ 5.0E+l4...
Jl§

11Targetc.c"',_. O.OE+OO '---.1...<:'----'---'--- -'--"'-'----'-1.5-1-0.500.5

1.5

5. Discussions
The plasma density and the K parameter, which havean important bearing on the properties of a plasma microundulator, were estimated.
The results shown in Figure 6 indicate that the volumeof Nd particles generated by the laser ablation wasexpanded to several tens cm 3 at several J1 s after the laser
irradiation. And the number of particles generated per laser
pulse was 6 X 10 14 n/pulse. Then, the density of Nd particles
was estimated to be the order of 10 13 cm- 3• Assuming that
efficiency of a resonant photoionization was about 10 %[3],the density of the obtained plasma micro-undulator wasestimated to be of the order of 10 12 cm- 3•
A K parameter of an electrostatic plasma microundulator is described by the following equation [4]K=n e2 A.

2ou(1)
8 rr 2£ omc2
where n0 is the plasma density, e is the elementary electriccharge, A u is the periodic length of the undulator, E 0 is
the dielectric constant in vacuum, m is the rest mass ofelectron, and c is the light velocity in vacuum. Using
Equation (1), A u=300 J1 m, and n0=10 12 em·', the K
parameter was estimated to be 4 X 10-5 . In a practicalundulator the parameter is more than 0.1. Accordingly, it isnecessary to increase the plasma density by using highpower lasers for the ablation and the ionization, andreducing the period after the irradiation for the ablation sothat the microscopic plasma-ripple is capable of acting as anundulator.

Angle 0 (rad)Figure 7. Measured distribution of Nd particles by ablation

4.3 Observation of Emission from Plasma Micro-Undulator
The formation of the plasma micro-undulator wasdemonstrated by the combination of laser interference andresonant photoionization techniques.
Figure 8 shows the pattern of emission from Ndparticles within the interference region with the periodic
length of about 300 and 500 J1 m, as discussed section 4.1. It
was confirmed that the pattern of emission by the resonant
photoionization corresponded to that of the interference
fringes.

6. Conclusions
Using the prism for the interference, the fringes wereobtained stably with the periodic length of 20 J1 m.
The emission from plasma corresponding to theinterference fringes, with the periodic length of 300 J1 m,
was observed by the irradiation of the interfered laser beam.
Accordingly, the formation of the plasma micro-undulator
was demonstrated by the combination of laser interference
and resonant photoionization techniques.
In future work we intend to increase the plasma
density and then demonstrate a plasma micro-undulator
with an electron beam for a synchrotron radiation.
References[1] K. R. Chen et al.: Phys. Rev. A, 45, 6 (1992) 4077
[2]Y. Suzuki: Kakuyugo Kenkyu, 68, 5 (1992) 488[3]T. Ikehata, Y. Suzuki et al. : JAERI-Research 95-028(1995)[4]Y. Suzuki: JAERI-Research 95-009 (1995)

Figure 8. Observed emission from plasma micro-undulator-112-