plasma should be treated kinetic theory, and the collisionless property makes
plasmas very different from neutral gases and fluids.
3. Compare the resistivity of plasma to the other conducting metals in the same unit
of (2.4.23). For example, cupper is 2 Â 10
À10 and stainless steel is 7 Â 10
À9 . In
high-temperature plasmas, the temperature goes to more than 1 keV, and
conductivity becomes much higher than such metals used in our life.
4. Tokamak plasma for fusion is heated up by the joule heating initially. It is,
however, difficult to increase the temperature to initiate fusion reaction
(10 keV), because of abrupt decrease of resistivity with increase of temperature.
Additional heating such as wave heating (RF heating) or neutral beam injection
heating is carried out.
5. Since the mean free path is electron energy dependent as shown in (2.4.20
0 ), high
energy component of electrons predominantly accelerated without collision in a
strong electric field. Such electrons called run-away electrons, and the
distribution function becomes to deviate from Maxwellian. The high-energy tail
component is observed in high-temperature plasma production with electric field
acceleration.
2.4.5 Relaxation Time to Thermal Equilibrium
Consider the case where the velocity distribution functions of electrons and ions are
not in Maxwell distribution. According to the statistical mechanics, the particles
exchange their momentum and energy through collisions and eventually become the
Maxwell distributions with the same temperature after substantial time passes. It is
well-known that the temperature equilibrium between ions and electrons takes a long
time because of large mass ratio; consequently two Maxwellian with T i and T e are
usually assumed.
In the case of molecular gas with same collision cross section σ and the same
mass, the relaxation time to Maxwell distribution τ M can be estimated
τ M ¼
ℓ
v
h i
¼
1
nσ v
h i
ð2:4:24Þ
Inserting the thermal velocity < v > to (2.4.24), it is reasonable to regard that the
particle velocity distribution relaxes to Maxwellian with the time scale of (2.4.24). In
fact, the relaxation time is proportional to 1/v, and it takes long time for particles
with low velocity to tend to Maxwell distribution; however, it is only proportional to
1/v and not so strong dependence on the velocity compared to the case of plasma.
In charged particle collision, it is characteristic that the collisional relaxation time
τ is proportional to the velocity as follows:
62
2 Laser Absorption by Coulomb Collision
plasmas very different from neutral gases and fluids.
3. Compare the resistivity of plasma to the other conducting metals in the same unit
of (2.4.23). For example, cupper is 2 Â 10
À10 and stainless steel is 7 Â 10
À9 . In
high-temperature plasmas, the temperature goes to more than 1 keV, and
conductivity becomes much higher than such metals used in our life.
4. Tokamak plasma for fusion is heated up by the joule heating initially. It is,
however, difficult to increase the temperature to initiate fusion reaction
(10 keV), because of abrupt decrease of resistivity with increase of temperature.
Additional heating such as wave heating (RF heating) or neutral beam injection
heating is carried out.
5. Since the mean free path is electron energy dependent as shown in (2.4.20
0 ), high
energy component of electrons predominantly accelerated without collision in a
strong electric field. Such electrons called run-away electrons, and the
distribution function becomes to deviate from Maxwellian. The high-energy tail
component is observed in high-temperature plasma production with electric field
acceleration.
2.4.5 Relaxation Time to Thermal Equilibrium
Consider the case where the velocity distribution functions of electrons and ions are
not in Maxwell distribution. According to the statistical mechanics, the particles
exchange their momentum and energy through collisions and eventually become the
Maxwell distributions with the same temperature after substantial time passes. It is
well-known that the temperature equilibrium between ions and electrons takes a long
time because of large mass ratio; consequently two Maxwellian with T i and T e are
usually assumed.
In the case of molecular gas with same collision cross section σ and the same
mass, the relaxation time to Maxwell distribution τ M can be estimated
τ M ¼
ℓ
v
h i
¼
1
nσ v
h i
ð2:4:24Þ
Inserting the thermal velocity < v > to (2.4.24), it is reasonable to regard that the
particle velocity distribution relaxes to Maxwellian with the time scale of (2.4.24). In
fact, the relaxation time is proportional to 1/v, and it takes long time for particles
with low velocity to tend to Maxwell distribution; however, it is only proportional to
1/v and not so strong dependence on the velocity compared to the case of plasma.
In charged particle collision, it is characteristic that the collisional relaxation time
τ is proportional to the velocity as follows:
62
2 Laser Absorption by Coulomb Collision