plasma Flashcards
quasi neutrality
a plasma is made up of atoms and molecules that have been ionised, therefore has a mixture of neutral and charged species. The opposite charged particles are strongly coupled and tend to electrically neutralize one and other on a macroscopic length scale
quasi neutrality equation
ne + ni = ni+
plasma is composed of
charged particles (ions and electrons) reactive neutrals, electric fields (from moving charged paricles ) and photons
thermodynamic equilibrium of plasma
plasma do not have to be in thermal equilibrium, electrons gain energy from the electric fields and are relatively hot compared to the heavier background gas and ions due to large mass difference between heavier particles and electrons there is poor energy transfer between these particles. The small percentage of electrons are hot, while the dominant background gas remains cold
the bohr model
Electrons in atoms and molecules can be raised to higher energy states electrons can only be in specific orbits around the central neucleus. These orbits have discrete energy states
light production
population of an excited level, through for example impac. excitation and subsequent relaxation of this excited state to a lower level, relasing a photon with energy equivalent to the difference in energy levels
degree of ionisation equation
I=ne/(ne+no)
ne = electron density no = number denisty of neutral atoms
EEDF
electron energy distribution function
the prob. of an electron to have a given energy density
difference between a plasma and gas
plasma has charged particles and its thus inflluenced by electric and magnetic fields
high energy electrons
sustain the plasma
general formula for energy distribution
f(E) = const exp (-E/KbT)
Total energy of particles equation
E=m|V|^2 /2 +e(phi)
elastic collisions
momentum chance, no energy change no particles change
inelastic collisions
momentum change
energy change
particles change
examples of inelastic collisions
photoionisation, De-excitation, Stimualted emission
collision cross-section
sigma , quantify the probability of a collision occuring between two or more particles (depending on energy) and has a dimension of m^2
mean free path
average distance a particle moves before it collides
time between collisions equation
tau =lambda/V
where v is the speed of particles travelling in beam
collision frequency equation
(greek v thing) =tau^-1 = ngsigmaV=ngk
k rate of interaction per atom of gas
reaction rate equation
Rab=nanbkab
the number of collisions between particles of type a and b given by rab units Rab= m^-3s^-2
energy thresholds
ionisation energy of atoms and molecules 10-20 ev
excitation 8-12
binding energy of molecules 1-6ev
coulombs law Force equation
F=1/4π(epsion) q1q2/r^2
coulombs law E
due to a single charge, electric field E
E=1/4π(epsilon) q/r^2
E=-dphi/dx
poisson equation
electric field due to a collection of charges is given by poissons equation
E(x)=1/epsilon ∫(sigma)dx
in differential form;
d^2 phi/dx^2 = -e/epsilon (ni-ne)
charged particles motion equation
F=md^2r/dt^2
d^2r/dt^2 =qE/m
Lorentz force
only exsits when charges are moving
F=1(E+V ^ B)
flux of charge
=nu
example of collective behaviour
plasma oscillations
plasma frequncys
an electric field can be set up in a plasma if there is disturbance to quasi neutrality , electrons move inresponse to the fields as they accelerate they over shoot zero field position due to their inertia an oscillation is set up in the plasma
debye shielding
plasma screen out electric fieldsm ions are repelled outwards and electrons inwards
debye length
length over which changes in potential are attneuated (lambda D)
debye length equation
Lambda D = √epsilon(Kb)(Te)/(ne)e^2
example of plasma breakdown
lightning
plasma breakdown
can be intentianol or unintentional
electrical breakdown occurs within gas (or mixture of gas such as air) when dielectric strength of the gas is exceeded
plasma ignition example
electron avalanche
plasma ignition
a seed electron undergoes ionisation when it ains sufficient energy from the applied electric field and is collides with a neutral gas/atom/molecuel.
ionisation event creates an electron pair
new elecron will in turn be accelereated in the electric field and undergo a collision creating another ionisation event and so on…
the original electron will also continue to be accelreated in the field towards the anode , ions created are accelreatewd in the oppoosite direction
gyro frequncy / cyclotron frequency
wc=-qB/m
The anglar frequency at which charged particles revolves about the magnetic field. inductive (indicates) the field strength, charge and mass of particles
plasma sheath
always forms in front of a surface, determines wall interaction and has a high electric field in order to confine the plasma, no electrons
development of sheath
when object is immeresed into plasma
electrons will reach the object first as they are lighter and more mobile
electrons fo to to object, object negatively charges due to excess electrons. as soon as it negatively charges electrons are repelled and positive ions are attracted
eventually the fluc of the positive nad negative charges balance
bohm velocity
the bohm velocity is the minimum velocity ions must have before they can enter into the sheath. They gain this in the pre-sheath region of the plasma between the bulk and the sheath
debye shielding
debye shielding is the plasmas ability to shield or screen out electric fields. the associated radius is the debye length. the length scale over which imposed potentials within a plasma decay as they are shielded by the plasma response.
higher density plasmas have samller debye length
reaction rate
the number of collisions between particles of type a and b given by Rab
rab=nanbkab
reaction rate in hydrogen plasma
H2+e- –> H2+ +2e-
R=nenH2Klz((Te))
elastic collisions
momentum change, no energy change or change in particles
inelastic collisions
momentum change, energy change, change of particles
e.g. exsitation, phtoionisation
collisions cross section
quantify the probability of a collision occuring between two or more particles (dependnt on enregy) and has dimension m^2
mean free path
the average distance a partile moves before it collides
energy thresholds
ionisation energy of atoms and molecules 10-2eV
excitation 8-12 eV
binding energy 1-6eV
plasma breakdown
electrical breakdown occurs withing a gas (or mixture of gases) when the dielectric strength of the gas is exceeded
plasma ignition
a seed electron undergoes ionisation when it gains sufficient energy from t he applied electric field and it collides with a neutral gas/atom/molecule. ionisation event creates an electron pair. new electrons will in turn be accelerated in the electric field and undergo collision creating another ionisation event and so on.
the original electron will als continue to be accelerated in the field towards the anode
low PD
electrons have a chance to gain large energies due to lack of collisions. However mean free path can be high compared with the gap size and they only have a small chance to collide in order to ionize
high PD
increased number of collisions reduce the electron energy and therefore make it more difficult to ionize-higher voltage required.
gyro frequency / cyclotron frequency
the angular frequency at which charged particles revolved about the magnetic field. indicates field strength, charge and mass of particles
low pressure plasmas and atmospheric plasma ( size comparison)
plasma dimensions : app are smaller. the Pachen curve shows that for breakdown to occur at similar voltages as the pressure increases there needs to be corresponding decrease in the dimension
low and atmospheric plasma mean free path comparison
higher pressure there is an increasing number of collisions, therefore the mean free path between colllisions decreases.
low and atmospheric plasma ion energies comparisons
the sheath will become more collisional at higher pressures, therefore the average ion energies will decrease.
ozone and hydrogen peroide plasma uses
sterilisation or decontamination as both ozone and hydrogen peroxide have antimicrobial/bacterial propoerties
comparison of electron temperature in a partially ionised gas and ion temperature
electron temperature is almost alwys higher then ion temperature
because
energy transfer between the lighter electrons ‘heated’ by external means and heavier ions is poor due to the large mass difference between them
since the power is predominantly coupled into the elctrons the electrons are ‘hot’ while the ions remain closer to room temp
higher power, as it increases the elctron density and hence creates more collisions
for a low temp plasma how would Te is approx Ti be reached
increase the pressure. increasing the collisionality will increase the enrgy transferr from the electrons to the ions
formation of a floating sheath (collisionless plasma time independent)
electrons have higher thermal velocity, due to a lower mass, than ions.
electrons reach the surface faster than ions.
the flux of positive and negative charges mucst equal
positive space charge builds up such that the electrons are pushed back into the plasma bulk and ions are accelerated towards the surface , in order to satisfy the condition of. a zero-net charge flux at the surface .
main assumptions of the high voltage matrix sheath model
the sheath region consists only of ions and no electrons (confined to bulk)
the ion density in the sheath is uniform i.e. it does not account for acceleration of the ions in the sheath field.
outline the concept of the townsend theory of electrical breakdown
seed electron created by background ionisation (e.g. radiation)
- applied external electric field accelerates free electrons
- due to the acceleration the electrons motions is a combination of their randdom thermal motion and a directional driftcaused by the action of the applied electrical field.
- if the electrons gain enough energy they can ionize the background gas through collisions.
- each ionisation event creates a new free electron, which in turn will be accelerated by the external field.
- formation of an electron avalanche
-the positive ions created from the ioisation are also acecelerated by the electric field and bomobard the electrode surface, leading to secondary electron production at the cathod surface further adding to the ionisation of the gas
typical discharge gap dimensions for plasmas sustained at atmospheric pressure
micrometer/millimeter as per PD scalling and the pachen curve with increasing pressure the gap distance must decrease.
plasma is excited by an external applied driving frequency, how do ions and electrons respond
electrons will readily resond, however, the ions cannot respond to the higher frequency
MHD
magnetohydrodynamics
list two assumptions MHD
single fluid picture , electrons and ions treated as an effective single fluid
strongly collisional plasma, time scale of collisions is much shorter than other characterisitic time scales, particle distribution close to maxwellian. length scales much longer than the larmor radii
larmor radius
is the radius of gyroation of a charged particle moving in a magnetic field
realtive size of the mean free path to the larmor radius. state the conditions under which the motions of charged particles will be domniated by the effects of a magnetic field
if the larmor radius is a lot smaller than the mean free path the particles are strongly magnetised
guiding centre drift velocity
when a particle trajectory is a helix around a guiding center, parallel to a magnetic field. this is known as the guiding center velocity.
how could a plasma be diamagnetic medium
gyration charges give rise to a magnetic moment. the magnetic moment vector is always opposite to the ambient field, thus the motion of charged particles reduce the influence of the applied magnetic field - diamagnetic medium
breakeven
the point in which the fraction of total fusion power produced that is available to be recycled as power input just exceeds the energy losses due to plasma transport radiative losses
iginition
is the point at which the alpha particle power resulting from fusion reactions is sufficient to overcome these plasma losses, at which point theres is no need for external heating to be applied to the plasma
E-mode
Emode is driven through the sheath dynamics, the electrons gain energy once during the applied rf cycle e.g. during the sheath expansion phase. Electrons gain sufficient energy to induce ionisation pressure
H-mode
H-mode is inductive mode and can be sustained due to the time-varying current in the inductive coil inducing a time varying magnetic field which in turn induces a time varying current in the plasma as soon as criteical plasma density is reached
limiters
limiters are plasma facing components that are designed to define the last closed flux surface of the core plasma. the location of the plasma-wall interaction can therefore be localized to the limiter interaction and the wall material can be chosen to minimize radiation losses. the major disadvante of limiter operation is that the plasma wall interaction is directly adjacemt to the core plasma
divertor
is a region of the tokamak remote from the hot core into which the exhaust plasma is channeled. typically it is formed by establishing a poloidal magnetic field null within the vacuu, vessel containing the plasma. it allows the region of interaction between the plasma and the first wall to be locally defined but with the advantage that it is remotefrom the main core plasma
advantages and disadvantages of direct drive
direct drive refers to inertial confinement fusion schemes in which the target capsule is directly illuminated by the high power laser used in such schemes. the directly absorbed laser power heats and implodes the targe,t, bringing it to the condition required for igintion
indirect laser fusion
in indirect drive, the primary laser impinge on a usaully cylindircal target known as hohlaraum. the fusion target capsule is loacted at the centre of the hohlraum and the laser-plasma interaction at the hohlraum generates an intense flux of x-rays which is absorbed by the target imploding it as in direct drive. the absorption of the x-rays at the target is more efffficient than for the direct absorbtion of laser light
mechanisms responsible for cell death through the application of low temperature atmospheric pressure plasma to biologica ltissue
eletroporation can be induced through the plasma generated high electric field leaving the cell permeable to certain RONS that can then enter into the cell. the plasma produced RONS can be toxic to cells and attack them in different ways e.g. through osixative stresss, induced DNA dmage , lipid peroxidation , disruption membrane integrity. plasma generated photons may also induce cytotoxic responses. synergies between all of these process can be important.
