Plasma and Fusion Research: Regular Articles

Volume 11, 3406012 (2016)

The Measurement of Plasma Structure in a Magnetic Thrust
Chamber∗)
Ryosuke KAWASHIMA, Taichi MORITA1) , Naoji YAMAMOTO1) , Naoya SAITO,Shinsuke FUJIOKA2) , Hiroaki NISHIMURA2) , Hiraku MATSUKUMA2) , Atsushi SUNAHARA3) ,Yoshitaka MORI4) , Tomoyuki JOHZAKI5) and Hideki NAKASHIMA1)
Department of Advanced Energy Engineering Science, Kyushu University,
6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan1)
Faculty of Engineering Sciences, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan2)
Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan3)
Institute for Laser Technology, 1-8-4 Utsubo-honmachi, Nishi-ku, Osaka 550-0004, Japan4)
The Graduate School for the Creation of New Photonics Industries, Hamamatsu 431-1202, Japan5)
Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan(Received 31 July 2015 / Accepted 3 November 2015)
Magnetic thrust chamber is a propulsion system controlling plasma by a magnetic field and is expected asa propulsion system of laser fusion rocket. This rocket obtains thrust due to the interaction between plasma andmagnetic field in the system. We examine the plasma structure in a magnetic thrust chamber by observing thelight emission from the plasma with several magnetic field strength. The experiments imply that anisotropic
plasma expansion is induced by the applied magnetic field and the velocity is decreased at the magnetic field
above 0.67 T.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: laser fusion rocket, magnetic thrust chamber, plasma, magnetic field, plasma emission
DOI: 10.1585/pfr.11.3406012

1. Introduction
Laser Fusion Rocket (LFR) is a spaceship utilizing an
extravagant amount of nuclear fusion energy and controlling fusion plasma with an external magnetic field. LFR
has a potential to produce large thrust and high specificimpulse simultaneously, so that it is expected as a rocketof interplanetary missions [1].
In interplanetary missions, long duration of the exposure of cosmic ray and living in an enclosed space deteriorate both physical and mental health of astronauts, andreducing mission period is indispensable. LFR is expected
to solve this issue because of its potential to generate large
thrust. In addition, the thruster can control the exhaustmass and velocity with wide range so that it enables an optimal design of the thruster depending on missions [2, 3].
In this paper, such a thruster is called “Magnetic ThrustChamber”.
The mechanism of thrust generation in a magnetic
thrust chamber is shown in Fig. 1. Plasma expands asshown in Fig. 1 (a). The plasma induces a diamagnetic current to sweep aside the magnetic field and magnetic field
is compressed [Fig. 1 (b)]. The compressed magnetic field
pushes back the plasma and the spaceship obtains the thrust
as a reaction force as described in Fig. 1 (c).
author’s e-mail: kawashima@aees.kyushu-u.ac.jp∗) This article is based on the presentation at the Conference on Laser
Energetics 2015 (CLE2015).

Fig. 1 The mechanism of thrust generation of magnetic thrustchamber. (a) Laser irradiates a target pellet to generate
nuclear reaction. The fusion plasma expands in a magnetic field generated by electromagnetic coil. (b) The
plasma induces a diamagnetic current to sweep aside themagnetic field and magnetic field is compressed. (c) The
compressed magnetic field pushes back the plasma andthe spaceship is accelerated by the reaction.

In previous researches, a magnetic thrust chamber has
been studied with numerical simulations and laser experiments. Nagamine et al. have simulated a plasma behavior
in magnetic field using a three-dimensional (3D) hybrid
code, treating ions as particles and electrons as fluid, respectively [4]. Maeno et al. combined a one-dimensional(1D) radiation hydrodynamic code with a 3D hybrid code
to simulate the creation process of a laser-produced plasma[5]. In addition, they examined the eﬀect of the magnetic

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c 2016 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 11, 3406012 (2016)

field strength on a plasma behavior [6]. Some experiments
in laser facilities with the energy of 4 J to 1 kJ have been
performed. Maeno et al. have directly measured thrustgenerated by a magnetic thrust chamber [7]. Yasunaga etal. have examined the interaction between magnetic field
and laser-produced plasma, and observed the diamagnetic
cavity [8]. However, the plasma structure in a magnetic
thrust chamber has never been observed experimentally.
In this paper, we examine the plasma structure in amagnetic thrust chamber by observing the light emissionof a laser-produced plasma with several magnetic fieldstrength. We observed the plasma deceleration.

2. Characteristics of Plasma Expansion

Fig. 2 Experimental setup. YAG laser irradiated the target andgenerated the plasma. The laser energy was 6.0 ± 0.8J, pulse width is 9.5 ± 0.5 ns and the wavelength was
1064 nm. The emission from the plasma was recordedwith ICCD cameras through a band pass filter and lens
( f = 200 mm).

Nikitin et al. have discussed the dynamics of the 3D
expansion of a spherical cloud of rarefied plasma into avacuum in the presence of a dipole magnetic field, in theframework of ideal MHD approximation. They described
how to determine the configuration and location of theplasma front which is caused by the retardation eﬀect [9].
In addition, they defined parameter that characterizes theinteraction between the expanding plasma and the dipolefield,
κ=12πE p R30
Ep=,EM
μ0 |μd |2where E p is the kinetic energy of ions, E M is the fieldenergy integrated beyond the spherical radius R0 (E M =(μ0 /4π)|μd |2 /(3R30 )). R0 is the distance from the magnetic
coil to the initial target position. μ0 is the vacuum magnetic
permeability and |μd | is the magnetic moment magnitude.
The critical value κc was found by Nikitin et al. for diﬀerent plasma locations. When κ is lower than κc , substantial
plasma deceleration will occur in all directions from theexplosion location. (When the plasma is located on theaxis of the magnetic field as in this experiment, the criticalvalue is κc = 0.4.) [10].

3. Experimental Setup
The experiment was performed at the Extreme UltraViolet (EUV) database facility of the Institute of Laser Engineering (ILE) in Osaka University. The experimental
setup is shown in Fig. 2. A plasma was created by focusing a 1064 nm Neodymium: Yttrium Aluminum Garnet (Nd:YAG) laser onto a polystyrene ([-CH2 -CH(C6 H6 )])n spherical target with a diameter of 500 µm. The
pulse width of laser is 9.5 ± 0.5 ns and the laser energy is
6.0 ± 0.8 J. The target is suspended by a carbon fiber attached to a glass rod to reduce the plasma formation ofglass rod. The distance between the coil surface and thetarget is 13 mm.
Magnetic field was generated by flowing current into
a 96-turn electromagnetic coil with the inner radius of
13 mm and outer radius of 25 mm. The current was gener-Table 1 Parameter κ in several magnetic field strength.

ated by a capacitor bank, in which three 32 mF capacitors
were connected in parallel. The magnetic field of 1.1 T wasgenerated at the initial target position by applying 500 V on
the capacitor bank.
We also calculated as shown in Table 1, implying thatplasma deceleration would occur over 0.46 T (κ < κc =0.4). In our experiment, E p = 6 J (We assume almost 100%of laser energy is converted to the kinetic energy of ions[11].), R0 = 13 mm, and μd = 11, 24, 48, 72, 94, and116 A·m2 with the magnetic field strength of 0.12, 0.23,0.46, 0.67, 0.89, and 1.1 T, respectively.
The emission from the plasma was collected with alens (focal length of 200 mm) and imaged onto intensified
charge coupled devices (ICCDs) at a wavelength of 660 nmwith a band-path filter with the width of 10 nm (FWHM).
Two ICCDs were used to take images of diﬀerent delay
times in a single laser-shot. The self-emission recorded
here is composed of H-alpha and thermal bremsstrahlung
emissions.

4. Result and Discussion
Figure 3 shows the initial magnetic field configuration
generated by the electromagnetic coil.
Figures 4 (a) - 4 (f) show the light emission from theplasma with the magnetic field of 0, 0.23, 0.46, 0.67,0.89, and 1.1 T, respectively, at 1.0 µs after plasma generation. In these figures, the target was set at the coordinate(x, y) = (0, 0) and was irradiated with the laser from left
side through the hole of the coil. The center axis of the3406012-2

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Volume 11, 3406012 (2016)

Fig. 3 Initial magnetic field configuration generated by electromagnetic coil.

magnetic field corresponds to the x-axis at y = 0.
Without the magnetic field, the plasma expands to −x
direction and ±y directions.
With the magnetic field, the plasma expansion to −x
and ±y direction is suppressed by the magnetic field. Comparing the plasma emission in +y direction with that in −ydirection, the plasma intensities in +y direction is higher
than that in −y direction. Since the high-energy plasma hit
the carbon fiber attached to a glass rod, the fiber is ablated
and the ablation plasma emits light.
Figure 5 (a) shows the light intensities from theplasma averaged from y = −4 to 4 mm along the x-axis at
1.0 µs in Figs. 4 (a) - 4 (f) with six diﬀerent magnetic fieldstrength. Without magnetic field, a peak of plasma intensity at x = −10 mm shows the plasma generated byFig. 4 The light emission from the plasma (a) without and with the magnetic field of (b) 0.23 T, (c) 0.46 T, (d) 0.67 T, (e) 0.89 T, and (f)1.1 T at 1.0 µs after laser irradiation. In these figures, target was set at the coordinate (x, y) = (0, 0) and was irradiated with thelaser from left side through the hole of the coil.

Fig. 5 (a) The light intensities from the plasma averaged from y = −4 to 4 mm along the x direction at 1.0 µs in Figs. 4 (a) - 4 (f). The lightintensity in x direction is higher as magnetic field strength is larger. (b) The light intensities from the plasma averaged from x = −5to 0 mm along the y direction at 1.0 µs in Figs. 4 (a) - 4 (f). The width with magnetic field is smaller than that without magnetic
field.

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Plasma and Fusion Research: Regular Articles

Volume 11, 3406012 (2016)sults to realize Laser Fusion Rocket. Experimental investigations on a thrust eﬃciency and the comparison between
numerical and experimental analyses will be our future
work. Nagamine et al. [5] examined thrust eﬃciency as
a function of criterion E B /E p . Here, E B is the magnetic
field energy and represented by E B = 1/2LI 2 (L: Inductance [µH], I: Current [A]). E p is the initial plasma kinetic
energy. They found thrust eﬃciency is the maximum whenE B is five times greater than E p . In a conceptual laser fusion rocket VISTA [1], a value of 5 is adopted for the ratio.

5. Conclusion
Fig. 6 Distance from initial target position (x, y) = (0, 0) to rising edges of the plasma intensity with several magnetic
field strength as a function of time.

the laser-produced plasma from the target surface colliding with the coil surface. The light intensity in +x direction is higher as the magnetic field strength is larger. Figure 5 (b) shows the light intensities from the plasma averaged from x = −5 to 0 mm along the y direction at 1.0 µs inFigs. 4 (a) - 4 (f) with six diﬀerent magnetic field strength.
The width of plasma expansion to ±y direction with magnetic field is smaller than that without magnetic field. Theintensity at x = 0 is higher as magnetic field strength islarger. This shows the plasma is suppressed by the magnetic field in y direction and does not expand across themagnetic field.
We examined the distance from initial target position(x, y) = (0, 0) to rising edge of plasma intensity along thedirection of 45◦ from x-axis as shown in Fig. 4 (f). We assumed the rising edge of plasma intensity shows the interaction between the expanding plasma and the magnetic
field. Figure 6 gives plots of the distance as a function oftime with several magnetic field strength. Figure 6 indicates the deceleration of plasma in the magnetic field over
0.67 T because the distance from initial target position torising edge of the plasma intensity with 0.46 T is larger
than that with over 0.67 T. As for 0.67 T, κ value of 0.077
is much smaller than the critical value κc = 0.4 because theplasma is ejected only in laser irradiation side and κc = 0.4
is overestimated in the present experiment. This indicatesthat the larger magnetic field with small κ is required todecelerate the plasma.
In the future plan, we will examine thrust eﬃciency
and compare the results of experiment with simulation re-We measured the plasma emission with and without
an external magnetic field to investigate the interaction between plasma and a magnetic field in a magnetic thrustchamber. The plasma expansion in both −x and ±y directions is suppressed by the magnetic field. In addition, thelight intensity in +x direction is higher as magnetic fieldstrength is larger. Therefore, the plasma expands to +x
direction more, which contributes to obtain thrust as themagnetic field strength is larger. We also calculated thecriterion and confirmed the plasma deceleration in a magnetic field over 0.67 T.

Acknowledgement
Authors would like to thank Mr. Eiji Sato for his exceptional support during this experiment. This work was
supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (Grant Numbers 25420852 and15K18283) and by the joint research project of the Institute of Laser Engineering, Osaka University (2014B1-31).
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