## 3 Plasma Sources
### Microwave Using Electron Cyclotron Resonance (ECR) Plasma Source  
As mentioned above, ECR is a method based on wave resonance which is used to achieve an over critically dense anisotropic plasma. In this case, the anisotropy is produced by the static magnetic field, which is introduced into the plasma chamber to act on the particles in conjunction with the electromagnetic field. This static field allows the microwave power to be absorbed by the electrons through a mechanism that is different to particle collision, hence being denoted as collision less absorption [45]. Microwave radiation along a magnetic field allows the waves to travel into the plasma absorption zone, even in the case of a dense plasma. As mentioned above, when there is a high plasma density, there would be total reflection of the incoming electromagnetic field in a non-ECR plasma system. The static magnetic fields, however, allow the electromagnetic radiation to propagate, even when the plasma density exceed the critical value [18].  
In this process, the high-frequency waves travelling through the plasma along the stationary magnetic field interact with the electrons in the plasma. The microwaves have limited interaction with the ions since they are slow moving. The electrons, on the other hand, being very mobile, are trapped and carried forward by the waves. If the velocity of the electrons matches the waves' phase velocity, the electrons are accelerated.  
Field experimentation involving several gases such as hydrogen, helium, argon, and oxygen has shown that the presence of the static magnetic component was crucial in the production of a dense plasma. In the case of such magneto-active plasmas, testing with a 1 kW MW source showed that it was only easy to initiate the plasma when a magnetic field was present at a value of \(\mathrm{B=B_{ce}}\), and hence the system was in ECR condition. At this value of static magnetic field, the electrons were therefore forced to gyrate around the magnetic flux lines at the Larmor radius. This circular trajectory radius, also called gyroradius, is determined through the fact that the particle speed multiplied by the magnetic flux density of the field is balanced by the centripetal force [45].  
\(\mathrm{B_{ce}}\), the value of magnetic induction necessary to achieve ECR, is outlined by Equation (5) [36].  
\[\mathrm{B_{ce}}=\frac{\omega m_{e}}{e} \tag{5}\]  
where  
\(\omega\) is the 2.\(\pi\).cyclotron frequency [Hz];  
\(\mathrm{m_{e}}\) is the mass of an electron;  
\(\mathrm{e}\) is the electron charge.  
The particles' velocity remains constant as the magnetic field's force is perpendicular to the motion at all times. Since the magnetic field cannot change the particle's kinetic energy, particles with higher energies, having higher velocities, orbit the magnetic field lines at a further distance, but complete the revolution at the same time as particles orbiting at lower orbits due to having lower energies [45].  
In a case where the cyclotron frequency f was equal to 2.35 GHz (\(\omega=14.76\) GHz), the value of magnetic field induction was \(\mathrm{B_{ce}}\)= 8.4 \(\times\) 10\({}^{-2}\) Tesla or 840 Gauss [36]. For a microwave frequency of 2.45 GHz, which is the one mostly used in industry, resonance occurs at a value of magnetic induction equal to 875 Gauss or 87.5 mT. The static magnetic field can be generated either through permanent magnets or else through electromagnets.  
ECR enables the achievement of collision-less plasmas at very low pressures, which can be below 1 Pa, where the ionization degree can reach 100%, thus achieving plasma densities of up to 1 \(\times\) 10\({}^{19}\) m\({}^{-3}\), even with pressures of 1 mTorr [36]. This is substantially more than what is achieved in plasmas operating at pressures of between 1 and 100 Pa, where the degree of ionization is less than 1%. At lower pressures below 1 Pa, a higher