Essay: Ion source and low energy beam transport line

2.1 Introduction
This chapter gives a brief overview of the 2.45 GHz microwave ion source and low energy beam transport (LEBT) system developed at VECC. This chapter is organized as follows: In section 2.2, the 2.45 GHz microwave ion source along with its different sub-systems is described; while the LEBT line associated with the microwave ion source and different beam diagnostic devices used for the measurement of ion beam parameters are described in section 2.3 and section 2.4, respectively. In section 2.5 the results of the measurements of total extracted beam current and atomic ion fraction as a function of its different operational parameters is described. The chapter concludes with a summary and outlook in section 2.6.
2.2 Description of the microwave ion source
The Microwave ion source (MIS) at VECC has been designed and constructed for 75 keV beam energy and 15 mA beam current. In the (MIS) at VECC, the hydrogen atoms are ionized by the electron impact ionization. The energy required for the ionization of atoms is gained by the electrons through an electric field setup in the plasma chamber by the incoming microwave power. The microwave power is transferred from the generator to the plasma chamber through a series combination of rectangular waveguides and impedance matching elements. The chamber is immersed in an axial non-confining magnetic field produced by two adjustable electromagnets. Finally, the generated ions are pulled out from the plasma chamber through an orifice in the plasma electrode by applying particular voltages to the extraction system. A schematic sketch of the ion source is shown in figure 3.1.
Figure 3.1: 1. Microwave coupling transformer, 2. Boron Nitride and Aluminum Nitride window, 3. Plasma chamber, 4. Boron Nitride disc, 5. Injection magnet coil, 6. Extraction magnet coil, 7, 8. Motorized shafts, 9. Plasma electrode, 10. Suppressor electrode, 11. Ground electrode.
The main subsystems of ion source are the plasma chamber and gas feeding system, the microwave system, the magnetic system and the extraction system. The details of these sub systems are described in the next few sub-sections.
2.1.1 Plasma chamber and gas feeding system
The ion source has a double walled plasma chamber made of stainless steel 304L. The chamber is 90 mm in diameter and 100 mm long. The longitudinal channels are made on the outer surface of the first wall of the plasma chamber for water cooling. In order to block the high flux of back streaming electrons coming from the plasma into the microwave coupling transformer, a series combination of Aluminium nitride (AlN) and Boron Nitride (BN) plate is used at the entry of the plasma chamber. The AlN plate is chosen to be placed in front of the plasma, due to its high thermal conductivity and low coefficient of thermal expansion. Due to the virtue of this property, AlN plate can easily sustain the heat load generated due to the back streaming electrons, without fracture. The BN plate is placed behind the AlN plate, as a precautionary measure, if the AlN plate breaks due to the thermal stress. Another BN plate is fixed on the plasma electrode of the ion source to reduce the generation of molecular ions, due to recombination reactions taking place in the plasma. The hydrogen gas is introduced into the plasma chamber, through an adjustable leak valve EVR 116 [1]. The gas flow is regulated by the leak valve and is kept in the range of 0.4 – 1 sccm for different operating conditions of the ion source. The plasma chamber and extractor region is pumped by two sets of scroll and turbo-molecular pump, each of 500 l/s pumping capacity. This pumping configuration gets a pressure of ~10−3 mbar in the plasma chamber and of ~10−6 mbar in the extraction region.
2.1.2 Microwave System
The microwave system of the ion source is shown in figure 3.2. The CW microwave power at a frequency of 2.45 GHz is generated by a 1.2 kW water cooled Magnetron and guided ahead through a WR 340 rectangular waveguide, operating in the TE10 dominant mode. A dual-directional coupler with a 60 dB coupling factor and a directivity of 30 dB is installed to record forward and reflected power arising due to the impedance mismatch between waveguide and plasma. A combination of manual 3-stub tuner and a four stub automatic tuner is used to adjust the amplitude and phase of the waveguide impedance to that of plasma impedance. A tapered waveguide WR340-WR284 adapter is used to match the input impedance of microwave coupler to the WR340 section of the microwave system.
Figure 3.2: 1. Magnetron, 2. Circulator, 3. Dummy load, 4. Dual directional coupler, 5. Three stub tuner, 6. E-plane bend, 7. Automatic tuner, 8. E-plane bend, 9. WR-340 to WR-284 transition, 10. Microwave coupling transformer, 11. Plasma chamber.
Finally a four-step double ridged microwave coupling transformer is connected before the plasma chamber. This transformer is designed to match the WR-284 waveguide impedance to the equivalent plasma impedance and to concentrate the electric field in the centre of the plasma chamber, leading to a high plasma density. Figure 3.3 (a) and (b), shows the front and back view of the water cooled microwave coupling transformer. The detailed design, simulation and characteristics of the coupling transformer will be described in the next chapter.
Figure 3.3: a. Plasma side, b. air side, c. side view, the cooling lines are visible in the photograph.
2.1.3 Magnetic System
The magnetic system of the ion source is composed of two axially movable solenoid coils as shown in figure 3.4. In most of the ion sources operating at 2.45 GHz, the distance between the two electromagnets is fixed in such a way so that the resonance field of 87.5 mT occurs at the location of two dielectric windows which are positioned at the two ends of plasma chamber [2-4].
Figure 3.4: 1. Extraction coil, 2. Injection coil, 3. d is spacing between the coils.
This feature helps to increase the plasma density, which further leads to a high extracted beam current from the ion source. A magnetic field profile is setup in the ion source that is almost flat and exceeds the resonance field along complete length of the plasma chamber, except at its two extremes. This field profile is measured by a Hall probe and is shown in figure 3.5. It is obtained by regulating the dc current flowing in each coil at 103 A and by positioning the coils at distance d of 128 mm.
Figure 3.5: The measured axial magnetic field along the axis of the plasma chamber.
2.1.4 Extraction system
Figure 3.6: a: Plasma chamber, b: Plasma electrode, c: Acceleration electrode, d: Ground electrode.
The ion beam extraction system of the MIS is arranged in an accel-decel configuration as shown in figure 3.6. This configuration has three electrodes, i.e. a plasma, acceleration and ground electrode. All the three electrodes are made of stainless steel. The insulators made of 99.5 % pure alumina ceramic are used to withstand the applied high voltage between the different electrodes. The ion beam is extracted from plasma chamber through a hole of 6 mm in diameter in the centre of the plasma electrode. The plasma electrode is shorted with the plasma chamber body and therefore it rises to the extraction potential of the ion source, which is usually kept at 75 kV. The acceleration electrode with an aperture diameter of 8 mm and length of 177 mm is placed at a distance of 16.5 mm from the plasma electrode. It is biased to a potential of ~ -2 kV during ion source operation. This negative potential repels the low-energy secondary electrons produced in the beam-line and prevents them to enter into the extraction region. Due to this phenomenon, an ion beam with a higher degree of space-charge compensation is extracted from the ion source. Finally, a ground electrode at zero potential is positioned 5.8 mm behind the acceleration electrode with an aperture diameter of 8 mm and a length of 235 mm.
Fig. 3.7 shows a photograph of assembled MIS showing the microwave feeding system, the electromagnets, the hydrogen gas feeding system and the extraction system.
Figure 3.7: 1. Isolation transformer, 2. 2.45 GHz water cooled magnetron, 3. Magnetron power supply and control unit, 4. Circulator and dummy load assembly, 5. Dual directional coupler and diode detector, 6. Four stub auto tuner, 7. H2 gas cylinder, 8,9. AC synchronous motor, 10. Water cooled double ridged waveguide, 11. Injection magnet coil, 12. Extraction magnet coil, 13. Extraction system, 14. High voltage deck.
2.2 Low Energy Beam Transport
Figure 3.8: 1. DCCT, 2. SM1, 3. ST, 4. SL, 5. FC1, 6. SM2, 7. Neon gas cylinder, 8. Gas dosing valve, 9. DC, 10. FC2.
The extracted ion beam from the MIS is focussed, selected and characterized in the LEBT system shown in Fig. 3.8. It consists of two solenoid magnets (SM1 and SM2), steering magnet (ST), vacuum equipments, water-cooled x-y slit (SL) and several diagnostic elements for ion beam characterization. The extracted ion beam from MIS primarily consists of H+, H2+ and H3+ ions. The total extracted beam current comprising of all the ionic species is measured by a DC current transformer (DCCT), denoted as I_DCCT throughout the thesis. In order to focus protons and reject molecular ions out of a mixture of ions coming from the ion source, a solenoid magnet (SM1) with a peak field of 0.4 T and effective length 0.4 m followed by a water cooled x-y slit installed at the waist location of the SM1 magnet is used. The current in SM1 magnet is setup to focus the protons at the location of water cooled slit (SL) with an aperture of 10 mm X 10 mm. Due to the dependence of focal length of the solenoid on the velocity of ions, the molecular ion diverges at the slit location and collides with the x and y jaws of slit [5]. The selected proton beam current is measured by a water cooled faraday cup (FC1) with electric suppression, installed just after the slit. The beam is focussed and transported further by a solenoid magnet SM2, with identical parameters as that of magnet SM1. Furthermore, beam characteristics like beam spot, beam emittance and beam profile is measured in the diagnostic chamber (DC), placed at a distance of 0.8 m after the SM2 magnet. Finally, a water cooled tapered faraday cup (FC2) is installed at the end of beam transport line to record the proton beam current and it also acts as a beam stop.
2.3 Beam Diagnostic elements
The ion beam transmission from the ion source to the end of LEBT line is dependent on different parameters of the ion source and LEBT system. In order to quantify the dependence of ion beam properties on these parameters different diagnostic devices like, DCCT, movable slits, Faraday cups, and an beam profile monitor are installed in the LEBT line of MIS. In this section, different beam diagnostic devices used during the measurements will be described.
2.3.1 Faraday Cup
Faraday cup (FC) is the most popular diagnostic tool for beam current measurement [6-8]. In the LEBT line of MIS two water cooled FC (FC1 and FC2) are installed. The FC1 is a standard FC with a 1 kW power dissipation capability and is placed just after the slit (SL), where a beam power above 500 W is regularly obtained. Figure 3.9 shows the photograph of FC1.
Figure 3.9: 1. Water cooled cup with aperture plate, 2. Pneuamatic cylinder, 3, 4. Limit swtitches, 5. Flange with BNC connector for current readout and electron suppression bias supply.
The FC1 is inserted into the beam path to measure the beam current and is retracted out of the beam line to transport the beam ahead. The movement of the FC1 is made with the pneumatically actuated and electrically controlled solenoid valve. The readout of the FC1 position is obtained by two micro switches provided on the water-cooled actuator assembly. The beam is stopped in a water-cooled cup installed in the bottom of the FC1 assembly. The beam stopper cup is electrically isolated and thermally conductive to the FC1 body. The maximum acceptable beam diameter of FC1 is fixed by an aperture plate installed in front of the beam stopper. After the first aperture plate a bias aperture plate is installed. It is negatively biased and reflects back the secondary electrons coming from the FC1 towards itself. The electrical connection to the beam stopper cup and bias aperture is brought outside vacuum through individual ceramic feed through. Finally, the FC1 signal is dropped across a precision resistor and developed voltage is readout, which is displayed on the graphical user interface of LEBT line. Another water cooled FC, FC2 is installed at the end of LEBT line. Figure 3.10 shows a photograph of the FC2.
Figure 3.10: a. Vacuum side, with an aperture plate of 20 mm diameter, b. Air side, with a water cooled conical beam stop.
It is a fixed FC and does not have any actuation mechanism. The beam stopper of FC2 is a tapered conical cup with longitudinal water cooling channels. It is mounted on a DN100 CF flange and isolated from the beam line with a Teflon block. The current readout of FC2 is obtained in a similar way as that of FC1.
2.3.2 Beam viewer
The primary method to acquire an image of the beam cross-section is the use of scintillating screens, where the scintillation produced by the impinging beam on the screen is captured through a video camera. Usually, this arrangement is known as beam viewer. The choice of a scintillating material depend on factors like light output, sensitivity and life time. The YAG: Ce and ZnS scintillators are generally used for low beam currents, whereas for high power ion beams, doped alumina ceramic is generally used [9,10]. Figure 3.11 shows the photograph of a beam viewer which is installed in the diagnostic chamber of the LEBT line.
Figure 3.11. 1. Alumina disc with x and y grid lines, 2. Vacuum bellow, 3. Pneuamatic actuator, 4. Water cooling line.
In the beam viewer, a 2 mm thick and 70 mm diameter alumina ceramic disc is used as a scintillating screen. This screen is marked with horizontal and vertical lines each spaced at a uniform distance of 5mm. The alumina ceramic disc is mounted on a water cooled plate made of copper. The water cooled plate is attached to a pneumatic actuator. The beam viewer is brought in the beam path by the actuator to capture the beam cross-section on the screen and after a few seconds the beam viewer is retraced back to allow the beam to travel upto FC2.
2.3.3 Direct-current current transformer
A DC current transformer (DCCT) is an indispensable diagnostic device for the non-intrusive measurement of dc beam current in high intensity machines. The DCCT works on the principle of zero flux detection combined with active feedback circuit [11,12]. The DCCT consists of two cores, which are driven in anti phase to saturation region by a modulation current. The excitation of these cores to saturation causes distortions in their modulation current waveforms. These distortions give rise to odd-harmonics in the frequency content of the modulation current waveform. However, due to opposite phase excitation of the two cores, the difference between their modulation current is zero, provided the cores have identical B-H characteristics. A DC current flowing through the cores magnetizes them with the same polarity and as a result both the cores do not reach saturation at the same time. Therefore, the difference between the modulation current of the cores is not zero, which gives rise to a second harmonic component of the modulation frequency. This second harmonic component is detected by a synchronous detection circuit. In order to increase the DCCT bandwidth, the signal proportional to the AC component of the beam current obtained from an AC current transformer is combined with the second harmonic detector signal. The two signals produce a compensating current, which passes through both the cores and generates a magnetic flux that always cancels the magnetic flux produced by the beam current. Therefore, the compensating current is always equal in magnitude to the beam current. The DCCT output is the voltage developed across a high precision resistor by the flow of compensating current through it. In the MIS the total extracted beam is current is measured by a modular parametric current transformer (MPCT) from Bergoz company [13]. The sensor head of DCCT as mounted on a beam pipe is shown in figure 3.12.
Figure 3.12: The sensor head of DCCT installed on a beam pipe.
A shielded cable connects the DB15 connector on the sensor head to the electronics box of DCCT. The electronics box is provided with circuits of modulation, sense and feedback required for the operation of DCCT. Three selectable ranges with a full scale value of 10mA, 20 mA and 30 mA are also provided on the electronics box for beam current measurement. The output voltage of DCCT is ± 5V, corresponding to the full scale value of beam current in the all the ranges. The calibration of DCCT output is done by passing a known value of current through it and recording its value on a current meter. During the regular operation of the ion source, the DCCT readout is displayed on a 3.5 digit digital panel meter.
2.3.4 Water cooled x-y slit
The beam defining slit (SL) is a combination of horizontal and vertical slits, with water cooling and maximum power dissipation capability of 1 kW. Each slit consists of two cylindrical jaws made of tantalum. The two slits are individually positioned driven through a combination of gear box and AC synchronous motors. The position readout of the two slits is obtained with their corresponding multi-turn potentiometer and current readout of the individual jaws is provided through four BNC connectors mounted in a set of two, on the horizontal and vertical slit mounting flange. In the MIS operation, an opening of 10 mm is kept between the horizontal jaws as well as the vertical jaws as shown in figure 3.13.
Figure 3.13: 1,2. Vertical jaws, 3,4. Horizontal jaws.
In each ion source setting, the current readout of the individual slit jaws are recorded and displayed on the control desk computer.
2.3.5 Non-interceptive beam profile monitor
A non-interceptive beam profile monitor is used to measure the beam profile of the high current beam extracted from the MIS. The high current proton beam produces fluorescence in the visible region of the light spectrum due to the excitation of residual gas molecules in the LEBT line. In order to capture the image of the beam induced fluorescence a four port diagnostic chamber with a glass viewing window and water cooled beam viewer is installed in the LEBT line after the SM2 as shown in figure 3.14.
Figure 3.14: 1. SM2 magnet, 2. Neon gas cylinder, 3. Gas dosing valve, 4. CCD camera, 5. Beam viewer installed in the diagnostic chamber.
The image of the beam induced fluorescence produced by the high current proton beam is captured through a CCD camera, installed in front of the viewing window. The captured image is analyzed and the beam profile is obtained by an algorithm implemented in a MATLAB program [14]. Moreover, the beam profile of space charge compensated proton can also be obtained with the injection of a Neon gas into the LEBT line. The gas injection can be done by a gas dosing valve, UDV 146 located just before the diagnostic chamber. This monitor is also used to estimate the beam emittance of the space charge compensated 75 keV, 5 mA proton beam. A detailed discussion about the beam emittance and space charge compensation measurements will be described in chapter 4.
2.4 Experimental Measurements
The microwave driven plasma ion sources are widely used for the generation of high current proton beam in many research projects [15-19]. The beam extracted from these ion sources consists of different ionic species of the injected gas. The currents of different ionic species extracted from the plasma are strongly dependent on different operating parameters of the ion source. At some operational settings of the ion source, the undesired ionic species extracted from the plasma may form a significant fraction of the total beam current and will mainly contribute towards the poor efficiency of beam transport in the LEBT line. Therefore, it is required to identify the operating parameters of the ion source, at which the maximum beam transmission is obtained in the LEBT line for a given microwave power. In order to find out the optimum operating parameters of MIS at VECC, the extracted beam current and proton fraction was measured as a function of gas flow rate, microwave power and magnetic field profile of the ion source. The results of this measurement are described in section 2.4.1.
Another parameter which plays a key role in the performance of ion source is its, plasma density. A higher value of plasma density aids to produce a higher extracted beam current from the ion source. The widely used method to increase the electron density in the ion source is to place an insulator disc with a high secondary electron emission coefficient at the entry and exit of the plasma chamber [20, 21]. The supply of cold electrons from the insulator into the plasma helps to increase the rate of ionization of the injected gas atoms and molecules. In the MIS, a 5 mm thick AlN plate is placed at the entry of the plasma chamber, whereas a 2 mm thick BN plate with an 8 mm diameter orifice is fixed on the plasma electrode. Moreover, the trace amount of water is also introduced into the plasma chamber, due to its effect on the extracted beam current and proton fraction [22]. In order to quantify the effect of adding insulator and water into the plasma chamber on the performance of the ion source, the extracted beam current and proton fraction were measured as a function of microwave power. The results of this measurement are described in 2.4.2.
The diameter of the plasma chamber has a deep influence on the plasma density and atomic ion fraction. Generally, the ion source designers choose a smaller diameter plasma chamber to obtain a higher electric field inside it, leading to a higher plasma density and a high extracted beam current from the ion source. However, due to the increased probability of wall recombination effects with smaller diameter plasma chamber, the atomic ion fraction is reduced. In order to understand the effect of these two competing plasma processes on the performance of the MIS, the extracted beam current and proton fraction was measured for two different aluminium liner inserted into the 90 mm inner diameter plasma chamber. The results of this measurement are described in 2.4.3.
2.4.1 Optimization of the operating parameters of the ion source
2.4.1.1 Gas pressure
In order to perform the experiment described here, the ion source was initially conditioned for several hours in the presence of , magnetic field profile ‘a’ , extraction and suppressor electrode voltages of 75 kV and -2kV at a pressure of ~ 4•10-7 mbar in the extraction region. In the conditioning process, the extraction voltage power supply drain current was regularly monitored and found to be ~ 0.2 mA. The hydrogen gas was then injected into the plasma chamber by adjusting the gas flow rate to setup a pressure of ~1.0 • 10−5 mbar in the extraction region. Thereafter, a microwave power of 100 W was introduced into the plasma chamber; thereby a plasma discharge was created in the chamber. The reflected power was always kept below 1% with the help of a four stub automatic tuner.
Once the current readout of DCCT was stable within ± 0.1 mA, then the data acquisition of extracted beam current and proton fraction was done as a function of microwave power in the range 100 W to 350 W, at four different operating pressures in the extraction region. The measurement results of this experiment are shown in Figure 3.15.
Figure 3.15: Variation of extracted beam current and proton fraction  as a function of microwave power at four different gas pressures (in mbar).
It can be seen from figure 3.15 that the extracted beam current for a fixed amount of microwave power, is higher for low pressure settings as compared to high pressure settings. This phenomenon is related to the reduction of ionization rate with an increase in rate the gas flow. In the case of high pressure setting at fixed microwave power, the cold electron density is increased in the ion source, which effectively brings down the average electron temperature in the plasma. Consequently, the ionization rate is reduced in the plasma, leading to a lower value of extracted beam current. Therefore, to increase the ionization rate at this high pressure setting a relatively high amount of microwave power is required to generate high density plasma with sufficient number of energetic electrons. A minor deviation from the observation reported here can be seen at a pressure setting of 1.1•10-5 mbar. This could be attributed to the fact that there is a insufficient gas flow to generate higher beam current as compared to the pressure setting of 1.8•10-5 mbar, at a fixed value of microwave power. A maximum beam current of 12 mA is extracted from the ion source at a pressure setting of 2.8•10-5 mbar as shown in figure 3.15. The extracted beam current for different pressure settings showed an increasing trend with an increment in the microwave power. However, in contrast to extracted beam current measurements, the proton fraction is found to be nearly constant upto a microwave power of ~ 250 W and drops with a further increase in microwave power. The maximum proton fraction and its slope decreased significantly at higher microwave power levels, for operating pressures in the range of 1.1•10-5 mbar to 4.6•10-5 mbar. This phenomenon occurs due to the fact that at a low operating pressure of 1.1•10-5 mbar, the available gas atoms in the plasma chamber are significantly ionized at a microwave power of ~ 180 W and a maximum proton fraction of ~ 70 % is reached. Above a microwave power level of 180 W there is a drop in η, which may be due to outgassing in the ion source, leading to a contamination of the plasma. As the gas pressure was increased to 1.8•10-5 mbar the proton fraction of ~64 % at a similar microwave level of 180 W was recorded. This reduction occurs probably due to the cooling of energetic electrons responsible for the gas ionization, as explained earlier in this section. In this case, the change in proton fraction above microwave power of 200 W was relatively very less as compared to the previous pressure setting. The main factor responsible for this phenomenon was the comparatively higher gas pressure in the plasma chamber as compared to the previous case This reasoning is also established by measurements at the other two operating pressure levels of 2.8•10-5 mbar and 4.6•10-5 mbar.
2.4.1.2 Drain current
In order to study the extraction system efficiency of MIS, the drain current of extraction and suppressor voltage power supplies were measured as a function of the extracted beam current at different operating pressure levels. The measurement was performed at similar values of extraction voltage and suppressor voltage as mentioned earlier in this section. A parameter ηex is defined as the ratio of extracted beam current to the extraction voltage power supply drain current. This parameter is taken as a measure for the extraction system efficiency. The variation of ηex with forward power at different pressure settings are plotted in figure 3.16.
Figure 3.16: Variation of extraction efficiency ηex as a function of microwave power at four different gas pressures (in mbar).
It can be easily seen from the above plot that at an applied microwave power of 300 W, ηex is above 80 % for different pressure settings, except in the case of 4.6•10-5 mbar, where ηex is 77 %. Another important characteristic of MIS, which can be observed here is that ηex increases with the increase in extracted beam current. The change in ηex is ~10 % for different pressure settings, except at 1.1•10-5 mbar, where the change is limited to ~ 4 %. The suppressor voltage power drain current at a particular pressure level was nearly constant with the change in microwave power. However, a drain current variation of 0.5 mA to 1.0 mA is recorded, under the MIS operation with a set of similar pressure values, mentioned earlier in this section.
2.4.1.3 Magnetic field
In this experiment, the effect of magnetic field profile on the extracted beam current is studied. The extracted beam current is recorded with a change in injection coil current (I_inj) from 97A to 107 A at the fixed values of extraction coil current (I_ex), microwave power and gas pressure. The experiment is carried out at a pressure of 2.8•10-5 mbar with an applied microwave power of 200 W. The reflected power is always maintained below 5 % by the auto tuner, at each set of coil currents. The result of this measurement is shown in figure 3.17 (a). Similarly, the extracted beam current is also recorded with a similar change in extraction coil current keeping I_inj, microwave power and gas pressure at constant value. This result is illustrated in figure 3.17 (b). The proton fraction is found to be nearly constant with the variation in injection and extraction coil currents. It can be seen here, that extracted beam current is very sensitive to the change in injection coil current as compared to extraction coil current. At the resonance magnetic field, a maximum beam current of 7 mA is extracted extracted from the MIS. Similarly, with a current variation of 97 A to 106 A in the extraction coil the extraction beam current was found to decrease with respect to its peak value. Although, the extracted beam current has increased from 1.5 mA to 3.8 mA, when the extraction coil current was changed from 98A to 97 A, but in this setting the plasma was very unstable and did not permit us to take further measurements.
Figure 3.17: (a) Variation of extracted beam current with the change in I_inj. (b) Variation of extracted beam current with the change in I_ex.
2.4.2 Water addition
In order to study the effect of the introduction of water into the plasma chamber on the extracted beam current and proton fraction, we have introduced trace amount of water in the 90 mm diameter plasma chamber. The extracted beam current and proton fraction is recorded as a function of microwave power at a pressure of mbar in the extraction region. The measurement results are shown in figure 3.18. It can be seen from the plot that extracted beam current increases from 7 mA to 9 mA with the addition of water in the plasma chamber. It is quite evident from the graph is that the extracted beam current tends to saturate in the range of forward power between 180W to 200W. The proton fraction shows an increasing trend at lower power levels and settles down to nearly same value as that obtained without the introduction of water in the plasma chamber. This probably happens due to the evaporation of water at higher power levels and as we could not maintain a constant supply of water vapours into the plasma chamber due to the experimental setup limitations, the proton fraction drops with the increase in microwave power.
Figure 3.18: Variation of extracted beam current and proton fraction as a function of microwave power for the case of water addition in the 90 mm diameter plasma chamber.
2.4.3 Different diameter chamber
In order to study the influence of plasma chamber diameter on the performance of ion source, we have performed an experiment to measure the extracted beam current and proton fraction as a function of microwave power for the case of two different aluminium liners with an inner diameter of 35 mm and 45 mm inserted into the plasma chamber. Figure 3.19 shows the photograph of a 45 mm inner diameter aluminium liner inserted into the plasma chamber.
Figure 3.19: 1. Plasma chamber, 2. aluminium liner of 45 mm diameter.
In the experiment described here, a higher field is expected for both the aluminium liners as compared to the 90 mm plasma chamber. Therefore the MIS is operated at a relatively lower gas pressure of mbar in the extraction region, to obtain a high ionization efficiency of the hydrogen gas. All the other parameters like magnetic field, extraction voltage and suppressor voltage are kept at similar values, as described in the sub-section 2.4.1.1. Then, the extracted beam current and proton fraction is recorded with a change in microwave power from 100 W to 300 W. Next, the same experiment is performed again with the 35 mm inner diameter aluminium liner inserted into the plasma chamber. The results of the measurement performed with the insertion of two aluminium liners in MIS, is shown in figure 3.20. It can be seen from the plot, that extracted beam current for the case of both aluminium cylinders is almost similar upto a microwave power of 200 W. However, for microwave power above 200 W, the extracted beam current for smaller I.D cylinder is more than the larger I.D cylinder. Moreover, the extracted beam current for smaller I.D cylinder saturates at lower forward power as compared to the case of larger I.D cylinder. The variation in the behaviour of the two aluminium cylinders probably arises due to the fact that, for a fixed amount of applied
Figure 3.20: Variation of extracted beam current and proton fraction as a function of microwave power for the case of 35 mm and 45 mm inner diameter cylinder.
microwave power a higher electric field is built up in the smaller I.D cylinder as compared to the case of larger I.D cylinder. The higher electric field in smaller I.D aluminium cylinder has a higher ionization probability and therefore, produces a higher amount of extracted beam current from the MIS.
It can be observed in figure 3.20 that, for the case of smaller I.D cylinder, η remains nearly constant at ~ 45% upto to a microwave power of 200 W and falls linearly to ~ 20 % at a microwave power of 300 W. However, in the case of other cylinder, η of ~ 37 % is recorded upto a microwave power of 180 W. The η drops down with a further increase in microwave power and reduces to ~ 28 % at a power of 300 W. The variation of proton fraction with aluminium cylinder I.D possibly occurs due to the two different competing phenomenon taking place in the hydrogen plasma. The first one is related with the generation of higher electric field in the smaller ID cylinder as compared to the other cylinder. As a consequence, high density plasma is generated in the smaller ID cylinder, leading to a high proton fraction from the MIS. Whereas, the second phenomenon is associated with the increase in recombination rate of ions with the decrease in plasma chamber diameter as described in [23]. However, in our case we have used BN disc in the plasma chamber to limit the recombination of ions to the wall, therefore with a 35 mm ID aluminium cylinder inserted into the plasma chamber, we are still able obtain a higher proton fraction as compared to the case of 45 mm ID aluminium cylinder.
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