The preionization stage, in the plasma accelerator developed in this thesis, is formed from a voltage grid supplied with a positive voltage pulse from a voltage generator. The voltage grid is placed between the grounded piezoelectric valve housing and the plasma accelerator; this is represented diagrammatically in diagram 3.3 and isometrically in diagram 3 .4. This preionizer unit works in a similar fashion to a cathode ray tube (eRT) [82], in that an electric field formed between the grounded piezoelectric valve housing and the voltage grid is used to accelerate electrons towards the deflecting electric field inside the plasma accelerator, this is represented diagrammatically in diagram 3.3.
The preionizer unit resides in the extension to the source chamber, refer to diagrams 2 . 1 4 and 2. 1 5 in section 2 .4.4. This unit comprises of a voltage grid which is directly attached to the skimmer, so that the two are held at the same potential, ceramic spacers separate the skimmer from the walls of the grounded vacuum chamber (the extension to the source chamber).
The operation of this preionizer unit relies upon the breakdown of the residual scattered gas left behind the main gas pulse produced from the piezoelectric valve. Upon a user specified time delay, after the neutral gas pulse has been produced from the valve, a square wave preionizing voltage pulse is sent to the voltage grid. The residual scattered gas then breaks down generating electrons, which are then accelerated towards the plasma accelerator. A short steel tube (with an inner diameter equal to the orifice of the skimmer) attached to the rear of the skimmer and held at the same potential allows the preionizing electrons to be directed towards the plasma accelerator, without being retarded by the electric field generated between the rear of the preionizing unit/skimmer and the grounded walls of the vacuum chamber, refer to diagram 3.3. The timing of the preionized voltage pulse (sent to the voltage grid) used to preionize the neutral hydrogen gas pulse within the plasma accelerator was experimentally determined
3 . 1 :
Overview -A new type of plasma accelerator the L icathrough trial and error; this aspect is discussed in section 3.3. Currently the efficiency of the preionizer device is unknown and remains an aspect of future experimentation.
There are two basic possibilities for the preionization of the neutral gas pulse at the plasma accelerator, either the gas pulse can be preionized before it reaches the plasma accelerator or the gas pulse can be preionized some where within the plasma accelerator. Depending upon the timing of the preionization pulse, with respect to the position of the gas pulse, the effect of preionizing different sections of the neutral gas pulse could be investigated; for instance the neutral gas pulse could be preionized just as it enters or leaves the accelerator. The timing of the preionizing pulse used in the experimentation of the Lica is discussed in section 3.3.
Piezo ele ctric Extension of the
valve s ourc e chamb er
E
.-
electrons +V.-
Voltage Skimmer E grid Plasma ac c elerator E +V EDiagram 3.3: Diagrammatic representation of the preionization process
piezoelectric
ionizer
grounded electrode (lOom in dia�t")
1
Diagram 3. 4: Isometric cross section of the valve, preionizer and the coaxial plasma accelerator
3 . 1 :
Overview - A new type of plasma accelerator the L icaThe operation of this plasma accelerator is a five step process: 1 ) the capacitor is charged, 2) then a gas pulse is produced from the piezoelectric valve, 3) the gas pulse is then preionized (essentially it i s a gas with some seed ionization not a plasma), 4) the preionized gas then breaks down in the presence of the high electric field within the plasma accelerator, 5) this causes the capacitor to discharge and the resulting plasma is then accelerated down the plasma gun through the action of the resulting Lorentz force (refer to section 1 .2 for a discussion on this acceleration process and to section 1 .3 for the various factors effecting the operation of the plasma accelerators).
The Lica was constructed around the experimental apparatus used to investigate the gas pulse characteristics of the piezoelectric valve covered in section 2.4 in its entirety. The Lica consists of a series of differentially pumped chambers: a source chamber, a main chamber, a flight tube and an ultra high vacuum chamber. The entire Lica setup is shown in an isometric cross sectional view in diagram 3.5. The essential components of the L ica are:
I ) A high voltage circuit, formed from a high energy capacitor and an energy dump connected to an oscilloscope
2) A piezoelectric valve and the pulsed circuit used to operate it 3) A preionization unit and a pulsed voltage driver
4) A plasma accelerator 5) A flight tube
6) An ion detector (a CDEM) and a multichannel scaler
These pieces of equipment and the associated devices used to operate them are discussed in section 3.3. 1 . Both the piezoelectric valve and the preionization unit are centred within the main chamber (facing towards the plasma accelerator) in an extension of the source chamber.
The CDEM and the oscilloscope were the main diagnostic devices used to gather results from the plasma accelerator. The CDEM i s used to determine the velocity profile of the plasma pulses produced from the plasma accelerator, by dividing the flight distance between the plasma accelerator and the CDEM by the time that the plasma counts were recorded. The oscilloscope was used to obtain the voltage trace (refer to section 3.3.2) from the capacitor circuit (refer to diagram 3 . 1 0 in section 3.3. 1 ).
Photomultipliers were tried as an additional means of measuring the velocity of the resulting plasma. I n diagram 3.5 it can be seen that there are two T-pieces separated by a distance of l m on the flight tube, it was here that two photomultipliers were attached. The final velocity of the
3 . 1 :
Overview - A new type of plasma accelerator the Licagenerated from these devices as the plasma passes each photomultiplier. Unfortunately the first photomultiplier was placed too close to the plasma accelerator, this resulted in the saturation of the first photomultiplier (refer to chapter 5 for a fuller discussion). So this means of determining the resulting velocity of the plasma could not be used in practice. An interferometer was also tried as a means of measuring the velocity of the plasma. However the v ibration of the flight tube from the mechanical pumps (refer to section 3.3. 1 ) was too great for this technique to be used, as all that was observed was the background vibration (refer to chapter 5 for more details). Spectral analysis to provide temperature and ion distributions within the plasma gun was not possible due to the time constraints placed upon this thesis.
Hgh voltage power supply
iezoelectric valve Cl nd Preionizer 4mflight Ullra high vacuum particle
Diagram 3. 5: Isometric cross sectional view of the Lica with the main components identified
3 .2 . 1 :
The electrical breakdown of gases3.2: Background on the generation of plasm as and their electrical properties
3.2. 1 : The electrical breakdown o f gases
At room temperature and atmospheric pressure gases, in the absence of electric fields, are very good insulators. But in the presence of a high electric field charged particles may gain sufficient energy between collisions to cause neutral molecules or atoms to become ionized upon impact. Electrons in the presence of an electric field lose little energy in an elastic collision with a molecule; instead their kinetic energy is transferred as potential energy that may result in the ionization of the impacted molecule. Because of this electron impact is the primary ionization mechanism in the electrical breakdown of gases, especially in the presence of high electric fields. The probability of ionization by electron impact is dependent upon the energy that the electrons gain in the direction of the applied electric field. The average energy gained �W over a distance Ae is given as [83]:
(3 . 1 )
where Ae is the mean free path of the electron in the direction o f the electric field,
E
i s the electric field strength and e is the elementary charge. The value for the mean free path of electrons in a particular gas (assuming all the electrons and molecules are in thermal equilibrium) can be calculated by knowing the mean free path for a molecule of gasAg .
The relationship between these two quantities is Ae =4fiAg
[83]. The value for Ag is directly proportional to the temperature and pressure; hence given the experimentally determined value AgO at a pressure ofPo
and temperature ofTo
the value for AI,' at a pressureP
and temperatureT
is given as: Ag g
=A
0Po
P�
T. [83]. oIn order for a molecule to be ionized upon impact the average energy gained must be greater than or equal to the ionization energy of the molecule e
V;,
whereV;
is the ionization voltage required for breakdown. It should be noted that electrons having energy below the ionization energy of the molecule may still lead to ionization, since if a molecule has impacts with several electrons it may gain enough energy to become ionized. Also, not all electrons that have energy above the ionization energy level will lead to the ionization of a molecule upon impact; this is because the electrical breakdown of a gas is a probability phenomenon and is generally expressed in terms of the ionization cross section (J'j which is a product of the probability of3.2. 1 : The electrical breakdown of gases
ionization upon impact
P;
and the molecular/atomic cross sectional area [83]. A plot of the ionization cross section as a function of electron energy is shown in graph 3 . 1 for various gases.1 4 , 2 . 8 w 0 . 6 o 4 0 . 2 iO 1 03 0.0 01
Heel ron en rgy (,eV )
Graph 3. J : Graph of the experimentally measured ionization cross section of hydrogen as afunction of electron energy determined by various groups, adaptedfrom [84].
Electrical breakdown in gases can be defined by two distinct classifications: those of a self sustained discharge and that of a breakdown discharge. In a self sustained discharge the current flowing through the ionized gas is solely dependent upon the initial ionization source, while a breakdown discharge is independent of the ionization source. The self sustained discharge can be further subdivided into two main mechanisms: primary ionization (primary ionization is used to describe any mechanism that leads to the ( initial) generation of electrons within the gas or gas with some seed ionization) and secondary ionization (secondary ionization is any process/mechanism that generates further electrons from primary ionization). Within the classification of secondary ionization mechanisms there is a special category that is referred to as cathode related effects and relates to any secondary ionization mechanism in which the cathode plays an important role in the mechanism.
Townsend's first ionization coefficient er Town is used to describe a pnmary self sustained
discharge within a gas in which there is already some seed ionization ( i.e. there is already the presence of some ions and more importantly electrons). Townsend's first ionization coefficient is based upon the assumption that the anode and cathode are separated by a distance
d
with a constant applied electric field and that the seed electrons are accelerated towards the anode. As the seed electrons move towards the anode they will collide with neutral molecules, it is3 .2. 1 :
The electrical breakdown of gasesassumed (in this model) that every electron has enough energy to cause ionization upon impact. After each collision more electrons will be generated and accelerated towards the anode in an ever increasing avalanche effect, which is approximated by an exponential function. Townsend's first ionization coefficient is simply interpreted as the coefficient in the exponential function, which gives the current (or number density of electrons) as a function of distance:
(3.2)
In equation (3.2) ia is the resultant current generated at the anode and io is the initial current generated at the cathode. The formula for Townsend's first ionization coefficient (refer to [83]
for the derivation) is given as:
In equation (3.3)
ATown
and BTown are constants, which are defined as:A Town