２０１５

Master's Thesis
Department of Advanced Energy Engineering Science,
Interdisciplinary Graduate School of Engineering Sciences,
Kyushu University

Title
Measurement of plasma structure in a Magnetic Thrust
Chamber

Name

Supervisor

Ryosuke Kawashima

Associate professor

Naoji Yamamoto

Assistant Professor

Taichi Morita

Table of Contents
1. Introduction
1.1 Background research……………………………………………….....…………
1.2 Laser Propulsion…………………………………………………………...………
1.3 Laser Fusion Rocket and Magnetic Thrust Chamber ...…………………...…
1.4 Principle of Magnetic Thrust chamber …………….…………………………...
1.5 Purpose of Research...……………………………………………………………

1
2
3
4
5

2. Laser absorption rate and plasma deceleration
2.1 Laser intensity……..……………………………………………….......................
2.2 Absorption Process and Absorption Rate………………………………………
2.3 Plasma deceleration………………………………………………………………

6
7
11

3. Measurement of plasma self-emission
3.1 Laser facility...……………………..…………………….…………………….
3.2 Source of magnetic field.………………………………….…………………….
3.3 Experimental setup and conditions ………………...…………………………
3.4 Results………………………………………………………….………………
3.5 The image of line plots along x and y direction………………………………

13
13
15
17
21

4. Measurement of electron density
4.1 Laser facility...……………………..…………………….…………………….
4.2 Experimental setup.………………………………….…………………….……
4.3 Experimental conditions ………………...……………………………………
4.4 Mach-Zehnder interferometry……………………………….………………
4.5 Analysis method of Mach-Zehnder interferometry……………………………
4.6 Electron density……………………………….…………………………………
4.6.1 Time development of electron density with several laser
conditions…………
4.6.2 Electron density with several laser energy………………………………
4.6.3. Electron density with one or six beams irradiation………………………
4.7 Comparison the results between two experiments……………………………

24
24
25
26
27
29
29
32
33
34

5 Summary36

References37

Acknowledgement

40Chapter 1: Introduction
1.1.

Background research

In 1961, President John F. Kennedy announced the Apollo program, a project toland a man on the Moon and return him to the earth safely by the end of the 1960s. On
Apollo 11 mission, astronauts Neil Armstrong and Buzz Aldrin landed on the Moon
and walked on the lunar surface. Since the mission, five subsequent Apollo missions
landed on the Moon by 1972. The Apollo program contributed the development of thetechnologies relating to a rocket and a manned space flight, especially electronics andtelecommunications. In addition, the space exploration has been greatly developed
and many discoveries have been found to date.
In 2010, a half of a century later since the announcement of President John F.
Kennedy, President Barack Obama announced the manned flight to Mars by the
middle of 2030s [1]. The Mars exploration is expected to provide new information to
elucidate the planet formation process. One of the reason that the planet formation
process has not been elucidated yet is that the initial conditions of the primitive solar
nebula (mass, temperature, and elemental composition) is still unknown. However,
the evidence of the primitive solar nebula has been already lost. Therefore, it isimportant to know the chemical composition and the isotopic composition of aterrestrial planet, such as the earth and Mars, to approximate the initial conditions.
Mars is the nearest terrestrial planet from the earth so that the Mars exploration is arealistic plan and is expected to elucidate the planet formation process.
The Mars exploration is also expected to forecast what will happen on the earth
in the future. Opportunity, an American unmanned Mars probe landed on Mars in
2004, found a convincing evidence that water existed on Mars in the past [2]. To exist
a liquid water stably, the thicker layer of the atmosphere than that at present isnecessary, so that it is thought that the thicker layer existed on primitive Mars and it
was blown away. By elucidating the reason of the disappearance of the Mars
atmosphere, we can forecast the possibility that the atmosphere of the earth will
disappear in the future and plan the measures. The Mars exploration is expected toprovide an information about the past and future earth.
In recent years, Mars is interested for not only the exploration but also the destinationof the human settlement. Mars One, a Not-for-Profit Organization (NPO) in the
Netherland, proposed to send four astronauts to Mars by 2025 and to settle them there1permanently [3]. Mars One recruited the members in 2013 and plan to start thetraining in 2015. Mars is getting to be a familiar with us.
However, the round trip to Mars with conventional rockets, mainly categorized into achemical rocket (high-thrust and low specific impulse) takes more than 500 days. Thelong mission time causes the physical and the mental damages on the astronauts, 1) aloss of a muscle and a bone mass due to a state of zero gravity, 2) the mental damage
due to living in a small room of a spaceship for a long time, and 3) the exposure of thecosmic ray. The problem of a muscle and a bone loss can be solved to some extent by
a continuous exercise and taking a protein and vitamin D [4]. However, withpresent-day technology, the complete solution of a mental damage and the exposureof the cosmic ray is not found. Therefore, a high-speed interplanetary spaceship ispreferable for a manned Mars exploration mission in order to shorten the mission time
as much as possible.

1.2 Laser Propulsion
As an interplanetary spaceship, a sail propulsion [5], a nuclear electric propulsion [6],and so on have been developed. In recent years, a laser propulsion has taken an
attention as the development of laser technology. Laser propulsion is a system
obtaining a thrust by controlling an exhaust direction of a laser-driven plasma. It has aunique characteristic, being able to control individually the energy source and thepropellant mass, resulting in the controllability of thrust parameters, such as specificimpulse, in wide range [7, 8].
With a conventional chemical rocket, its momentum per propellant mass isdetermined constantly because the generated energy of a propellant depends on its
chemical potential. Therefore, the performance of the rocket can be determined
automatically by selecting the propellant.
On the other hand, with laser propulsion, the energy injected into a propellant can be
determined voluntarily. That is, it can determine either utilizing a lot of a fuel orgetting a lot of the exhaust velocity (i.e. energy) to obtain a momentum. For example,
a high specific impulse mode can be set for an orbital transfer vehicle, requiring thesaving of a propellant. And a high momentum coupling coefficient mode can be set
for a ground launch vehicle, requiring a high thrust instead of a high specific impulse.
For an interplanetary flight, the required thrust and specific impulse are varied
depending on the orbit of the spacecraft. Therefore, laser propulsion can be designed
to be optimized during transferring an orbit.

2

1.3.

Laser Fusion Rocket and Magnetic Thrust Chamber

Laser Fusion Rocket (LFR) is one of the laser propulsion which utilizes alaser-driven fusion plasma and converts the thermal energy into the kinetic energy.
According to Einstein’s mass-energy equivalence, written as 𝐸 = 𝑚𝑐 2 , a fusion
reaction releases the energy of 10 MeV which is a hundred thousand times larger thanthat released by a chemical reaction. Therefore, LFR can achieve large thrust, andreduce the mission time of the round trip to Mars up to 100 days. As a design concept,
a Vehicle for Interplanetary Space Transport Application Powered by Inertial
Confinement Fusion (VISTA) is proposed by Lawrence Livermore National
Laboratory (LLNL) [9]. Since then, the study for the realization of LFR, such as
shielding from the neutron radiation, has been conducted [10-14].
As the propulsion system of LFR, a “Magnetic Thrust Chamber” is being thoughtin the proposal. Magnetic thrust chamber obtains a thrust by transferring the random
velocity components of the high-temperature plasma into the momentum withmagnetic field. Since a fusion plasma has high energy, the damage on the chamber
wall due to the collision of the plasma is concerned. However, magnetic thrustchamber can control the plasma to avoid the damage on the chamber wall. As a result,the lifetime improvement of the propulsion system can be expected. The mechanismof the thrust generation of magnetic thrust chamber is described in the next section.
Finally, the merits of LFR are summareized below.
1) Since a fusion plasma releases a high energy, LFR can generate a high thrustand voyage with high speed, resulting in the reducing the mission time of theinterplanetary flight.
2) The relation between the laser energy and the propellant mass is independent
with a laser-produced plasma, so that the plasma exhaust velocity and masscan be controlled in wide range. The plasma mass can be controlled bycontrolling a ratio of propellant mass to a total target mass. Therefore thesystem can be arranged depending on missions (i.e. a variability of theimpulse bit).
3) The damage on the chamber wall can be reduced by controlling the directionof the fusion plasma, resulting in the lifetime improvement of the magnetic
thrust chamber.

3

1.4.

Principle of Magnetic thrust chamber

Figure 1.4 shows a mechanism of a thrust generation of a magnetic thrust chamber.
(a) An electromagnetic coil generates a magnetic field. The plasma expands inthe magnetic field of the coil.
(b) Diamagnetic current cancel out the initial magnetic field of the coil. The
plasma compresses the magnetic field around the plasma.
(c) The compressed magnetic field pushes back the plasma and the spaceshipis accelerated by the reaction.
For a realization of LFR, demonstrating magnetic thrust chamber is essential.
However, magnetic thrust chamber has never been demonstrated in the proposal of
LLNL. Therefore, our group aims to demonstrate a magnetic thrust chamber and our
final goal is to construct the magnetic thrust chamber obtaining a thrust with a fusion
plasma.

Fig. 1.4 The mechanism of thrust generation of magnetic thrust chamber [4]. (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 the magnetic field and magnetic field
is compressed. (c) The compressed magnetic field pushes back the plasma and thespaceship is accelerated by the reaction.

4

1.5.

Purpose of research

Our group has utilized the simulation codes, 3-D hybrid PIC code and a 1-D
radiation hydrodynamic code [15-17]. 3-D hybrid PIC code simulates the movementof a plasma in a magnetic field by treating ion and electron as a particle and a fluid,
respectively. A radiation hydrodynamic code simulates the formation process of alaser-produced plasma. Miwa modified the 1-D radiation hydrodynamic code into 2-D
radiation hydrodynamic code and simulated the behavior of the laser-produced plasma
depending on the shape of the target pellet [18]. Tominaga used 3-D hybrid PIC codeand 2-D radiation hydrodynamic code to simulate the spatial distribution of the ion
current in a magnetic thrust chamber and could simulate the plasma behavior in therear area (See Fig. 1.5) [19]. However, the thrust deduced from the numerical code isthree times larger than that measured by experiment. In addition, the ion current
profile at high angle of simulation codes behaves differently with that of experiment.
Moreover, the ion current at low angle of simulation codes has not matched with thatof the experiment quantitatively yet.
Our group has also conducted the experiments of a magnetic thrust chamber.
Maeno et al confirmed that magnetic thrust chamber generated a thrust due to theinteraction between the plasma and the magnetic field of a permanent magnet [20].
Yasunaga et al measured the time variation of the magnetic flux density between amagnet and a target and found the diamagnetic cavity [21]. Maeno et al investigatedthe dependence of the incident laser wavelength and energy on the impulse of amagnetic thrust chamber [22]. They found that the impulse with the laser frequency ofω (the fundamental wavelength of 1,053 nm) was smaller than that of 2ω and 3ω,which the impulses of 2ω and 3ω were similar. They also found that the impulse
increased as the incident laser energy increased, however, the experimental values of
the impulses were far from the theoretical values. Therefore, to unravel the difference,the state of the plasma and a magnetic field (e.g. density, temperature, spatial
distribution of plasma, and time variation of magnetic flux density) needs to beobserved. Yasunaga et al. have examined the interaction between magnetic field andlaser-produced plasma, and observed the diamagnetic cavity [23]. Hinaga used
Faraday type charge collectors to measure the ions and observed that the momentumof the ions was changed by a magnetic field. He also conducted the experiment using
Thomson parabola and measured the carbon ion with the degree of ionization of 3, 4,
and 5 [24]. However, the plasma structure in a magnetic thrust chamber has neverbeen observed experimentally, so we examine the plasma structure in a magnetic thrustchamber by observing the light emission of a laser-produced plasma with several magnetic fieldstrength and electron density with several magnetic field.

5Chapter 2
Laser absorption rate and plasma deceleration
2.1.

Laser intensity

Laser intensity, I, is an index representing a performance of a pulsed laser[25-32]. When a laser (with the laser pulse width of τ, the laser energy of EL, and thelaser spot radius of rspot) irradiates a solid as shown in Fig. 2.1, laser intensity
represents the laser power per unit area, written as Eq.(2.1).
𝐼=𝐸𝐿
2𝜋𝑟𝑠𝑝𝑜𝑡𝜏(2.1)

Fig. 2.1. The diagram of irradiating a target with laser spot radius of rspot.
After the laser irradiates a solid, the incident laser is reflected at the point of thecritical density (See Section 2.1.2). This critical density, nc, is written as,
4𝜋 2 𝑚𝑒
𝑛𝑐 =
,
(2.2)
𝜇0 𝑒 2 𝜆2
where me is the electron mass, μ0 is a magnetic permeability at vacuum, e is anelementary charge, and λ is a laser wavelength. Then, the laser wavelength can be
written as,
2𝜋𝑐
𝜆=
,
(2.3)
𝜔
where c is the speed of light.
6

2.2.

Absorption Process and Absorption Rate

If the high-power laser irradiates a solid, the electron oscillates by electric field of
the incident laser. The oscillation of the electron gives the ion the photon energy of
the incident laser due to a collision of electron with the photon or the ion [25-32]. As
Fig. 2.2 shows, the plasma, with the electron density decreasing in +x direction
exponentially, is produced by the incident laser coming from left. This absorption
process is called inverse bremsstrahlung. The absorption rate of the incident laser
energy is derived here.

Fig. 2.2. The diagram of irradiating a solid with a laser:
The electron density decreases exponentially as leaving from the solid. The criticalpoint is where the electron density is equal to the critical density.
When the laser with its electric field, E, irradiates a plasma, the equation of motionof the electron can be written as Eq.(2.4), including the effects of the dispersion andcollision with the ion.
𝑑𝒗𝑚𝑒
= −𝑒𝑬 − 𝑚𝑒 𝜐𝑒𝑖 𝒗 ,(2.4)
𝑑𝑡
where v is an electron velocity and νei is collisional frequency of ion and electron.
Assuming that the ion is at rest, the current density, j, can be written by only electron
motion.
𝒋 = −𝑛𝑒 𝑒𝒗 = 𝜎𝑬 ,(2.5)
where σ is the electrical conductivity.
Assuming that the electric field changes as 𝑬 = 𝐸𝑒𝑥𝑝(−𝑖𝜔𝑡 + 𝑖𝒌 ∙ 𝒓), where k is awavenumber vector and r is a position vector. Eq.(2.4) can be rewritten as follows.
𝑑𝒗𝑒𝐸+ 𝜈𝑒𝑖 𝒗 = −𝑒𝑥𝑝(−𝑖𝜔𝑡 + 𝑖𝒌 ∙ 𝒓)
𝑑𝑡𝑚𝑒
Solving this equation for v.
𝑒𝑬
𝒗=−
(2.6)
𝑚𝑒 (𝜈𝑒𝑖 − 𝑖𝜔)7Substituting Eq.(2.6) into Eq.(2.5), j and σ can be obtained as follows:
2
𝑖𝜔𝑝𝑒
𝜀0 𝑬
𝒋=
𝜔 + 𝑖𝜈𝑒𝑖

𝜎=

(2.7)

2
𝑖𝜔𝑝𝑒
𝜀0
𝜔 + 𝑖𝜈𝑒𝑖2
where ωpe is a plasma frequency, which can be expressed as 𝜔𝑝𝑒= 𝑛𝑒 𝑒 2 /𝜀0 𝑚𝑒 .
The laser intensity can be written by using a group velocity of an electromagnetic
wave, vg.

𝐼 = 𝑣𝑔 𝜀0 𝑬2(2.8)

The energy conservation law holds between an energy provided by a laser and a jouleheat consumed by a plasma. With using Eq.(2.7) and Eq.(2.8), the following equation
can be derived.
𝛻 ∙ 𝑣𝑔 𝜀0 𝑬2 = −𝑅𝑒(𝒋 ∙ 𝑬∗ )
2
𝑖𝜔𝑝𝑒
𝜀0
= −𝑅𝑒(𝑬 ∙ 𝑬∗ )𝜔 + 𝑖𝜈𝑒𝑖2
𝜈𝑒𝑖 𝜔𝑝𝑒
𝜀0 𝑬2=−2𝜔 2 + 𝜈𝑒𝑖(2.9)

= −𝛼𝑐 𝜀0 𝑬2 ,(2.10)
where αc is an energy absorption rate in a plasma and * stands for complex conjugate.
Equation (2.9) implies that a plasma is not heated up if νei=0 (i.e. no dispersion andcollision due to the ion, resulting in no joule heat). Define an absorption coefficient,Ka, by the following equation. The absorption coefficient is an index how much light
is absorbed into a medium when light irradiates into the medium.
𝛼𝑐𝐾𝑎 =(2.11)
𝑣𝑔
The following equation describes the propagation equation of the electromagnetic
wave varied with 𝑒𝑥𝑝(−𝑖𝜔𝑡 + 𝑖𝒌 ∙ 𝒓) from Maxwell equation. Here, Fouriertransform (𝜕⁄𝜕𝑡 = −𝑖𝜔) is used.
∇ × (∇ × E) − 𝜇0 𝜀0 𝜔2 𝐸 + 𝜇0 𝜀0

2
𝜔𝑝𝑒
𝜔
𝐸=0
𝜔 + 𝑖𝜈𝑒𝑖

𝛻 2 𝐸 − 𝛻(𝛻 ∙ 𝐸) +where ε is a permittivity of plasma and is expressed as,𝜔2
𝜀𝐸 = 0 ,𝑐2(2.12)2𝜔𝑝𝑒
𝜀 = 1−.
𝜔(𝜔 + 𝑖𝜈𝑒𝑖 )The dispersion relation to an electromagnetic wave can be derived from Eq.(2.12) and8can be written by the following equation. The propagation direction of the wave andthe direction of the electric field are perpendicular, resulting in 𝒌 ∙ 𝑬 = 0. Fouriertransform (𝛻 = 𝑖𝒌) is used.
𝜔𝑝𝑒 2
𝑐𝑘
𝜔
( )2 = 1 − ()(2.13)
𝜔𝜔
𝜔 + 𝑖𝜈𝑒𝑖
By solving Eq.(2.13) for k, the real part kr, the real wavenumber, and the imaginary
part ki, a spatial decrement, are obtained. In addition, the group velocity of anelectromagnetic wave, vg, can be obtained from kr. Assuming 𝜔 ≫ 𝜈𝑒𝑖 , Eq.(2.13)
becomes as,𝜔𝑝𝑒 2𝜔
𝑘𝑟 = √1 − ()(2.14)
𝑐
𝜔
𝜔𝑝𝑒 2
𝜕𝜔
𝑣𝑔 =
= c√1 − (
)
𝜕𝑘𝑟
𝜔
𝑘𝑖 =

𝜔𝑝𝑒 2
1 𝜈𝑒𝑖 𝜔𝑝𝑒 2() /√1 − (
) .
2 𝑐 𝜔
𝜔

(2.15)

Equation (2.14) implies that the incident laser reflects at the point of the criticaldensity. When 𝜔 > 𝜔𝑝𝑒 (i.e. 𝑘𝑟 > 0), the incident laser propagates in the region of alow density plasma with being absorbed as shown in Fig. 2.2. However, at the point of
the critical density when 𝜔 = 𝜔𝑝𝑒 (i.e. 𝑘𝑟 = 0), the electromagnetic wave cannot
propagate forward anymore and reflects at this point.
The collisional frequency of electron and ion can be expressed as𝜈𝑒𝑖 =𝑍 2 𝑒 4 𝑛𝑖 ln Λ3(2𝜋𝑘𝑇𝑒 )3/2 𝜀02 √𝑚𝑒,(2.16)

where Z is the degree of ionization, ni is the ion density, lnΛ is a Coulomb logarithm,
and Te is the electron temperature.
Absorption coefficient has a relation of 𝐾𝑎 = 2𝑘𝑖 . The absorption coefficient Ka
could be obtained by using this relation, Eq.(2.15), and Eq.(2.16).
𝐾𝑎 ==𝑍 2 𝑒 4 𝑛𝑖 ln Λ𝜔𝑝𝑒 2
1 𝑛𝑒 𝑒 2√1 − (/)
2𝜔
3(2𝜋𝑘𝑇𝑒 )3/2 𝜀02 √𝑚𝑒 𝑐𝜔 𝜀0 𝑚𝑒
𝑍 2 𝑒 6 𝑛𝑒 𝑛𝑖 ln Λ3𝜔 2 𝑐𝜀03 (2𝜋𝑚𝑒 𝑘𝑇𝑒 )3/2 [1 − (𝜔𝑝𝑒 /𝜔)2 ]1/2

Then the absorption fraction, ηa, could be derived from the absorption coefficient.
Assuming the plasma whose electron density on the surface of a solid is linearlyvaried with 𝑛𝑒 = 𝑛𝑐 (1 − 𝑥⁄𝐿) as shown in Fig. 2.3 and that the laser irradiates theplasma with the incident angle θ against the direction of the electron density gradient.
The variation of the laser intensity in the x direction can be expressed byLambert-Beer law as,𝑑𝐼= −𝐾𝑎 𝐼 .
(2.17)𝑑𝑥9

By considering that the incident laser reflects at the point of the critical density (i.e.
the path of the incident laser is 2L), the absorption fraction can be obtained byintegrating Eq. (2.17).
𝐿𝜂𝑎 = 1 − exp (−2 ∫ 𝐾𝑎 𝑑𝑥) ,(2.18)

0where L is the characteristic length of the density gradient.

Fig. 2.3. The incident laser propagating in plasma with incident angle of θ:
Assuming that the electron density decreases linearly, the incident laser is reflected at
the critical point and the most laser energy is absorbed there.
Next,

the

electric

field

of

the

incident

laser

is

modifiedby

𝐸 = 𝑅𝑒(𝐸(𝑥) 𝑒𝑥𝑝(−𝑖𝜔𝑡 + 𝑖𝑘𝑦 𝑦)) [27]. The propagation equation to the electricfield can be obtained by WKB (Wentzel-Kramers-Brillouin) approximation.
d𝑣𝑔𝑥
𝑑𝐸(𝑥)
𝐸(𝑥) + 2𝑣𝑔𝑥
+ 𝜈𝑒𝑖 𝐸(𝑥) = 0dx𝑑𝑥
𝑘𝑥 𝑐 2
𝑣𝑔𝑥 =
𝜔
𝑘𝑥 =

𝜔2
𝜔
√1 − 𝑝𝑒 − sin2 𝜃𝑐𝜔2,where vgx is a group velocity in x direction, and kx is a wavenumber in x direction, andky is a wavenumber in y direction. ky is expressed as 𝑘𝑦 = (𝜔/𝑐)𝑠𝑖𝑛𝜃.
The absorption fraction can be derived in the same way with the derivation of theformer absorption fraction. Then, the absorption fraction can be obtained as follows.
32𝜈𝑒𝑖𝑐 𝐿
𝜂𝑎 = 1 − exp {−cos 5 𝜃} ,15𝑐where νeic is a collisional frequency of electron and ion at the critical point.
When the incident laser irradiates in perpendicular to the surface of the solid (i.e.
θ=0), the absorption fraction can be simplified as1032𝜈𝑒𝑖𝑐 𝐿} .
15𝑐
The absorption fraction can be represented as a function of laser intensity in W/cm2
as follows [27].
𝐿𝜔𝜂𝑎 ln(1 − 𝜂𝑎 )−1 = 1011
(2.19)𝑐𝐼
Assuming L=50 μm, the absorption fraction as a function of the incident laser
intensity is obtained by Eq.(2.19), as shown in Fig. 2.4.
𝜂𝑎 = 1 − 𝑒𝑥𝑝 {−Fig. 2.4. The absorption fraction with respect to incident laser intensity.

2.3.

Plasma deceleration

Parameter κ shows plasma deceleration in a magnetic field. Nikitin et al. havediscussed the dynamics of the 3D expansion of a spherical cloud of rarefied plasma
into a vacuum in the presence of a non-uniform external magnetic field of dipole
structure, in the framework of ideal MHD approximation, and described how todetermine the configuration and location of the plasma front which are caused by theretardation effect [34]. In addition, they defined an energetic criterion κ thatcharacterizes the interaction between the expanding plasma and the dipole field,
which is the same as adopted here and κ is given as𝐸𝑝
12𝜋𝐸0 𝑅03𝜅==,𝐸𝑀
𝜇0 |𝜇𝑑 |2
where 𝐸𝑝 is the kinetic energy of ions, 𝐸𝑀 is the field energy integral of the dipole
beyond the spherical radius 𝑅0 (𝐸𝑀 = (𝜇0 ⁄4𝜋)|𝜇𝑑 |2 ⁄(3 𝑅03 )), 𝑅0 the distance from11the magnetic coil to the explosion location, 𝜇0 is the vacuum magnetic permeability,and |𝜇𝑑 | 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 the explosion location. (Whenthe plasma is located at the axis, the critical value is 𝜅𝑐 = 0.4) [34]12Chapter 3
Measurement of plasma self-emission
3.1.

Laser facility

This experiment was conducted at the Extreme Ultra-Violet (EUV) databasefacility of the Institute of Laser Engineering (ILE) at Osaka University. Fig. 3.1 showsa vacuum chamber. A single beam irradiates a target to produce a plasma.

Fig. 3.1 The vacuum chamber of EUV facility

3.2.

Source of magnetic field

Magnetic field was generated by flowing current into an electromagnetic coilwith 96-turn cupper wire with the inner radius of 13 mm, outer radius of 25 mm andthe thickness of 10 mm. The current was generated by a capacitor bank, in which
three 32 mF capacitors were connected in parallel as shown in Fig. 3.2. The magnetic
field of 1.1 T was generated at the initial target position by applying 500 V on thecapacitor bank and flowing current of about 1100 A. The current was measured with acurrent probe. To calculate the magnetic field strength, a magnetic field was measured
by using a Gauss meter before the experiment and a current was also recorded
simultaneously. Therefore, the relation between a current and a magnetic field can beobtained. Then, by recording the current flowing in the coil during the experiment, the13

magnetic field can be calculated by using the relation.
Figure 3.3(a) shows the coil we used and Fig. 3.3(b) is the field strength at atarget position as a function of time. The field lasts about 10 ms. In the experiment,
the laser irradiates the target at the peak of the magnetic field strength. The time-scaleof the experiment is in micro-second which is much smaller than the time duration of
the magnetic field. Therefore, the field strength can be considered as constant during
the plasma expansion.
Figure 3.4 shows the initial magnetic field configuration generated by theelectromagnetic coil. Initial target position corresponds to the coordinate (x, y) = (0,0).

Fig. 3.2 Capacitor bank for generating current

Fig. 3.3 The electromagnetic coil and the time variation of magnetic field:
The size of the coil used in this experiment is inner radius of 13mm, outer radius of
25 mm, and thickness of 10 mm.

14

Fig. 3.4 Initial magnetic field configuration generated by the electromagnetic coil

3.3.

Experimental setup and conditions

The experimental setup is shown in Fig. 3.5. A plasma was created by focusing
a 1064 nm Neodymium: Yttrium Aluminum Garnet (Nd:YAG) laser onto apolystyrene ([-CH2-CH(C6H6)-])n spherical target with a diameter of 500 µm (Fig.
3.6). The pulse width of the laser is 9.5±0.5 ns and the laser energy is 6.0±0.8 J as
shown in Table. 3.1. The target is suspended by a carbon fiber attached to a glass rod
to reduce the plasma formation of the glass rod. The distance between the coil surface
and the target is 11 mm.
The emission from the plasma was collected with a lens (focal length of 200mm) and imaged onto intensified charge coupled devices (ICCDs) at a wavelength of
660 nm with a band-path filter with the width of 10 nm (FWHM). Two ICCDs wereused to take images of different delay times in a single laser-shot. The self-emission iscomposed of H- and thermal bremsstrahlung emissions.
The relation between the magnetic field strength and parameter 𝜅 are as shownin Table. 3.2. Plasma deceleration was assumed to occur in the magnetic field of over
0.46 T.

15

Fig. 3.5 Experimental setup

Fig. 3.6 Polystyrene target pellet with the diameter of 500 μm.
Table. 3.1 Condition of Nd: YAG Laser
Number of incident laser

1 beam/shot

Wavelength1064 nm

Pulse width

9.5±0.5 ns

Laser energy

6.0 ±0.8 J

Table. 3.2 Parameter 𝜅 with several magnetic field strength
The magnetic field strength [T]
𝜅
0.23
0.67
0.46
0.17
0.67
0.077
0.89
0.044
1.1
0.029

16

3.4.

Results

Figures 3.7-3.12 show the light emission from the plasma at 0.1 s, 0.2 s, 0.3
s, 0.5 s, 1.0 s, 1.5 s after plasma generation with the magnetic field of 0, 0.23,
0.46, 0.67, 0.89, and 1.1 T, respectively. In these figures, the target is set at thecoordinate (x, ) = (0, 0) and is irradiated with the laser from left side through thehole of the coil. The center axis of the magnetic field corresponds to the x-axis at y =0.
Without the magnetic field, the plasma expands to -x direction and ydirections. With the magnetic field, the plasma expansion to –x and y direction issuppressed. Comparing the plasma emission in +y direction with that in –y direction,
the plasma intensities in +y direction is higher than that in –y direction. Since thehigh-energy plasma hit the carbon fiber attached to a glass rod, the fiber is ablated andthe ablation plasma emits light.

Fig. 3.7 The light emission from the plasma without and with the magnetic field of
0.23 T, 0.46 T, 0.67 T, 0.89 T, and 1.1 T at 0.1 s after laser irradiation.

17

Fig. 3.8 The light emission from the plasma without and with the magnetic field of
0.23 T, 0.46 T, 0.67 T, 0.89 T, and 1.1 T at 0.2 s after laser irradiation.

Fig. 3.9 The light emission from the plasma without and with the magnetic field of
0.23 T, 0.46 T, 0.67 T, 0.89 T, and 1.1 T at 0.3 s after laser irradiation.

18

Fig. 3.10 The light emission from the plasma without and with the magnetic field of
0.23 T, 0.46 T, 0.67 T, 0.89 T, and 1.1 T at 0.5 s after laser irradiation.

Fig. 3.11 The light emission from the plasma without and with the magnetic field of
0.23 T, 0.46 T, 0.67 T, 0.89 T, and 1.1 T at 1.0 s after laser irradiation.

19

Fig. 3.12 The light emission from the plasma without and with the magnetic field of
0.23 T, 0.46 T, 0.67 T, 0.89 T, and 1.1 T at 1.5 s after laser irradiation.

20

3.5.

The image of line plots along x and y direction

Left figures in Figs. 3.13 (a)-(f) show the light intensities from the plasma
averaged from = −4 o 4 mm along the x direction as shown in Fig. 3.7-Fig. 3.12
with six different magnetic field strength. Black line shows the plasma intensity
without the magnetic field. The red, green, blue, light blue, and pink lines show theplasma intensity with the magnetic field strength of 0.23 T, 0.46 T, 0.67T, 0.89 T, 1.1
T, respectively. The small peak of intensity without and with the magnetic field of
0.23 T observed at x = -11 mm because the high speed plasma reaches the surface ofthe coil at 0.2 s. The intensity with the magnetic field of 0.23 T is not as high as thatwithout magnetic field, meaning the plasma is decelerated. Plasma expands to –x
direction as time passes, but the shift of intensity is small and not so different among
the magnetic field strength over 0.67 T. This shows the energy of magnetic field ismuch larger than the kinetic energy of plasma. More plasma expands to +x direction,as magnetic field strength is larger from 0.5 s. Second peak is also observed in +x
direction from 1.0 s. This means that the plasma increases in the direction which
contributes to obtaining the thrust.
Right figures in Figs. 3.14 (a)-(f) show the light intensities from the plasma
averaged from x = −5 o 0 mm along the y direction as shown in Fig. 3.7-Fig.
3.12 with six different magnetic field strength. The width of plasma expansion to ydirection with magnetic field is smaller than that without magnetic field. The intensity
at x = 0 is higher as magnetic field strength is larger. This shows the plasma issuppressed by the magnetic field in y direction and does not expand across themagnetic field.

21

Fig. 3.13 The light intensities from the plasma along x direction at (a) 0.1 s, (b) 0.2
s, (c) 0.3 s, (d) 0.5 s, (e) 1.0 s, (f) 1.5 s after laser irradiation.,22

Fig. 3.14 The light intensities from the plasma along y direction at (a) 0.1 s, (b) 0.2
s, (c) 0.3 s, (d) 0.5 s, (e) 1.0 s, (f) 1.5 s after laser irradiation.,23

Chapter 4
Measurement of Electron density
4.1.

Laser facility

The measurement of electron density was conducted at laser facility in Osaka
University, called “Gekko XII facility”. Figure. 4.1 shows a vacuum chamber. GXII
facility is one of the biggest laser facility in the world. Plasma is generated by thespherical symmetric irradiation of lasers to target. (They have twelve beams and one
or six beams irradiate target in this experiment).

Fig. 4.1

4.2.

The vacuum chamber of GXII facility

Experimental setup

Electron density was measured by using Mach-Zehnder interferometer.
Experimental setup is shown in Fig. 4.2. Mach-Zehnder interferometer is a device
used to determine the relative phase shift variations between two collimated beams
derived by splitting light from a single source. A light beam from a probe laser (Verdi
V5, DPSS laser, wavelength: 532 nm) is split by first beamsplitter. One of thempasses through the plasma and the other passes through the vacuum. And then two
beams are recombined by second beamsplitter. These two beams interfere with eachother and make fringes. These fringes are measured by two ICCDs. We also use
24

Digital Delay and Pulse Generator (DG535) to adjust the laser timing and shutter
timing of two ICCDs. Laser energy is 2 W in this experiment, so we could get enoughlight intensity to measure fringe shift by the light refraction through the plasma.

Fig. 4.2

4.3.

Experimental setup (Mach-Zehnder interferometer)

Experimental conditions

In this experiment, electron density with several laser energy was measured
without and with the magnetic field strength of 1.1 T on the target position. Glasslaser was used to generate plasma and laser condition is as shown in Table 4.1.
Number of incident laser is 1 beam or 6 beams. The wavelength of laser is 1053 nm.
Pulse width is 1.3 ns. Laser energy is 10, 50, 100, 600 J/beam. Target is polystyrene
spherical target with the diameter of 500
and the thickness of 7
as shown in
Fig. 4.3. The ratio of plasma energy to magnetic energy, 𝜅 is calculated and shownin table. 4.2. 𝜅 is obtained by using the equation as shown the chapter of 4.2. Laser
energy is converted to the kinetic energy of plasma as shown in Table. 4.3.
Table 4.1 The condition of Glass laser
Number of incident laser
1 beam or 6 beam
Wavelength1053 nm
Pulse width
1.3 nsLaser energy
10, 50, 100, 600 J/beam25

Fig. 4.3

Target composed of polystyrene

Table 4.2 Parameter 𝜅 in several magnetic field
Laser energy and number of laser𝜅
10 J × 6
0.29
50 J × 6
1.11
100 J × 6
1.75
600 J × 1
0.79
600 J × 6
4.73

Table 4.3 Conversion of laser energy to kinetic energy of plasma
Conversion efficiency ofLaser energy and Laser intensity
Total kinetic energy
laser energy to kinetic2
number of laser[W/cm /beam]of plasma [J]energy of plasma [ ]10 J × 6
100
60
3.9 × 1012
13
50 J × 676
228
2.0 × 1012100 J × 6
100
360
3.9 × 1015600 J × 1
27
162
2.4 × 1015600 J × 6
27972
2.4 × 10

4.4.

Mach-Zehnder interferometry
Assuming target is ionized perfectly, refractive index is represented as follows.
11  pe 2 r  2  ne r  2
N  1   1 
nc  2 


(4.4.1)

Probe laser irradiates plasma in parallel to x-axis apart y from x-axis as shownin Fig. 4.4. 𝑛𝑒 is number density of electron. 𝑛𝑐 = 𝜀0 𝑚𝑒 𝜔2 /𝑒 2 is cut-off density
against electromagnetic wave angular frequency is . In this condition, 𝑛𝑐 = 1.1 ×
1015 𝜆−2 [𝑚−3 ], [ ] is the wavelength of probe laser in vacuum.
If spatial variation of refractive index of plasma is slow against incident
wavelength, Phase difference 𝜙( ) is represented as follows by using
26

WKB(Wentzel-Kramers-Brillouin) approximation.

  y    k 0  k p dx z1z2

𝑘𝑝 = 2𝜋⁄𝜆21  N dz
 
z1

z2

(4.4.2)and 𝑘0 is wave number in plasma and vacuum. N is refractive

index of plasma. 𝜙( ) is obtained as follows in the case of 𝑛𝑒 ≪ 𝑛𝑐 .

y 

1
 z
2 a
2
2 2






n
r
dz

n
r
r

y
rdr r  y ee
nc znc y1(4.4.3)2

𝑛𝑒 is obtained from 𝜙( ) by using abel inversion assuming plasma iscylindrical symmetric against the x-axis.
ne  

Fig. 4.4

4.5.

n c

  y 2  r 2  21 dy  y  r 

a d y 2 r

(4.4.4)dy

Optical path of probe laser in plasma

Analysis method of Mach-Zehnder interferometry

Fig. 4.5(a) and (b) are the fringe images detected by the ICCDs. Figure 4.5(a) is
the image of reference shot which shows the distribution of fringes before plasma
generation. Fig. 4.5(b) is the image of main shot which shows the distribution offringes at 200 ns after laser irradiation. Fringe shifts were measured in Fig. 4.5(b)because refractive index of air and plasma is different. Phase difference is calculated
from these fringe shift. Phase difference between light fringes is 2 .

27

Fig. 4.5 The images of fringes measured by Mach-Zehnder (a)The distribution offringes before laser irradiation in Reference shot (b)The distribution of fringes at t=
200 ns after laser irradiation in main shot
“Neutrino” is used for the calculation of the phase shift from the distribution offringes. At first, the number of fringes is calculated by wavelet transformation. At
second, the phase map is obtained by unwrapping the phase data. These work aredone in reference data and main data. At third, the phase difference is calculated bysubtracting the reference data from main data as Fig. 4.6(a). Finally, electron density
is calculated as shown Fig. 4.6(b) by using abel inversion assuming plasma iscylindrical symmetry with z-axis.

Fig. 4.6 (a) The distribution of phase difference at t = 200ns after laser irradiation(b) The distribution of electron density at t =200ns after laser irradiation

28

4.6.

Electron density

4.6.1. Time development of electron density with several laser conditions
Figure 4.7 and 4.8 show time development of electron density with six laser
irradiation of 10 J/beam with the magnetic field strength of 1.1 T and without themagnetic field, respectively. Electron density is measured in this experiment, but
plasma behavior including ion can be measured because electron follows ion. In Fig.
4.7, plasma is stopped by the magnetic field and formed the structure like a shell at t =200 ns and 300 ns. Electron density is about 5.0×1017 cm-3 there. The electron density
decreases and that structure almost vanishes at 400 ns. In Fig. 4.8, shell structure likea shell by electron is not observed. Electron already expands outside of the field ofview. Black region is a region where neutral atoms exist because neutral particle showphase difference opposite to electron deducted by electrons. Similar structure of black
region was found with and without magnetic field, that is, neutral particle doesn’tinteract with magnetic field, of course.

Fig. 4.7 Time development of electron density with six laser irradiation of 10
J/beam with the magnetic field of 1.1 T at (a) 50 ns, (b)100 ns, (c)200 ns, (d)400 ns
after laser irradiation.

29

Fig. 4.8 Time development of electron density with six laser irradiation of 10
J/beam without magnetic field at (a)50 ns, (b)100 ns, (c)200 ns, (d) 400 ns after laser
irradiation
Time development of electron density with six laser irradiation of 100 J/beam
to CD target shell of without and with the magnetic field strength of 1.1 T as shown in
Fig. 4.9 and Fig. 4.10. CD shell target composed of carbon and deuterium with thediameter of 500
and the thickness of 7
was used in this shot because of theshortage of polystyrene shell target. Electron is stopped by the magnetic field andformed the structure like a shell at t = 200 ns and 300 ns. Electron density is about
5.0×1017 cm-3 there. The electron density decreases and that structure almost vanishes
at 400 ns. Then, electron expands to +x direction which contributes to obtain thrust. In
Fig. 4.10, structure like a shell by electron is not observed. Electron already expands
outside of the field of view.

30

Fig. 4.9 Time development of electron density with six laser irradiation to CD
target of 100 J/beam with the magnetic field strength of 1.1 T

Fig. 4.10 Time development of electron density with six laser irradiation to CD
target of 100 J/beam without the magnetic field31

4.6.2. Electron density with several laser energy
The distribution of electron density with several laser energy, 10 J×6, 50 J×6,100 J×6 with the magnetic field strength of 1.1 T at 50 ns and 200 ns is shown in Fig.
4.11. The distribution of electron density with several laser energy, 10 J×6, 50 J×6,100 J×6 without the magnetic field at 50 ns and 200 ns are shown in Fig. 4.12. With
the magnetic field strength of 1.1 T, plasma expands wider as laser energy is higher at
50 ns after laser irradiation. Then, plasma forms shell structure at 200 ns. The shellstructure extends more to +y direction as laser energy is higher. Without magnetic
field, plasma is generated and expands at 50 ns, but plasma expands outside of the
field of view at 200 ns as shown in Fig. 4.12.

Fig. 4.11 The distribution of electron density with several laser energy with themagnetic field strength of 1.1 T at 50 ns and 200 ns (a) 50 ns and 10 J×6, (b) 50 ns
and 50 J×6, (c) 50 ns and 100 J×6, (d) 200 ns and 10 J×6, (e) 200 ns and 50 J×6, (f)200 ns and 100 J×6

32

Fig. 4.12 The distribution of electron density with several laser energy without themagnetic field at 50 ns and 200 ns (a)50 ns and 50 J×6, (b) 50 ns and 100 J×6, (c) 50
ns and 100 J×6, (d) 200 ns and 10 J×6, (e) 200 ns and 50 J×6, (f) 200 ns and 100 J×6

4.6.3. Electron density with one or six beams irradiation
The distribution of electron density with one or six beams irradiation with themagnetic field strength of 1.1 T at 50 ns and 200 ns (a) 50 ns and 600 J×1, (b) 50 ns
and 100 J×6, (c) 200 ns and 600 J×1, (d) 200 ns and 100 J×6 is as shown in Fig. 4.13.
The distribution of electron density with one or six beams irradiation without themagnetic field at 50 ns and 200 ns (a) 50 ns and 600 J×1, (b) 50 ns and 100 J×6, (c)200 ns and 600 J×1, (d) 200 ns and 100 J×6 is as shown in Fig. 4.14. Plasma is
expanding in a magnetic field at t = 50 ns and is stopped by the magnetic field at t =200 ns. Kinetic energy of plasma with 100 J×6 irradiation is larger than that with 600
J×1, so the distribution of plasma with 100 J×6 irradiation is larger than that with 600
J×1 irradiation.

33

Fig. 4.14 The distribution of electron density with one or six beams irradiation
without the magnetic field at 50 ns and 200 ns (a)50 ns and 600 J×1, (b) 50 ns and
100 J×6, (c) 200 ns and 600 J×1, (d) 200 ns and 100 J×6

4.7.

Comparison the results between two experiments

Fig. 4.15 shows the comparison of plasma expansion between two experiments
about the measurement of plasma self-emission and electron density by using
Mach-Zehnder interferometer. Fig. 4.15(a) shows light intensity of plasma
self-emission with the magnetic field of 0.23 T with one beam of 6 J at 200 ns afterlaser irradiation (κ=0.67). This figure is calculated from light intensity of plasma
self-emission by using abel inversion. Fig. 4.15(b) shows the distribution of electron
density with the magnetic field strength of 1.1 T with one beam of 600 J at 200 ns
after laser irradiation (κ=0.79). Similar surface of the plasma expansion is observed inboth figures with similar κ. Fig. 4.16 shows the peak positions as a function of theangle θ derived from Fig. 4.15. Both results show similar tendency in the range 0 < θ< 60◦, while the peak positions from 600 J case are further than those from 6 J case at60◦ < θ. It is because plasma energy in the range (0 < θ < 60◦) is estimated by laser
energy and laser absorption rate and plasma interacts with magnetic field in the range(60◦ < θ).

34

Fig. 4.15 Comparison of the plasma expansion between two experiments in thesimilar value of 𝜅. (a) Light intensity of plasma self-emission with the magnetic fieldstrength of 0.23 T with one beam of 6 J at 200 ns after laser irradiation (b) The
distribution of electron density with the magnetic field of 1.1 T with one beam of 100
J/beam at 200 ns after laser irradiation.

Fig. 4.16 The peak positions as a function of the angle θ derived from Fig. 4.15

35

Chapter 5: Summary
Plasma emission with and without an external magnetic field was measured toinvestigate the interaction between plasma and magnetic field in a magnetic thrustchamber. The plasma expansion in both –x and y direction is suppressed by themagnetic field. In addition, the light intensity in +x direction is higher as magnetic
field strength is larger. Therefore, more plasma expands to +x direction, which
contributes to obtain thrust as magnetic field strength is larger. We also calculated thecriterion 𝜅 and plasma deceleration was measured with magnetic field of over 0.23 T.
Electron density was also measured by using Mach-Zehnder interferometer withseveral laser energy. Electron is suppressed by magnetic field and forms the structure
like a shell in a magnetic field of 1.1 T at 200 ns and 300 ns after laser irradiation.
Then, plasma expands to +x direction. Electron expansion for +y direction is smaller,
as laser energy is larger.
Comparison of the plasma expansion in a magnetic field between two
experiments are conducted in similar 𝜅. As a result, similar structure of the plasma
expansion is observed in both figures with similar 𝜅. It means that the structures
obtained from both experiments show more or less same, under the condition of thesimilar ratio of the plasma energy to the magnetic field energy. It shows 𝜅 is theimportant factor to establish the scaling rule for Laser Fusion Rocket.

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Acknowledgments
I would like to express the deepest appreciation to my supervisors; Professor
Hideki Nakashima, Associate Professor Naoji Yamamoto, and Assistant Professor
Taichi Morita.
I would like to express the appreciation to my advisers; Dr. Shinsuke Fujioka of
Osaka University, Dr. Atsushi Sunahara of Institute for Laser Technology, Dr.
Yoshitaka Mori of The Graduate School for the Creation of New Photonics Industries,
Dr. Tomoyuki Johzaki of Hiroshima University, Dr. Akifumi Yogo of Osaka
University, and Dr. Hiroaki Nishimura of Osaka University. I would also like toexpress the appreciation to Mr. Eiji Sato and GEKKO technical crews for their
exceptional support during the experiments.
I would like to appreciate Mr. Hasuo and Mr. Yamaguchi of Applied Quantum
Physics and Nuclear Engineering in Kyushu University for their support and advice of
manufacturing experimental equipment. I would like to appreciate Mrs. Takako
Mawatari who take care of my paperwork.
I would like to thank my labmates who conduct an experiment together in Osaka
University; Hiroshi Tominaga and Naoya Saito. In addition, I thank my labmates; Lee
Hou-Yi, Koichi Ushio, Yu Okuda, Kohei Takase, Kazuhiko Nakano, Atsushi
Yamaguchi, Kensuke Iijima, Tomohiro Ichimaru, Bunsuke Ueno, Yusuke Egawa,
Masafumi Edamoto, Satoshi Miura, Yutaro Itadani, Masataka Iwamoto, Yutaro
Kawahara, Hirotaka Fuchigami
This work was supported by the Japan Society for the Promotion of Science (JSPS)
KAKENHI (Grant Numbers 25420852 and 15K18283) and by the joint research
project of the Institute of Laser Engineering, Osaka University (2014B1-31).
Finally, I would like to express the deepest gratitude to my family for their kind
support and help.

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