**4.3 Er-field evolution with time**

In **Figure 8(a–c)**, the radial electric field (Er) variations are shown for three different z-planes of the cylindrical cavity. The figures also show the evolution of the Er-field profile at different time instances after the MW launching. **Figure 8(a)** gives the Er profile close to the ECR surface (z = �28 mm), which is also near to the MW launch point (z = �60 mm). Another figure (**Figure 8**(b)) shows the Er-field profile along the central plane (z = 0) of the cavity, whereas **Figure 8(c)** gives the Er-field evolution along the plane z = 28 mm that is close to the ion beam extraction point.

In all the figures, a strong inhomogeneity in the Er-field profile signifies the power absorption locations. Taking a time instant t = 67 ns in all the figures, the presence of the Er-field throughout the cavity can be visualized since the plasma density at this time has not reached above the critical density. As the plasma density crosses the critical density with advancement of time (>500 ns), the Er-field profile is being modified following the density evolution pattern. The inhomogeneity in the Er-field profile is more pronounced in **Figure 8**(b) than in the case of other figures. The difference in the radial variation of the Er-field comes from the variation of resonance magnetic field contour as shown in **Figure 3**. The strong inhomogeneous part of Er-field initially very near the ECR surface indicates that the electrons are accelerated by the ECR phenomenon. Then after >2 μs, this inhomogeneous part gets shifted toward the UHR fUHR ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi fpe <sup>2</sup> <sup>þ</sup> fce<sup>2</sup> q� � � � regions where the MW frequency matches the fUHR . The Er-field value after �<sup>2</sup> <sup>μ</sup>s at r = 0 mm and z = �28 mm location is negligible and so the plasma density reached overdense state, whereas for some regions (UHR regions), as shown in **Figure 8(a)**, the Erfield component is showing significant intensity. The high value of Er-field, near the

location (B < *BECR*), represents that the plasma is still underdense in this region during time t = �3 μs. Therefore, the dual conditions (B < BECR and ne < ncrit) are satisfied for the UHR heating process. This phenomenon is also being reflected in **Figure 7(a)** and **(b)** as discussed earlier. **Figure 7(b)** shows the electrostatic electric field to grow

500 ns. The electrostatic electric field, the component of the total electric field, is getting increased due to the increase of the plasma ambipolar potential. So this gives a clear evidence of power transferring from MW coupling to the electrostatic wave coupling to the plasma. Since the electrostatic wave does not face any density

*Evolution of Microwave Electric Field on Power Coupling to Plasma during Ignition Phase*

*DOI: http://dx.doi.org/10.5772/intechopen.92011*

Er*-field evolution and its pattern during plasma generation (a) r = 28 mm, (b) r = 0 mm and (c) r = 28 mm.*

**Figure 8.**

**57**

with the plasma density and correspondingly the MW electric field decreases after

*Evolution of Microwave Electric Field on Power Coupling to Plasma during Ignition Phase DOI: http://dx.doi.org/10.5772/intechopen.92011*

500 ns. The electrostatic electric field, the component of the total electric field, is getting increased due to the increase of the plasma ambipolar potential. So this gives a clear evidence of power transferring from MW coupling to the electrostatic wave coupling to the plasma. Since the electrostatic wave does not face any density

decreased to �10–12 eV. This range lies close to the thermal electron temperature. Therefore, the reduction of the high energy component of the plasma bulk temperature can be argued as the minimal occurrence of ECR heating process and correspondingly the initiation of the electrostatic heating mechanisms in the same time frame [29–31, 33]. The plasma reaches steady state after 2000 ns (2 μs) (**Figure 7a**).

) than the critical density. During the same instant of time, the plasma

In order to investigate the different coupling mechanisms involved during the plasma evolution process and their impacts on the plasma parameters, the behavioral pattern of the different components of the electric field (Er and Ez) is shown in **Figures 8–10**. The power coupling mechanisms are ECR, UHR and polarity reversal related to ES heating. The particular type of coupling mechanism is understood from the electric field evolution pattern throughout the cavity. The electric field characters can be useful to characterize the plasma parameters based on the theoretical and experimental proofs. The proofs say the higher the plasma density, the

In **Figure 8(a–c)**, the radial electric field (Er) variations are shown for three different z-planes of the cylindrical cavity. The figures also show the evolution of the Er-field profile at different time instances after the MW launching. **Figure 8(a)** gives the Er profile close to the ECR surface (z = �28 mm), which is also near to the MW launch point (z = �60 mm). Another figure (**Figure 8**(b)) shows the Er-field profile along the central plane (z = 0) of the cavity, whereas **Figure 8(c)** gives the Er-field evolution along the plane z = 28 mm that is close to the ion beam extraction

In all the figures, a strong inhomogeneity in the Er-field profile signifies the power absorption locations. Taking a time instant t = 67 ns in all the figures, the presence of the Er-field throughout the cavity can be visualized since the plasma density at this time has not reached above the critical density. As the plasma density crosses the critical density with advancement of time (>500 ns), the Er-field profile is being modified following the density evolution pattern. The inhomogeneity in the Er-field profile is more pronounced in **Figure 8**(b) than in the case of other figures. The difference in the radial variation of the Er-field comes from the variation of resonance magnetic field contour as shown in **Figure 3**. The strong inhomogeneous part of Er-field initially very near the ECR surface indicates that the electrons are accelerated by the ECR phenomenon. Then after >2 μs, this inhomogeneous part

> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi fpe

q� � � �

Therefore, the dual conditions (B < BECR and ne < ncrit) are satisfied for the UHR heating process. This phenomenon is also being reflected in **Figure 7(a)** and **(b)** as discussed earlier. **Figure 7(b)** shows the electrostatic electric field to grow with the plasma density and correspondingly the MW electric field decreases after

frequency matches the fUHR . The Er-field value after �<sup>2</sup> <sup>μ</sup>s at r = 0 mm and z = �28 mm location is negligible and so the plasma density reached overdense state, whereas for some regions (UHR regions), as shown in **Figure 8(a)**, the Erfield component is showing significant intensity. The high value of Er-field, near the location (B < *BECR*), represents that the plasma is still underdense in this region

<sup>2</sup> <sup>þ</sup> fce<sup>2</sup>

regions where the MW

At the saturation, the plasma density is approximately 2 times more (�1.3 �

electron temperature and its gradient reduce in the location where the magnetic

10<sup>17</sup> m�<sup>3</sup>

point.

field is relatively lower.

*Selected Topics in Plasma Physics*

more the power absorbed by the plasma.

**4.3 Er-field evolution with time**

gets shifted toward the UHR fUHR ¼

during time t = �3 μs.

**56**

also be transmitted into the extracted ion beam performance in terms of spike or instabilities. It was experimentally demonstrated by Ropponen et al. [1] that a sharp transient in the ion current density during the preglow mode can cause a sharp fall

*Evolution of Microwave Electric Field on Power Coupling to Plasma during Ignition Phase*

Since the inhomogeneous part of Er-field is shifting toward the off-resonance locations as shown in **Figure 8(b)**, it can also produce high-energy electrons there if certain conditions are satisfied as per Gammino et al. [34]. The conditions say the MW electric field has to be above a certain transition value for being capable to energize the electrons that also depend on the magnetic field gradient present in the

The generated high-energy electrons can interact with the slow extraordinarytype microwave and produce the cyclotron range instability in the plasma [1]. It was also proved experimentally by Mansfeld et al. [35] that the extracted ion beam current from the microwave ion source can gain oscillation due to the presence of cyclotron-type instability of plasma during the afterglow operation mode. The slow extraordinary mode microwave is produced from the mode conversion of ordinarytype microwave near the UHR region. As the mode conversion layer is present in the present plasma cavity, the ordinary mode microwave crosses the evanescent layer and some part of its energy is converted into the slow X mode. Since the plasma is confined in the cavity under mirror magnetic field configuration, the injected MW will have two components, extraordinary mode and ordinary mode. **Figure 9(a)** shows the MW ordinary mode is propagating toward the overdense plasma region from the underdense launching point. At some point, it will encounter a cut-off corresponding to the ordinary-type MW. At the cut-off, some part of the ordinary mode MW energy is evanescently transformed into a slow extraordinary mode following the CMA diagram concepts. For that reason, a bend in the MW propagation in the slow extraordinary (X) mode is seen in the electric field simulation (**Figure 9b**). This slow X mode then propagates toward the UHR region and hence the electric field is being accumulated there, as shown in **Figure 9(a)**. The accumulation of the electric field at this location increases its intensity at some plasma condition and can cross the corresponding parametric decay threshold condition. The parametric decay of the slow X mode near the UHR region can generate

in the high energy component of the plasma electron temperature.

*DOI: http://dx.doi.org/10.5772/intechopen.92011*

ion- and electron-type electrostatic waves as per the literature [36].

**4.4** *Ez***-field evolution in plasma with time**

**59**

As a supportive evidence of the generation of electrostatic ion wave, **Figure 8(b)**

The spatio-temporal evolution of the axial component (Ez) of the electric field is shown in **Figure 10** throughout the cylindrical cavity along the planes r = 0 mm and 28 mm, respectively. It is seen that only a portion of single wavelength of the wave electric field is present that has significant intensity throughout the plasma cavity after the immediate instant (67 ns) of MW launching. The magnitude of the Ez field becomes almost zero throughout the cavity as soon as the plasma starts creating

shows that the inhomogeneous part of the Er-field is shifting toward the offresonance region with a velocity of 1250–1500 m/s. This range of velocities falls in the range of ion acoustic speed. Based on the published reports [37], the electric field propagating perpendicularly with respect to the external magnetic field as in the present case (**Figure 9a**) encounters the UHR layer in the overdense plasma state. In the present computation, the ordinary mode (O mode) electric field after converting into the slow extraordinary mode (X mode) shows a bending in the perpendicular direction and reaches the UHR region. This is known as 'O – slow X' conversion process that is responsible for generating ion waves and makes the inhomogeneous electric field to shift at the same ion acoustic speed [38].

same location.

#### **Figure 9.**

*(a) Propagation and mode conversion of MW* E~*-field corresponding to an ordinary (*O*)-type resonant mode are shown and (b) Phase angle (*≈ *tan* �<sup>1</sup> *Im E*~ *<sup>=</sup> Re <sup>E</sup>*<sup>~</sup> <sup>Þ</sup> *of MW* <sup>E</sup>~*-field.*

#### **Figure 10.**

*Spatio-temporal evolution of Ez-field profile obtained along the axis of the cylindrical cavity. A sharp change in the two places for both the figures implies the power absorption locations that can be checked with Figure 5 described above. (a) r = 0 mm, (b) r = 28 mm.*

barrier, it is capable of penetrating the core dense plasma and transferring the energy through damping to the plasma particles. As demonstrated by other groups [23, 28], this change in the power coupling mode from the electromagnetic to electrostatic case is a signature of the UHR heating. Due to the heating at UHR, the plasma electron temperature and also the density are possible to enhance further.

It can be observed in **Figure 8** that the magnitude of the Er-field at time t = 67 ns (**Figure 8**) is greater than 7 kV/m along different z-planes within the cavity near the ECR locations. This intensity is sufficient for creating ionization in the Argon gaseous particles. In the later instant (�2 μs) of time, the magnitude of Er-field changes in between 3.75 kV/m and 0.15 kV/m on the ECR regions. This type of nonlinearity in the Er-field has also been demonstrated by Hopwood et al. [4] before. The power coupling phenomena involved during the plasma evolution can

#### *Evolution of Microwave Electric Field on Power Coupling to Plasma during Ignition Phase DOI: http://dx.doi.org/10.5772/intechopen.92011*

also be transmitted into the extracted ion beam performance in terms of spike or instabilities. It was experimentally demonstrated by Ropponen et al. [1] that a sharp transient in the ion current density during the preglow mode can cause a sharp fall in the high energy component of the plasma electron temperature.

Since the inhomogeneous part of Er-field is shifting toward the off-resonance locations as shown in **Figure 8(b)**, it can also produce high-energy electrons there if certain conditions are satisfied as per Gammino et al. [34]. The conditions say the MW electric field has to be above a certain transition value for being capable to energize the electrons that also depend on the magnetic field gradient present in the same location.

The generated high-energy electrons can interact with the slow extraordinarytype microwave and produce the cyclotron range instability in the plasma [1]. It was also proved experimentally by Mansfeld et al. [35] that the extracted ion beam current from the microwave ion source can gain oscillation due to the presence of cyclotron-type instability of plasma during the afterglow operation mode. The slow extraordinary mode microwave is produced from the mode conversion of ordinarytype microwave near the UHR region. As the mode conversion layer is present in the present plasma cavity, the ordinary mode microwave crosses the evanescent layer and some part of its energy is converted into the slow X mode. Since the plasma is confined in the cavity under mirror magnetic field configuration, the injected MW will have two components, extraordinary mode and ordinary mode. **Figure 9(a)** shows the MW ordinary mode is propagating toward the overdense plasma region from the underdense launching point. At some point, it will encounter a cut-off corresponding to the ordinary-type MW. At the cut-off, some part of the ordinary mode MW energy is evanescently transformed into a slow extraordinary mode following the CMA diagram concepts. For that reason, a bend in the MW propagation in the slow extraordinary (X) mode is seen in the electric field simulation (**Figure 9b**). This slow X mode then propagates toward the UHR region and hence the electric field is being accumulated there, as shown in **Figure 9(a)**. The accumulation of the electric field at this location increases its intensity at some plasma condition and can cross the corresponding parametric decay threshold condition. The parametric decay of the slow X mode near the UHR region can generate ion- and electron-type electrostatic waves as per the literature [36].

As a supportive evidence of the generation of electrostatic ion wave, **Figure 8(b)** shows that the inhomogeneous part of the Er-field is shifting toward the offresonance region with a velocity of 1250–1500 m/s. This range of velocities falls in the range of ion acoustic speed. Based on the published reports [37], the electric field propagating perpendicularly with respect to the external magnetic field as in the present case (**Figure 9a**) encounters the UHR layer in the overdense plasma state. In the present computation, the ordinary mode (O mode) electric field after converting into the slow extraordinary mode (X mode) shows a bending in the perpendicular direction and reaches the UHR region. This is known as 'O – slow X' conversion process that is responsible for generating ion waves and makes the inhomogeneous electric field to shift at the same ion acoustic speed [38].

## **4.4** *Ez***-field evolution in plasma with time**

The spatio-temporal evolution of the axial component (Ez) of the electric field is shown in **Figure 10** throughout the cylindrical cavity along the planes r = 0 mm and 28 mm, respectively. It is seen that only a portion of single wavelength of the wave electric field is present that has significant intensity throughout the plasma cavity after the immediate instant (67 ns) of MW launching. The magnitude of the Ez field becomes almost zero throughout the cavity as soon as the plasma starts creating

barrier, it is capable of penetrating the core dense plasma and transferring the energy through damping to the plasma particles. As demonstrated by other groups [23, 28], this change in the power coupling mode from the electromagnetic to electrostatic case is a signature of the UHR heating. Due to the heating at UHR, the plasma electron temperature and also the density are possible to enhance further. It can be observed in **Figure 8** that the magnitude of the Er-field at time t = 67 ns (**Figure 8**) is greater than 7 kV/m along different z-planes within the cavity near the ECR locations. This intensity is sufficient for creating ionization in the Argon gaseous particles. In the later instant (�2 μs) of time, the magnitude of Er-field changes in between 3.75 kV/m and 0.15 kV/m on the ECR regions. This type of nonlinearity in the Er-field has also been demonstrated by Hopwood et al. [4] before. The power coupling phenomena involved during the plasma evolution can

*Spatio-temporal evolution of Ez-field profile obtained along the axis of the cylindrical cavity. A sharp change in the two places for both the figures implies the power absorption locations that can be checked with Figure 5*

*(a) Propagation and mode conversion of MW* E~*-field corresponding to an ordinary (*O*)-type resonant mode are*

*<sup>=</sup> Re <sup>E</sup>*<sup>~</sup> <sup>Þ</sup> *of MW* <sup>E</sup>~*-field.*

**Figure 10.**

**58**

**Figure 9.**

*described above. (a) r = 0 mm, (b) r = 28 mm.*

*shown and (b) Phase angle (*≈ *tan* �<sup>1</sup> *Im E*~

*Selected Topics in Plasma Physics*

after the MW launching. Therefore, the Ez field does not play any significant role in accelerating the electrons, and if this would happen, the electrons losses will occur in the axial direction. **Figure 10(a–b)** evidences that the Ez field becomes almost zero everywhere except near the ECR surface after 8 μs. The Ez-field magnitude shows minimum values where the plasma density is of maximum values.

It can be observed in **Figure 10(a)** that the polarity in the inhomogeneous part of the electric field is getting opposite for two different time instances, 20 and 85 μs. The reversal in the polarity of electric field occurs near the ECR surface. The polarity reversal is caused by the ambipolar field produced from the plasma density gradient. The plasma density gradient is computed from the electron momentum equation using drift-diffusion approach in the present FEM model [38]. The shifting of the inhomogeneous part of the electric field is in the inward direction. The speed of displacement of the inhomogeneous part of the electric field is estimated in the range of <sup>10</sup><sup>3</sup> m/s that lies in the range of ion sound speed. Similar shifting at the same velocity is also observed before in **Figure 8**(b) corresponding to the Er*-*plots. These observations indicate that the plasma density gradient near the ECR surface is accompanied by the generation of ion acoustic waves that are electrostatic in nature [39, 40]. Hence, the electric field polarity reversal associated with electrostatic ion wave heating is being initiated during this period of the plasma evolution after the microwave launch.

### **5. Validation with experiment**

Experiments are performed to cross-check the above-mentioned plasma parameters obtained during the gas ignition moment. The present section of the chapter provides the details of the experimental methods, analysis of the experimental results and also a comparative study of the experimental data with the simulation.

impedance of WR284 waveguide (four-step ridge waveguide) having 220-mm length is made comparable to the typical plasma impedance by embedding four ridge sections on both the inner side walls of it. The ridge sections of different lengths but same width (48 mm) are placed consecutively at the broader walls of the waveguide. As shown in **Figure 11**, the ridge waveguide is mechanically connected to one side of a high voltage (HV) break cum vacuum window, made of Teflon having a dimension of 35 mm diameter and 6 mm thickness, whose other side is connected to the ion source cavity (**Figure 11**). The other side of the cavity is connected to the conventional pierce geometry-based 3-grid extraction system, housed in a drift duct vacuum chamber that consists of a plasma grid, an extraction grid and a grounded grid (**Figure 11**). The ion source is evacuated by a combination of Turbo Molecular Pump, which is backed by a dry-scroll rotary pump, connected to the drift duct vacuum chamber in the downstream side of the extraction system. The gas feed system comprises a needle valve, mass flow controller and the required gas cylinder. In the present experiment, nitrogen gas is used because of availability. Other gases, like argon or hydrogen, can also be used. The experiment is carried out for the MW power in the range of 50–700 W and gas pressure in the range of <sup>1</sup> <sup>10</sup><sup>3</sup> mbar to 1 <sup>10</sup><sup>4</sup> mbar. The Langmuir probe (LP) diagnostics is used to characterize the plasma parameters within the pressure range varying from 2

*Evolution of Microwave Electric Field on Power Coupling to Plasma during Ignition Phase*

All the MW power ranges mentioned throughout the chapter are considered as

, power ratio) at the directional coupler ports

set power. The difference of forward and reflected power is considered to be plasma-absorbed power. MW reflection varies from 5 to 10% within the above set power range. These ranges of plasma reflection with similar experimental set-ups and operating environments are reported in [10]. The accuracy at low set power levels of magnetron is tested by repeated measurements of its output power (forward) before the experiment is performed. An extra component, named isolator with water-cooled dummy load (make: National Electronics, Model: 2722-162- 10311, isolation: 26 dB, reflection rating: 6.5 kW) is placed in the experimental setup (not shown in **Figure 11**) before the HV break. The power and carrier frequency signals are measured by microwave spectrum analyzer (model: FSH8, make: ROHDE & SCHWARZ, band: 100 Hz–8 GHz) at the directional coupler port.

<sup>10</sup><sup>4</sup> to 1 <sup>10</sup><sup>3</sup> mbar [23].

**Figure 11.**

*Schematic view of the experimental system.*

*DOI: http://dx.doi.org/10.5772/intechopen.92011*

Signals are attenuated by 60 dB ( 106

**61**

#### **5.1 Methods of experiment**

Experiments are carried out in a microwave ion source system that has similar system configuration, magnetic field distribution, MW conditions and also the operating conditions. The simulated temporal plasma parameters, such as the plasma density and hot electron temperature, are validated with the experiment [23]. In the present experiment, MW-plasma reactor of the experimental set-up is a cylindrical cavity (**Figure 11**) of 107-mm length and 88-mm diameter. The plasma in the reactor is generated by coupling microwave through the electron cyclotron resonance (ECR) heating as well as off-ECR heating methods, as discussed before. The complete experimental set-up consists of a cylindrical cavity, microwave system, ion beam extraction system and two pairs of ring magnets (each magnet has pole strength 1.38 T) assembly [23]. The plasma cavity/reactor is surrounded by two pairs of ring magnets to generate a mirror-type magnetic field to confine the plasma inside the cavity.

To generate plasma, the MW is produced by a magnetron (power: 0–2 kW, make: Richardson Electronics, Model no. NL10250-7), which is operated either in continuous or in pulsed mode. MW power is fed to the reactor through a combination of a four-step ridged waveguide, a HV break and vacuum window assembly, an impedance tuner unit, directional coupler and an isolator with water dummy load (**Figure 11**). The plasma impedance is matched by a 3-stub tuner to get maximum E-field at the center of the MW-plasma reactor. MW is coupled to the ~ cylindrical plasma reactor by a four-step ridged waveguide (WR 284) [23]. The

*Evolution of Microwave Electric Field on Power Coupling to Plasma during Ignition Phase DOI: http://dx.doi.org/10.5772/intechopen.92011*

**Figure 11.** *Schematic view of the experimental system.*

after the MW launching. Therefore, the Ez field does not play any significant role in accelerating the electrons, and if this would happen, the electrons losses will occur in the axial direction. **Figure 10(a–b)** evidences that the Ez field becomes almost zero everywhere except near the ECR surface after 8 μs. The Ez-field magnitude

It can be observed in **Figure 10(a)** that the polarity in the inhomogeneous part of the electric field is getting opposite for two different time instances, 20 and 85 μs. The reversal in the polarity of electric field occurs near the ECR surface. The polarity reversal is caused by the ambipolar field produced from the plasma density gradient. The plasma density gradient is computed from the electron momentum equation using drift-diffusion approach in the present FEM model [38]. The shifting of the inhomogeneous part of the electric field is in the inward direction. The speed of displacement of the inhomogeneous part of the electric field is estimated in the range of <sup>10</sup><sup>3</sup> m/s that lies in the range of ion sound speed. Similar shifting at the same velocity is also observed before in **Figure 8**(b) corresponding to the Er*-*plots. These observations indicate that the plasma density gradient near the ECR surface is accompanied by the generation of ion acoustic waves that are electrostatic in nature [39, 40]. Hence, the electric field polarity reversal associated with electrostatic ion wave heating is being initiated during this period of the

Experiments are performed to cross-check the above-mentioned plasma parameters obtained during the gas ignition moment. The present section of the chapter provides the details of the experimental methods, analysis of the experimental results and also a comparative study of the experimental data with the simulation.

Experiments are carried out in a microwave ion source system that has similar system configuration, magnetic field distribution, MW conditions and also the operating conditions. The simulated temporal plasma parameters, such as the plasma density and hot electron temperature, are validated with the experiment [23]. In the present experiment, MW-plasma reactor of the experimental set-up is a cylindrical cavity (**Figure 11**) of 107-mm length and 88-mm diameter. The plasma in the reactor is generated by coupling microwave through the electron cyclotron resonance (ECR) heating as well as off-ECR heating methods, as discussed before. The complete experimental set-up consists of a cylindrical cavity, microwave system, ion beam extraction system and two pairs of ring magnets (each magnet has pole strength 1.38 T) assembly [23]. The plasma cavity/reactor is surrounded by two pairs of ring magnets to generate a mirror-type magnetic field to confine the

To generate plasma, the MW is produced by a magnetron (power: 0–2 kW, make: Richardson Electronics, Model no. NL10250-7), which is operated either in continuous or in pulsed mode. MW power is fed to the reactor through a combination of a four-step ridged waveguide, a HV break and vacuum window assembly, an impedance tuner unit, directional coupler and an isolator with water dummy load (**Figure 11**). The plasma impedance is matched by a 3-stub tuner to get maximum E-field at the center of the MW-plasma reactor. MW is coupled to the

cylindrical plasma reactor by a four-step ridged waveguide (WR 284) [23]. The

shows minimum values where the plasma density is of maximum values.

plasma evolution after the microwave launch.

**5. Validation with experiment**

*Selected Topics in Plasma Physics*

**5.1 Methods of experiment**

plasma inside the cavity.

~

**60**

impedance of WR284 waveguide (four-step ridge waveguide) having 220-mm length is made comparable to the typical plasma impedance by embedding four ridge sections on both the inner side walls of it. The ridge sections of different lengths but same width (48 mm) are placed consecutively at the broader walls of the waveguide. As shown in **Figure 11**, the ridge waveguide is mechanically connected to one side of a high voltage (HV) break cum vacuum window, made of Teflon having a dimension of 35 mm diameter and 6 mm thickness, whose other side is connected to the ion source cavity (**Figure 11**). The other side of the cavity is connected to the conventional pierce geometry-based 3-grid extraction system, housed in a drift duct vacuum chamber that consists of a plasma grid, an extraction grid and a grounded grid (**Figure 11**). The ion source is evacuated by a combination of Turbo Molecular Pump, which is backed by a dry-scroll rotary pump, connected to the drift duct vacuum chamber in the downstream side of the extraction system. The gas feed system comprises a needle valve, mass flow controller and the required gas cylinder. In the present experiment, nitrogen gas is used because of availability. Other gases, like argon or hydrogen, can also be used. The experiment is carried out for the MW power in the range of 50–700 W and gas pressure in the range of <sup>1</sup> <sup>10</sup><sup>3</sup> mbar to 1 <sup>10</sup><sup>4</sup> mbar. The Langmuir probe (LP) diagnostics is used to characterize the plasma parameters within the pressure range varying from 2 <sup>10</sup><sup>4</sup> to 1 <sup>10</sup><sup>3</sup> mbar [23].

All the MW power ranges mentioned throughout the chapter are considered as set power. The difference of forward and reflected power is considered to be plasma-absorbed power. MW reflection varies from 5 to 10% within the above set power range. These ranges of plasma reflection with similar experimental set-ups and operating environments are reported in [10]. The accuracy at low set power levels of magnetron is tested by repeated measurements of its output power (forward) before the experiment is performed. An extra component, named isolator with water-cooled dummy load (make: National Electronics, Model: 2722-162- 10311, isolation: 26 dB, reflection rating: 6.5 kW) is placed in the experimental setup (not shown in **Figure 11**) before the HV break. The power and carrier frequency signals are measured by microwave spectrum analyzer (model: FSH8, make: ROHDE & SCHWARZ, band: 100 Hz–8 GHz) at the directional coupler port. Signals are attenuated by 60 dB ( 106 , power ratio) at the directional coupler ports before coming to microwave spectrum analyzer through the high frequency (0–40 GHz), low loss and low VSWR cable of length 1 m.

To generate plasma in the low power range, magnetron's low power testing is required. In the present experiment, low power testing ensures the variation of full width at half maximum of 2.45 GHz frequency is within 5–12 MHz (within the specified 25 MHz bandwidth as per Sairem data). Also, the set power fluctuations are within 2–5% as shown in **Figure 12(a)**, which is considered to be stable operating conditions. After performing the inverse Fourier transform of the MW spectrum analyzer data, a graph of detected MW power vs. time is shown in **Figure 12 (b)** to verify the power levels at the first few milliseconds for the comparison of plasma parameters with the simulated data. The rise time of the pulse is obtained as 2.2 μs.

The magnetron's pulse response at a fixed set power, 200 W, is obtained by taking the inverse Fourier transform of the microwave spectrum analyzer data. The magnetron's rise time is �2.2 μs. This exercise of measuring the pulse response of a particular set power can help to pick up the temporal values at different set power

*Evolution of Microwave Electric Field on Power Coupling to Plasma during Ignition Phase*

To compare the simulated hot electron temperature and density, their parameters are noted down at different instant of time during the plasma evolution. Taking the time instant to be same as that of the simulation, the set power values are noted down in an experimental datasheet. Then, a single Langmuir probe measures the plasma floating potential at the noted set power values as mentioned above [23]. The Langmuir probe measurement is performed in steady state plasma condition. Since the real-time measurement of the plasma parameter requires in the ns-time scale very sophisticated and expensive hardware that has faster time responses (ns range), steady state Langmuir probe measurements are performed to avoid

The line plots are shown in **Figure 13(a)** for the simulated hot Te. The simulated data point lies in a region that is very close to the experimental data points. The simulated hot Te is approximately 78 eV at 70 W of plasma absorbed power during time t = 600 ns. One can also observe that the set power for the magnetron reaches 50 W at the same time instant. The measurement of the hot Te by a Langmuir probe diagnostic at the same set power level shows it to be � 72 eV. Similarly, the hot Te was measured for two other power levels, 70 and 130 W. The hot Te at these two power levels are � 36 and 28 eV, respectively. Hence, the experimental results fit well with the simulated values described above [23]. On the other hand, the experimental data for the plasma density are also shown in **Figure 13(b)**. The plasma densities for the above set power levels are observed to be �1.8–2 times less than the simulated data. The deviation of the measured plasma density from the simulated values is caused due to the difference in the absorbed power that is absorbed by the plasma in both the cases (experiment and simulation). The experimental results for the plasma density are shown in **Figure 13(b)** for three different set powers. The measured plasma density (�1.1 �1017m�<sup>3</sup><sup>Þ</sup> at set power of 200 W agrees well with

*(a) Simulated temporal hot* Te *variation along with experimental data, taken at discrete set power values. Plasma-absorbed power during simulated hot* Te *evolution is fixed at 70 W. Experimental set powers 50, 70 and 130 W correspond to plasma-absorbed powers at 40, 50 and 70 W, respectively [14]. Pulse rise of set power is also shown to verify the power levels during the ns to few μs periods. (b) Simulated temporal plasma*

*density variation along with experimental results, obtained at the same set power values.*

levels following the pulse response of magnetron [23].

*DOI: http://dx.doi.org/10.5772/intechopen.92011*

**5.2 Comparison of experimental results with simulation**

the simulated plasma density (�1.3 �1017m�<sup>3</sup>Þ.

**Figure 13.**

**63**

those expensive diagnostics.

The magnetron's output (set power) in the low power range is checked in time scale prior to the Langmuir probe diagnostic [23]. The response of the magnetron set power at 2.45 GHz frequency is recorded at the directional coupler port by a microwave spectrum analyzer (SA) circuitry. The circuitry consists of a high frequency cable, a band pass filter, and spectrum analyzer and FSH4 View software.

#### **Figure 12.**

*(a) Percentage error of the magnetron set power fluctuations. Data for 280 W have been benchmarked with the Sairem company data and (b) variation of the detected MW power vs. time for the set power of 200 W.*

#### *Evolution of Microwave Electric Field on Power Coupling to Plasma during Ignition Phase DOI: http://dx.doi.org/10.5772/intechopen.92011*

The magnetron's pulse response at a fixed set power, 200 W, is obtained by taking the inverse Fourier transform of the microwave spectrum analyzer data. The magnetron's rise time is �2.2 μs. This exercise of measuring the pulse response of a particular set power can help to pick up the temporal values at different set power levels following the pulse response of magnetron [23].

To compare the simulated hot electron temperature and density, their parameters are noted down at different instant of time during the plasma evolution. Taking the time instant to be same as that of the simulation, the set power values are noted down in an experimental datasheet. Then, a single Langmuir probe measures the plasma floating potential at the noted set power values as mentioned above [23]. The Langmuir probe measurement is performed in steady state plasma condition. Since the real-time measurement of the plasma parameter requires in the ns-time scale very sophisticated and expensive hardware that has faster time responses (ns range), steady state Langmuir probe measurements are performed to avoid those expensive diagnostics.

#### **5.2 Comparison of experimental results with simulation**

The line plots are shown in **Figure 13(a)** for the simulated hot Te. The simulated data point lies in a region that is very close to the experimental data points. The simulated hot Te is approximately 78 eV at 70 W of plasma absorbed power during time t = 600 ns. One can also observe that the set power for the magnetron reaches 50 W at the same time instant. The measurement of the hot Te by a Langmuir probe diagnostic at the same set power level shows it to be � 72 eV. Similarly, the hot Te was measured for two other power levels, 70 and 130 W. The hot Te at these two power levels are � 36 and 28 eV, respectively. Hence, the experimental results fit well with the simulated values described above [23]. On the other hand, the experimental data for the plasma density are also shown in **Figure 13(b)**. The plasma densities for the above set power levels are observed to be �1.8–2 times less than the simulated data. The deviation of the measured plasma density from the simulated values is caused due to the difference in the absorbed power that is absorbed by the plasma in both the cases (experiment and simulation). The experimental results for the plasma density are shown in **Figure 13(b)** for three different set powers. The measured plasma density (�1.1 �1017m�<sup>3</sup><sup>Þ</sup> at set power of 200 W agrees well with the simulated plasma density (�1.3 �1017m�<sup>3</sup>Þ.

#### **Figure 13.**

before coming to microwave spectrum analyzer through the high frequency

To generate plasma in the low power range, magnetron's low power testing is required. In the present experiment, low power testing ensures the variation of full width at half maximum of 2.45 GHz frequency is within 5–12 MHz (within the specified 25 MHz bandwidth as per Sairem data). Also, the set power fluctuations are within 2–5% as shown in **Figure 12(a)**, which is considered to be stable operating conditions. After performing the inverse Fourier transform of the MW spectrum analyzer data, a graph of detected MW power vs. time is shown in **Figure 12 (b)** to verify the power levels at the first few milliseconds for the comparison of plasma parameters with the simulated data. The rise time of the pulse is obtained

The magnetron's output (set power) in the low power range is checked in time scale prior to the Langmuir probe diagnostic [23]. The response of the magnetron set power at 2.45 GHz frequency is recorded at the directional coupler port by a microwave spectrum analyzer (SA) circuitry. The circuitry consists of a high frequency cable, a band pass filter, and spectrum analyzer and FSH4 View software.

*(a) Percentage error of the magnetron set power fluctuations. Data for 280 W have been benchmarked with the Sairem company data and (b) variation of the detected MW power vs. time for the set power of 200 W.*

(0–40 GHz), low loss and low VSWR cable of length 1 m.

as 2.2 μs.

*Selected Topics in Plasma Physics*

**Figure 12.**

**62**

*(a) Simulated temporal hot* Te *variation along with experimental data, taken at discrete set power values. Plasma-absorbed power during simulated hot* Te *evolution is fixed at 70 W. Experimental set powers 50, 70 and 130 W correspond to plasma-absorbed powers at 40, 50 and 70 W, respectively [14]. Pulse rise of set power is also shown to verify the power levels during the ns to few μs periods. (b) Simulated temporal plasma density variation along with experimental results, obtained at the same set power values.*
