This week is the third week for this project. According to the guide of supervisor, we know that the circuit can be changed without using the batteries. So, we build the circuit as the figure shows.

Figure 1: The equivalent circuit of the low pressure plasma

In the previous week, the ion current we calculated is too small, so the we change the value of some parameters. We set n to be 3.72*10^15(m^-3), A is 1000(cm^2), Te is 3.1eV. Because the gas we choose in this project is argon, one argon has 40 protons, the M need to be 40*1.67*10^-27(kg). At last we assumed the ion current to be 1.5mA.

Figure 2: The current which the constant current source supply is 1.5mA

To make sure that the whole circuit can work successfully, first we test the   function of current sources. We measured the voltage of zener diode and the resistor and the current in the circuit. The actual voltage of zener diode and the resistor is 6.12V and the current is 1.51mV. These data we measured are similar with the value we calculated. Then, we connected other components to the circuits. After connecting the circuit, we use the oscilloscope to display the wave forms of the plasma and the power supply. To make some contrast experiments, we changed different frequencies, amplitudes and add the capacitors to find the change law of the voltage.

Figure 3: The voltage between the zener diode and the resistor is nearly 6.2 V

In the second week experiment,we calculated the exact value of each components in the current source which is represented the ion current. We calculate the ion current using the equation Ii=neA(kTe/M.40)^1/2. Finally, the ion current is nearly equal to 0.38 to 0.40mA. In the current source, the zener diode is about 6.2V to 6.8V. The resistance is about 1Kohm. The DC voltage is about 9V. Therefore, we connected the current source on the breadboard. The technician told us the battery which we need in the electron current will be arrive in the third week, so we just complete the current source part in this week.

Today, we calculate out the ion current in the circuit which the current source

need to supply.

 n is the density of the plasma. e is one electron charge. k is the Boltzmann constant which is 1.3807x10^-23. Te is the electron temperature.M is the mass of the proton. We use the value of the variable in the reference book and finally we find that the current should be nearly 2.4 mA.

 Therefore, we find the technician to rewrite our experimental component.

 The components are 4 zener diodes, 4 666ohm resisters, 4 PNP transistor, 4 NPN transistor, and 2 6Mohm resisters.The components mentioned above are used in the current source circuit. The other components we need is 2 diodes, 2 4.5v batteries, 3 3nF capacitors, and a dc power supply and a RF power supply.

   In this week's experiment, the supervisor told us that we do not need to connect the circuit on the breadboard. What we need to do is to determine the type and the size of the components in our circuit. For instance, the capacitors,the radio frequency voltage, the diodes and the transistors. And then, we ought to calculate the value of the ion current and electron current by using the equation given in the reference book.At last, we should draw our circuit clearly on the paper which will be connected later.
   Through this project, we need to firstly know some basic knowledge of the plasma.A plasma is a collection of free charged particles moving in random directions. The plasma is electrically neutral. The plasma is usually called the fourth state of the matter. It has the following features. 1.The plasma is driven electrically. 2. Charged particle collisions with neutral gas molecules are important. 3. The boundaries at which surface losses are important. 4. Ionization of neutrals sustains the plasma in the steady state.5. The electrons are not on thermal equilibrium with the ions.

           Figure one: The equivalent circuit of the low pressure plasma

           Figure two: The circuit that can supply the constant current

 During the experiment, we should measure the voltage and the current in the plasma.Two parallel circuits are connected to the radio frequency voltage. The ideal diode represents the electron current. The current source is corresponding to the steady ion current.The capacitor is the sheath in the equivalent circuit. The experiment will be implemented in two situations. The first situation is that the parallel circuit with no capacitor which is the condition that there is no displacement current in the sheaths. The other situation is the circuit have the circuit with the capacitor which means take the displacement current into account.In the laboratory, the current source can be replaced by the circuit shown in the second figure