An experimental investigation of the proton fraction in a three electrode 2.45 GHz ECR ion source

The ion beam extracted from a proton ion source contains H+ ion along with H2 + and H3 + molecular ions. In this work, the ionic fractions in the extracted beam of ECR ion source were measured using an analyzing magnet. A computerized proton fraction measurement setup is described in detail with instrumentation and control for pulsed operation of the ECR ion source. A timing diagram has been incorporated for trigger synchronization illustrating the process of pulsing the plasma, controlling the analyzing magnet parameters and acquiring data about the magnetic field and beam current. The proton fraction was measured in two cases, in the first case using SS304 plasma chamber and in the second case incorporating aluminium (Al) cylinder and boron nitride (BN) plates inside the plasma chamber. In the first case, the proton fraction was found to be in the range of ∼60% and in the second case because of the presence of secondary electron donors it increased to 93%. In order to understand the enhancement of H+ fraction in the hydrogen plasma, plasma parameters (ne , Te and EEDF) and its dependence on microwave power and neutral gas pressure were investigated.


Introduction
Electron Cyclotron Resonance Ion Sources (ECRIS) are widely used for proton sources as the front end of high-power accelerators such as LEHIPA, ESS, FAIR and Peking University [1][2][3][4][5].In Low Energy High Intensity Proton Accelerator (LEHIPA), 50 keV proton beam is accelerated to 20 MeV in two different types of cavities.The first stage uses a Radio Frequency Quadrupole (RFQ) to accelerate 50 keV beam to 3 MeV [6,7], and the second stage uses a Drift-Tube LINAC (DTL) to accelerate 3 MeV beam to 20 MeV [8].In August-2023, LEHIPA successfully accelerated the proton beam to its target energy of 20 MeV with a peak current intensity of 2 mA.
An upgraded 2.45 GHz ECR ion source test bench has been designed and developed to achieve beam current in the range of ∼ 10-15 mA, beam emittance ∼ 0.2 •mm•mrad and proton fraction of > 80% requested by the LEHIPA.The first stage is the analysis of ECR plasma using the Langmuir Probe [9], the second stage is the analysis of pulsed ion beam extraction approaches [10].This paper discusses proton fraction characterization using an analyzing magnet and its enhancement.The beam from ECRIS is focused, steered and diagnosed in the Low Energy Beam Transport (LEBT) line [11].The magnetic field in the LEBT solenoid magnets is set to focus and transport H + beam within 5 mm at the entry of RFQ.So, by adding aperture discs at suitable locations can partly filters out the unwanted H + 2 and H + 3 molecular ions.The RFQ is designed to accelerate and focus only 50 keV H + ion beam.The higher species present in the beam such as H + 2 and H + 3 are not accelerated.In the long run, this may damage the accelerating structure.Thus, for safe operation of RFQ, ion sources must have proton fractions more than 80%.So, our effort is to provide such a beam for injection into the RFQ.The next section discusses the ECR ion source followed by proton fraction experimental setup, beam dynamics simulation, instrumentation and control followed by results and discussion.

ECR Ion Source
The ECR ion source with three electrode extraction geometry is shown in figure 1.The key components of the ion source are microwave system, plasma chamber, extraction electrodes, electro-magnet coils, precision gas leak valve and vacuum pumping systems.Microwave generator is a 2.45 GHz magnetron with microwave power output up to 2 kW in continuous (CW) and pulsed modes.The waveguide line consists of magnetron, three port circulators with water load, dual directional coupler with diode detectors, four stub tuners, 1 st waveguide E-plane bend, high voltage waveguide break, quartz window, 2 nd waveguide E-plane bend and ridge waveguide.For measurement of microwave forward and reflected power, a dual directional coupler with coupling factor of 60 dB is used in combination with passive diode detectors.Four stub auto tuners are used to match the load impedance (plasma) to the source impedance (generator).Waveguide high voltage break isolates ground potential waveguide lines from 50 kV high voltage waveguide lines.A quartz window is used as a vacuum termination without interfering with microwave waveguide link.The ridge waveguide is designed to match waveguide impedance to plasma impedance of 50 Ω and increase the electric field in the plasma chamber.-2 - The plasma chamber is made of SS304 and has an inner diameter of 90 mm and a length of 100 mm.The vacuum system consists of a dry roughing pump and a turbo molecular pump.The base vacuum achieved in an ion source chamber is of the order of 5 × 10 −7 mbar with a leak rate of 5 × 10 −9 mbar L/s.The vacuum in plasma chamber is two orders higher (5 × 10 −5 mbar) than that in the ion source chamber (5 × 10 −7 mbar).There is a gas insert tube of 3.2 mm inner diameter connected to the plasma chamber.A precision gas leak valve is plugged into the other end of the gas insert tube.The FDGSI make MFH2.300hydrogen generator is used to generate hydrogen gas with a purity of 99.99999%.Hydrogen gas is injected into the plasma chamber using a precision gas leak valve such that the operating gas pressure in the plasma chamber is in the range of 1 × 10 −3 mbar to 1 × 10 −5 mbar.
on Source at IADD   Three electrode extraction geometry consists of 1 st plasma electrode, 2 nd suppressor electrode and 3 rd ground electrode with apertures of 8 mm, 13 mm and 13 mm respectively.The gap between 1 st -3 -and 2 nd electrode is 17 mm and 2 nd and 3 rd electrode is 2 mm.The layout of three electrode extraction geometry is shown in figure 2. The plasma electrode is at 50 kV potential and suppressor electrodes at −4 kV potential with respect to ground electrode.Two electro-magnet coils mounted above the floating plasma chamber are isolated by a perspex cylinder.For 2.45 GHz microwave frequency, the required magnetic field for ECR phenomena is 875 G. On the central axis of the plasma chamber, 875 G magnetic field is generated by electro-magnet coils.The magnetic field over the plasma chamber is plotted for different electro-magnet coil currents to generate a magnetic field of 800-1000 G, as shown in figure 3.

Experiment setup
The experimental setup for proton fraction characterization in figure 4 consists of three electrode ECRIS, vacuum chamber, focusing solenoid, Direct Current Current Transformer (DCCT), bending magnet, 0.5 mm vertical slit and faraday cup.The beam current entering the bending magnet is measured by DCCT.A hall probe is positioned in the good field region of a bending magnet to measure its magnetic field.the ion in kilograms,  is the potential in volts and  is the magnetic field in tesla.Table 1 shows the key parameters of the bending magnet.In order to bend 50 keV beam to 90 • , a magnetic field of 1054 G is required for H + , 1490 G for H + 2 , and 1826 G for H + 3 in a bending magnet.

Beam optics simulation
The beam optics simulation of the experimental setup was performed in TRACEWIN code.The beam line elements for the simulation are drift-1 (700 mm), solenoid magnet, drift-2 (690 mm), edge focused 90 • bending magnet, drift-3 (60 mm) and beam stop.The input parameters for TRACEWIN -5 -

JINST 19 T01006
were 50 keV, 10 mA ion beam, 0.2 •mm•mrad rms-normalized beam emittance and 8 mm beam size at the ground electrode of the ion source.Focusing solenoid effective length is 560 mm with a magnetic field of 1250 G. Figure 5 shows the beam envelopes in transverse planes  and .The solenoid magnet focuses 50 keV H + beam from the ion source, and bending magnet bends the 50 keV H + beam.The beam size in the  and  plane is 40 mm at the entrance of bending magnet.This is well within the drift tube aperture (120 mm) of the bending magnet.Figure 5 shows a beam profile in  and  plane at the ground electrode (0 meters), at the entrance of bending magnet (1.96 meters) and at the exit of bending magnet (2.8 meters).

Instrumentation & controls
The schematic of proton fraction measurement instrumentation and control is shown in figure 6.
The computer-based instrumentation is incorporated to control the power supplies of the ion source, focusing solenoid magnet and 90 • bending magnets.Further it acquires and store data from these systems.The Data Acquisition (DAQ) module has a provision of four analog inputs, two analog -6 -outputs, and four timers/counters.The analog input sampling rate of DAQ is 2 MS/s with trigger capability.For the pulsed operation of an ion source, plasma is pulsed and the high voltage extraction power supplies are kept in DC mode.The trigger synchronization schematic of the measurement is shown in figure 7. The DAQ is programmed to generate a trigger pulse that trigger's an analog output-1 pulse of 0-10 V with pulse width same as the trigger pulse.The magneton receives this analog signal and generates microwave power based on a calibration value of 0-10 V, equivalent to 0-2000 Watts.A current power supply made by DANFYSIK with a 480 A, 50 V rating was used to energize the bending magnet.In order to control the bending magnet power supply with precision, serial communication option was utilized.The bending magnet power supply current was increased in each cycle of trigger pulse that corresponds to magnetic field increase from say 950 G to 955 G in first cycle and finally to 1950 G at the end of the cycle.A current pre-amplifier of range ±200 μA with proportional output of ±10 volts, was used to measure the faraday cup current after the vertical slit of 0.5 mm (width) by 80 mm (height).The bending magnet's magnetic field is measured with a FW Bell's 5180 gauss meter.The gauss meter operates in the range of ±3 kG and gives a linear output of ±3 V. DAQ records the voltage output of the current meter on Analog Input-1 (AI-1) and the gauss meter on Analog Input-2 (AI-2) in trigger mode for a 2 ms pulsed beam as show in figure 7.
The timing diagram of proton fraction scan is shown in figure 7 and described as follows: 1.At the beginning of the scan, bending magnet's magnetic field is set to 950 G.
2. At the initiation of trigger pulse, plasma generation and beam extraction take place.The beam is bent according to the magnetic field of bending magnet.For one set point of magnetic field, 100 data points of beam current are recorded during a period of 2 ms.
-7 - 3. For each magnetic field, a software program calculates the maximum and average faraday cup currents.
4. The bending magnet field is increased by ∼ 5 G and the process continues until it reaches 1950 G.
5. Upon completion of the proton fraction scan, peak and average current w.r.t.magnetic field is plotted in a GUI and stored in a csv file.
In both, peak current data and average current data, the measured proton fraction is in the same range.In this study, proton fractions were calculated based on average faraday cup current data.

Results and discussion
Proton fraction experiments were conducted under two cases.In the first case only SS304 plasma chamber is considered, and in the second case an Al cylinder and BN plates were added in the plasma chamber.

SS304 plasma chamber
Experiments were conducted with SS plasma chambers at operating gas pressures of 1 × 10 −5 mbar, with microwave power of 860 W (reflected power of 200 W), extractor potential of 50 kV, and suppressor potential of −2.5 kV. Figure 8 illustrates the waveforms of trigger pulses, forward microwave power, reflected microwave power, and beam current measured using DCCT for a pulsed beam of 2 ms.The best-achieved fractions were 60% of H + , 33% of H + 2 and 7% of H + 3 for a 50 keV, 8 mA beam as shown in figure 9.

Steps to improve proton fraction
Our aim is to obtain high intensity hydrogen beam with high H + fraction.To achieve this, it is important to control the main physical processes of hydrogen plasma inside the source chamber.The -8 - dominant species inside the source chamber are H + , H + 2 , H + 3 , H − ions, electrons, H atom and H 2 molecules.To obtain high intensity H + dominated beams, the wall processes such as recombinative desorption process and secondary electron emission of the wall material play an important role besides the volume processes that happens inside the source chamber.For H + ion sources, a wall material with high secondary electron emission coefficient is considered superior.
The main formation and loss processes with threshold energy  Th in the ECR hydrogen plasma are listed in eq. ( 5.1) to eq. (5.9) and the cross-sections for the processes are discussed in [12].To increase H + fraction in the plasma, the processes resulting H + must be dominant.
• Two step ionization hydrogen molecules H 2 to H atom eq.(5.5) followed by H atom to H + ion eq.(5.6).
• H + ion can be lost by ambipolar diffusion and the recombination at the chamber walls.
The formation of H + , H + 2 , H + 3 mainly depend on number of H 2 molecules, H atoms, number of electrons, energy of electrons, electron energy distribution function, cross section of the process and threshold energy of the processes.The plasma density (  ) and temperature (  ) are controlled by ECR plasma operating gas pressure, applied microwave power and magnetic field over the plasma chamber.The behaviour of plasma parameters were studied with applied microwave power by using a langmuir probe.The probe measurement shows that with increase in microwave power from 400 W to 1200 W, the plasma density (  ) increases from 5.5 × 10 15 m −3 to 7.9 × 10 15 m −3 and plasma temperature -10 -  (  ) decreases from 10.5 eV to 6.8 eV respectively.The plot of   and   with respect to microwave power is shown figure 10.The derived Electron Energy Distribution Function (EEDF) of Langmuir probe  −  curve is plotted in figure 11 for microwave power of 400 W, 800 W and 1200 W. The mean electron energy for 800 W and 1200 W is 8.6 eV and 6.8 eV respectively.

Aluminum and Boron Nitride plates in the plasma chamber
As per literature [13][14][15][16][17][18][19] the use of Boron Nitride (BN), Aluminum (Al) and Alumina (Al 2 O 3 ) as plasma chamber wall materials increases the number of secondary electrons in the plasma.The secondary electron emission coefficient of BN, Al and Al 2 O 3 are 2.9, 1.6 and 6.3 respectively.
The experiments have been conducted with an Al cylinder of 85 mm length and 5 mm thickness and two BN plates of 2 mm thicknesses and 90 mm in diameter located inside the stainless-steel plasma chamber as shows in figure 12.The electrons collisions with Al cylinder (plasma chamber wall) and BN plates (waveguide flange and extractor electrode) generates secondary electrons which leads to increase number of secondary electrons in the plasma.Proton fraction experiments with Al and BN plates in the plasma chamber were conducted with system parameters as follows: operating pressure from 5 × 10 −6 mbar to 1 × 10 −5 mbar, microwave power of 860 W, extractor potential 50 kV, suppressor potential −2.5 kV and focusing solenoid field of 1250 G.
The proton fraction at pressure 1 × 10 −5 mbar increases from 60% (figure 9) for stainless steel chamber wall material to 79% (figure 13) for Al and BN plates chamber wall materials.The proton fraction scans are shown in figure 13 and plotted in figure 14 for various operating gas pressures.The result in figure 13 shows that varying pressure has significantly less influence on H + fraction than the choice of wall materials.At lower pressure 5 × 10 −6 mbar the H + fraction increase to 83% while H + 2 and H + 3 reduces to 16% and 1% respectively.The decrease of proton fraction with pressure say from 83% at 5 × 10 −6 mbar to 79% at 1 × 10 −5 mbar is due to higher recombination rate at higher pressures.
-12 -  fraction scan for 1.5 kW microwave power is shown in figure 15.The H + fraction increase to 93% and H + 2 reduce to 7%.The dramatic increase of H + fraction as the microwave power increase to 1500 W can be explained by the hydrogen plasma processes inside the plasma chamber.H + ions are produced by two multiple ionization processes eq.(5.5)-(5.6)and eq.(5.8)-(5.9).For low temperature plasma, processes eq.(5.5) and eq.(5.6) predominate the generation of H + ions.This is seen in figure 10 as microwave power increases plasma temperature decreases.In this case more hydrogen can be produced by process eq.(5.5) as the microwave power (electron density in figure 10) increases, since the dissociation degree of hydrogen is proportional to electron density.Additionally, these high-density hydrogen atoms are ionized to protons, which causes increase in H + fraction.Therefore, higher microwave power and lower pressure is important to produce high H + fraction in proton ion sources.

Conclusion
The proton fraction of the beam extracted from 2.45 GHz ECR ion source has been characterized.A computerized instrumentation of control system is developed to measure the proton fraction of pulsed ion beam.The proton fraction with SS plasma chamber was found to be of the order of 60%.To increase the H + fraction in pulsed plasma, the behavior of   and   were studied by using Langmuir probe.Al cylinder and BN plates were added to the plasma chamber to increase the number of secondary electrons.The number of electrons, their energy, distribution and cross-section for the H + formation processes in the plasma has been enhanced by operating ECR plasma at higher microwave power and lower pressure with efficient microwave power coupling to the plasma.The proton fraction with Al and BN plates along with ECR plasma optimization rises to 83% for 50 keV, 8 mA beam for 860 W microwave power.When microwave power is increased to 1.5 kW, beam current increases to 20 mA and proton fraction increases to 93%.

Figure 3 .
Figure 3. Magnetic field profile at the center axis of plasma chamber as a function of coil current.

1 Figure 5 .
Figure 5. Tracewin simulation results: 50 keV H + ion beam envelope in  and  plane in (a), beam profile in  and  plane at ground electrode in (b), at entrance of bending magnet in (c), and at the exit of bending magnet (d).

Figure 8 .
Figure 8. Waveform of trigger pulse, beam current of 8 mA, microwave forward power of 860 W and reflected power of 200 W for plasma  on time of 2 ms.

Figure 9 .
Figure 9. Proton fraction scan with SS plasma chamber at microwave power of 860 watt, operating gas pressure of 1 × 10 −5 mbar and beam parameters of 50 keV, 8 mA.

Figure 10 .
Figure 10.Plot of plasma density (  ) and plasma temperature (  ) with respect to microwave power.

Figure 11 .
Figure 11.Plot of Electron Energy Distribution Function (  ) for microwave power of 400 W, 800 W and 1200 W.

Figure 1 : 1 Figure 12 .
Figure 1: Boron Nitride plates and Aluminum cylinder in the plasma chamber

Figure 13 .
Figure 13.Proton fraction scan with aluminum cylinder and boron nitride plates in the plasma chamber for different operating gas pressure at microwave power of 860 W and beam parameters of 50 keV, 8 mA.

Figure 14 .Figure 15 .
Figure 14.Plot of H + , H + 2 and H + 3 fraction with respect to operating gas pressure.