For this project we studied briefly the effects of an electrostatic field imposed upon a polarized particle. We designed an electrical device that allowed us to have a voltage difference of appoximatly 30kV between our cathod/anode. This difference of voltage (noted as “V” in the calculations), is found between an anode and a cathoded at a given distance (noted “d”).

From our limited “understanding” of Townsend discharges realised that we will need to create an electrostatic field sufficiently intense to accelerate the composition of air ( N2, O2, CO, Ar…) over our cathode/anode distance with enough final kinetic energy where the “ion/ion” or “ion/electron” or “ion/atom” collision would ionize.

We can simplify this system and consider the acceleration of a single particle in an electric field:

• Since:

E =ΔV/d where E is the electric field

• We know that:

ma=qE where “q” is the particle charge and “m” &“a” are the mass & acceleration of a given particle

• therefore:

v = V*q*T / d*m where “v” is the velocity of the charged particle & T is the time in seconds

• solving for T:

T=sqrt(2d²*m/q*V)

• Finally:

v= sqrt(2V*q/m)

This means that kinetic energy of the particle: Ec= qV

Ec must be higher than that of the ionization energy Ei in order to ionize. This energy must also be high enough to incite at least 2 ionizations in order to trigger a “townsend cascade”. We have looked at the ionisation energies of multiple elements and decided that we would like most to study the ionisation of nitrogen as it is inert and relatively safe. The down sides are that atomic N does not exist in nature and we will have to procede with N2. This choice also allows us to build the machine using nothing but “air” and the change to pure N2 will actually be an improvement on the system as Oxygen (21% of the air) requires more energy to ionize. Logistically, we also have relatively easy access to liquid nitrogen of which we hope to create a nitrogen rich environment.

This graph shows us the relatively high ionisation energy of Nitrogen.

More accurately the Ei/mol of nitrogen are:

1st: 1402.3 kJ/mol 2nd: 2856.0 kJ/mol 3rd: 4578.1 kJ/mol 4rd: 7475.O kJ/mol

-Calculate the voltage (energy) required to ionise 1 partcile

We now have two different generators:

We are still studying the relation of amperage/voltage required to ionize a given volume(mass) of air/nitrogen. We currently understand that a high voltage is required to ionize a gas. However we observed that a more powerful generator was able to ionize larger volumes of gas at a time. It was obvious that a generator of relatively High voltage was needed. Due to our limited knowledge of electronics we decided to simply make the highest voltage that we could in our FABlab with the materials made available to us.

Through some preliminary experimenting with some Spectral Lamps we realised that a glow-discharge state was desired as it gave us the most “responsive” plasma under the submission of magnetic fields. We present here a “qualitative graph” of the different “states” of neon plasma at 1 torr (133 Pa = 0.0013 Bar)

The first thing that was made obvious is that we will need a generator with variable control and a means to measure our system if we would like to reliably create a stable glow-discharge. However what is less obvious is the relation that pressure/distance will play on the the system. We were interested in creating plasma across a distance in the order of a few centimeters. This size will allow us to test and record the movements with the equiptment made available to us with a relatively good precsion. The machines of the FABlab SU all work within a tolerance of about 100microns and therefore we can work with a relative 1% margin of error.

After Paschen's Law we were able to find a curve that represents the “break-down” voltage of N2 plasma in respects to the product of Pressure*Distance. We will see further on that our current vacuum limit is somewhere between 1,000 Pa - 10,000 Pa (7.5 torr - 75 torr)

From this we calculated that our product: 7.5<p*d<75 or roughly 10^1<p*d<10² This calculation reaffirms that we will in fact need somewhere between 10kV-100kV in order to create a glow-discharge plasma.

The experiment for the Townsend Breakdown Voltage is explained here and is one of the first experiments that we wish to do on our system for the reasons of personal interest and machine calibration.

We built two different generator systems to fulfill this requirement. We started with a High Voltage DC power-supply which offered higher voltage and greater control. The second generator was a Neon Sign Transformer (NST) powered Tesla coil.

The problems, benefits, and disadvantage of each are explained just below.

To create our DC High-Voltage generator we used a relatively simple NE555 timer and applied a astable circuit (page 10 of datasheet). The output of this pin was then linked to gate of Power-Mosfet IRF220. The drain and source of the mos were then in series between a 60W (5Amp max) lab generator. The Drain and Source of the Mosfet are then placed in parallel with the leads of a recycled FlyBack transformer salavaged from an old TV screen.

-include .gbr and .drl files ASAP-

This circuit is fairly straight forward to make and will require:

Breadboard or CIF 1 NE555 timer 2 Capacitors (0.01uf and C2 (explained below)) 3 Resistors (30ohm for Ra, 1Kohm for Rb & 10ohm to place in series before the Mos gate) 1 IRF220 PowerMos 1 Fly-back Transformer

Since the signal frequency (square wave) can be determine using the formula:

f= 1.44/(Ra+2*Rb)*C2

Since: (Ra+2*Rb)=2030

C2=1mF ⇒ f=1 Hertz C2=100uF ⇒ ff=10 Hertz C2=10uF ⇒ ff=100 Hertz

Since the Flyback transformer's datasheet indicated that it's peak operating frequency is that of 50Khz

we needed a C2=0.0000000096 F = 9.6nF

We must keep in mind that the duty-cycle of the output signal is:

D=Rb/(Ra+2*Rb)

By choosing the resistances that we did, we can have a our desired 50Khz signal while at the same time keeping a near 50% duty-cycle. Another final note is that we can keep the same duty cycle and frequency by increasing the Resistors by a factor X as long as we decrease C2 by a factor X. This maybe desirable to protect our NE555 from higher amps.

We will improve upon our current circuit by replacing Ra with a variable-resistance. This will allow us to adjust the frequency while at the same time minimally impact our duty-cycle.

This photo on the left demonstrates the relation

• Liste à puce

As all the capacitors we had were not rated for HV we had to build our own, so we first tested different techniques: …

We then decided to do this in a more methodical way:

• Used the CIF to machine aluminium foil:

• Bought some glass :

• And used polyurthane spray to glue the foil (polyurethane is also a good dielectric):

• Cleaned everything with Alcohol :

• And we had all our plates in less than 3 hours:

To create a HV voltage probe for this project we simply applied the classic “Voltage Divider” :

Where we used: Our Resistors (R1 & R2) were specialised High Voltage Resistors We selected the MOX-2-12 series as they resisted up to 20kV

Since our volt-meter only is reliable upto 2kV on its own, we needed to divide our input power by a factor of 10 to be able to read 20kV of our generator.

The values chosen were 100Mohm for R1 & 10Mohm for R2 (we had to add more resistors :80kOhm to R2)

R2 was then placed in parallel with the standard lab Multi-metre which we measure to have had an impedance of approximatly 13Mohm (we can call it R3).

Placed in Parallel with the Multi-meter: R2'= R2*R3/R2+R3 = 5.65Mohm

Finally we have an output voltage of: V(out)=V(in)*(R2'/R2'+R1)

V(out)= V(in) *0.053

where V(out) < 2kV Therefore, we have built a very basic high-voltage probe that can measure up to 37.7kV which surpasses our initial design goal.

-add margin of error-