Generation of hydrogen ionic plasma superimposed with positive ion beam

In this study, a hydrogen ionic plasma with relatively low residual fractional electron concentration ( n e / n + ∼ 10 − 2 ) is generated using an aluminum plasma grid for the production of negative hydrogen ions and a control grid for negative ion extraction and electron removal. The ionic plasma is composed of negative and positive ions, containing molecular ions. Negative ions are in part produced using positive ions with several electron volts. A positive ion beam with 50 eV or more contributes to increase the density of the ionic plasma. The positive ion beam energy and the control grid bias voltage are tuned in such a way that a high-density ionic plasma is maintained.


I. INTRODUCTION
Conventional plasmas consist of electrons and positive ions, and the asymmetric diversity of collective plasma phenomena is caused by their large mass difference. In contrast, pair plasmas consisting of only positive-and negative-charged particles with equal mass have attracted special attention because the particle mobility in electromagnetic fields is almost the same and the pair plasmas have spatiotemporal symmetry. Positrons have been discussed as a species of antimatter, in the context of CPT invariance, highenergy physics and astrophysics, and pair plasmas consisting of positrons and electrons have been investigated theoretically. [1][2][3][4][5][6][7][8][9][10][11] Pair plasmas represent a new state of matter with unique thermodynamic properties markedly different from those of the conventional electron-ion plasmas. Non-neutral plasmas of positrons have been experimentally generated in laboratories. [12][13][14][15][16][17][18][19][20][21][22] Positron-electron pair plasmas have been generated by injecting low-energy electrons into positron plasmas and pair creation using a high intensity laser.
To experimentally clarify the collective properties of such a plasma, the time scale of pair annihilation must be much longer than the period of plasma oscillation. Owing to the high rate of pair-annihilation and their loss to the vacuum chember, positronelectron plasmas are too short-lived for much to be learned about them. A fullerene pair-ion plasma, consisting of positive and negative ions with an equal mass (C + 60 and C − 60 ), has been generated 23,24 and investigated experimentally. 25,26 The response frequency of this pair-ion plasma, based on such massive ions, lies in a narrow frequency range below 50 kHz, so they are not suitable for studying phenomena at higher frequencies. The need to study lighter ions has led to a hydrogen pair-ion plasma source [27][28][29] because hydrogen atomic ions, H + and H − , are the lightest ions.
Negative hydrogen ions are produced by referring to the negative-ion source developed in neutral beam injection (NBI), typically used to heat fusion-oriented plasmas. [30][31][32][33][34][35][36] When cesium, Cs, is added to the source, the work function on the surface of metal electrodes decreases, causing a tendency for electronic transition to positive ions and hydrogen atoms approaching the surface, in turn causing an increase of negative ions. [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] However, Cs is unsuitable because it easily becomes a positive ion and coexists in the plasma, becoming an impurity positive ion. It has been shown that even if the work function is relatively high, negative ions occur when the incident angle of positive ions on the metal surface is large. [27][28][29] Negative ions are produced using a plasma grid. 52 Positive ions are introduced into the extraction apertures of the plasma grid, and negative ionization of positive ions is performed in the process of reflection from the inner walls of apertures. It is clear that alminum is a metal ARTICLE scitation.org/journal/adv with relatively high negative ion yield. The role of the aluminum oxide layer (work function ∼4.3 eV) on the surface for negative ion production 53 has, however, not been clarified. By passing positive and negative ions through apertures of an aluminum plasma grid and then through an extraction grid with magnetic fields to deflect and remove electrons, a plasma consisting of positive and negative ions can be generated. The density of this ionic plasma is too low (∼ 10 7 cm −3 ) for clarification of the collective physical properties such as wave propagation, 26 so it is necessary to improve the density, which is the purpose of this study. Figure 1 shows a schematic view of the experimental setup with two sections for generating a hydrogen plasma. One section has a sidewall biased at a dc voltage V driver with a cross section of 16×16 cm and an axial length of 10 cm. The other section has a grounded sidewall with a cross section of 25×25 cm and an axial length of 17 cm. The hydrogen plasmas are generated by a filamentdriven dc arc discharge between the filament cathodes and the sidewall anode. 54,55 The plasmas in the biased section at V driver > 0 V and in the grounded section are called the driver plasma and the target plasma, respectively. In the target section, there are four cathodes consisting of U-shaped tungsten filaments, each having a diameter of 0.7 mm, length of 16 cm, height of 6 cm, and width of 2.5 cm. These filaments are negatively biased at a discharge voltage V d with respect to the grounded sidewall anode, where V d = −70 V is the maximum condition in the plasma density per unit discharge power. Azimuthal line-cusp magnetic fields near the chamber wall are generated by permanent magnets attached to the outside chamber wall. The target plasma is generated in a region free of magnetic-fields. As the line-cusp magnetic fields reduce the plasma loss to the sidewall, these fields contribute to the increase in the plasma density. In the driver section are two U-shaped tungsten filament cathodes folded at 90 ○ , each having a diameter of 0.7 mm, length of 20 cm, height of 8 cm, and width of 2.5 cm. The cathodes are biased at V dd + V driver , where V dd = −80 V. A mesh (50 mesh) made of SUS304 stainless steel is installed between the driver plasma and the target plasma. The plasma separation mesh is biased at a dc voltage Vgt < 0 V to prevent electrons passing from the target plasma to the driver plasma. The hydrogen gas flow is controlled by a mass flow meter, and the typical gas pressure in the target section is 0.06 Pa.

II. EXPERIMENTAL APPARATUS
Fast electrons are emitted from the filament cathodes which have an energy of 70 eV corresponding to the cathode sheath potential in the target plasma. Positive ions are produced when these fast electrons collide with hydrogen molecules, resulting in what is called collisional ionization. As the discharge current density is not high enough, most of the fast electrons consume kinetic energy for collisional ionization near the filament, and the fast electron density in the target bulk plasma is low. The plasma potential of the target plasma is approximately +5 V with respect to the grounded sidewall. The plasma potential varies within a range of several volts depending on the spatial position and the surface condition of the sidewall. On the other hand, the driver plasma potential changes in proportion to V driver because the sidewall and the filament cathodes are biased at V driver . In the sheath formed in front of the separation grid, positive ions in the driver plasma are accelerated and injected into the target plasma. An aluminum mesh without caesiation produces a relatively high negative ion yield, therefore aluminum is used in place of the usual molybdenum for the plasma grid material. The aluminum plasma grid has a thickness of 20 mm and a diameter of 130 mm, and is biased at a dc voltage V PG . It has nine apertures each with a diameter of 13 mm. The hydrogen plasma is introduced into the apertures and positive ions are negatively ionized on the inner wall of the apertures. The plasma grid will be referred to as Al-PG. For NBI performed at the National Institute for Fusion Science, Japan, the aperture diameter of the plasma grid made of molybdenum was 12 mm or 14 mm in the negative ion sources. 56 The For analyzing the types of charged particles arriving in the extraction current, a compact magnetic sector mass spectrometer is used, as shown in Fig. 2(a). It is a single-focusing mass spectrometer in which a single magnetic sector is used to apply a magnetic field that differentiates ions on the basis of their mass-to-charge ratio. 57,58 The miniature electromagnet coil in this spectrometer, built for the purpose, has a diameter of 8 cm, an axial length of 5 cm, and an iron-core diameter of 3 cm. The mass spectrometer simply consists of a pair of electromagnet coils and a yoke that surrounds them; the iron-core gap is 1.7 cm. The coil is wound with AIW having a diameter of 0.3 mm, where AIW is polyamide-imide copper wire, and the maximum conductor resistance is 262.9 Ω/km. The upper and lower discs of the coil and the outer peripheral plate are water-cooled, indirectly cooling the AIW. Because the cooling capacity is modest, it is only possible to run current through it continuously for approximately 1 min. When the coil current is 0.5 A, the deflecting magnetic flux density at the center between the iron cores is 443 mT. Charged particles pass between parallel copper plates with an interelectrode spacing of 1.5 cm where they are deflected by the Lorentz force. A tungsten wire collector with a diameter of 0.5 mm is installed, with a cover plate around it so as not to measure stray charged particles coming from behind the collector. The angular spread of the ion beam is normally refocused at a resolving slit, whose width determines the resolution of the sector mass spectrometer. The wire collector serves in place of the resolving slit. The current of the charged particles Ic is measured using the wire collector. The entrance of the mass spectrometer has a nozzle shape with an inner diameter of 1.5 cm. The entrance electrode, called an acceleration electrode, has an aperture of 2 mm in diameter to define the beam. Charged particles incident thereon are led between the parallel copper plates. The direction of the magnetic field B d which deflects the positive ions is defined as positive (B d > 0), and the magnetic field for measuring negatively charged particles is set to B d < 0. If the collector and the other electrodes is biased at a different voltage, an electric field inside the mass spectrometer is formed. The particle trajectory changes owing to the local acceleration and deceleration of charged particles and the E × B drift due to the deflection magnetic field. The acceleration electrode, entrance nozzle, parallel copper plates, cover plate, and wire collector are biased at the same voltage of Van.
To analyze the kinetic energy of positive ions irradiated on Al-PG, a compact electric sector energy analyzer is fabricated, as shown in Fig. 2(b). [59][60][61][62][63] To perform mass spectrometry of positive ions having specific kinetic energy and perform energy analysis, these ions must aligned, or realigned, with the z axis. The energy analyzer uses electric sectors with a particle trajectory deflection angle of 90 ○ , connected in tandem forming a sort of "S" shape. The curvature radii of the inner and outer electrodes of the sector are 10 mm and 15 mm, respectively, and the electrode interval is 5 mm. The inner and outer electrodes are biased at dc voltages of V E and Van with respect to grounded (V E < Van < 0 V). The electric field formed by the electrode voltage difference Van − V E deflects positive ions towards the inner electrode, so that only positive ions with kinetic energy balanced with the deflection electric field can pass through the sector ARTICLE scitation.org/journal/adv electrodes. This is all to evaluate the energy distribution of positive ions irradiated on Al-PG, not to generate negative ions. Thus, a 1 mm thick SUS316 stainless steel plate is used as a substitute electrode for Al-PG. The extraction aperture of positive ions is limited to 2 mm in diameter. The electrode is biased at a dc voltage of V PG . For extraction and acceleration of positive ions and orbital convergence by the Einzel lens effect, an acceleration electrode and a deceleration electrode are installed with an insulation distance of 1 mm. Both electrodes have the same structure as the substitute electrode, with an aperture diameter of 2 mm and a thickness of 1 mm made of SUS316 stainless steel. The acceleration and deceleration electrodes are biased at Vex and Van, respectively. Positive ions passing through the electric sectors reach a limiter plate with 0.5 mm thickness set in front of a collector. These ions are then detected at the collector electrode. Both the limiter plate and the collector are biased at Van.

A. Positive ion beam injection
The deflection magnetic field for electron removal is applied in the vicinity of CG. Figure 3 shows the z axial profiles of magnetic flux density in the direction perpendicular to the z axis at r = 0 cm. There is an option of using either a magnetic shielding plate having a thickness of 3 mm or a copper plate having a thickness of 1 mm. The two curves make a comparison between these two cases. When the shielding plate is used, the magnetic flux density reaches a maximum value of 74 mT at z = +1.5 cm and decreases to 3 mT at z = −0.7 cm. The magnetic field near the exit of the CG apertures weakens, and nearly no magnetic field appears in the downstream region. With the copper plate, a strong magnetic field is applied near the exit of the CG apertures, but the maximum magnetic flux density is approximately halved. Because the presence of the deflection magnetic field near the exit of the CG apertures affects particle trajectories in the mass analysis and the energy analysis, the magnetic shielding plate is used in the follwing measurements.
Mass spectrometry is used to analyze the types of positive ions in the discharge plasma. Mass spectrometry of positive ions is performed by changing the hydrogen gas pressure in the target section, where the driver plasma is not generated. Figure 4 shows the B d spectra of the collector current Ic at Van = −250 V, as in Fig. 2(a). However, Al-PG and CG are removed here, and the plasma is directly irradiated to the acceleration electrode in order to reduce ion species conversion and ion loss owing to collision of positive ions with the electrodes. The peaks at B d = 105 mT, 145 mT, and 180 mT indicate protons H + and molecular H + 2 and H + 3 ions, respectively. As the gas pressure decreases, the plasma density decreases even if the discharge power is constant. Ic is normalized to the H + ion peak current I + H , as it focuses on the ratio of the molecular ion peak current to I + H . In the B d spectra, that is, in the mass spectra, when the gas pressure is low (0.008 Pa), the H + 2 ion current I H + 2 is relatively high. As the gas pressure increases, I H + 2 decreases, and I H + 3 increases. There is almost no mass peak of H + 2 ions at 0.3 Pa. An increase in the gas pressure increases the collision frequency with neutral hydrogen molecules and tends to broaden the mass peaks. I + H and I H + 3 increase as the gas pressure increases; however, they do not change significantly at 0.04 Pa or more. In contrast, I H + 2 decreases as the gas pressure increases. The thermoelectrons emitted from the filaments are accelerated in the cathode sheath, becoming fast electrons with an energy of 70 eV. The fast electrons quickly collide with H 2 molecules and H atoms in the vicinity of the filaments. Coupled rate equations were numerically solved to estimate the particle densities. The major reactions with the fast electrons considered in the rate equations are as follows. 64 The production yields from these reactions in the low range of the fast electron density satisfy (A) > (C) > (B) > (D). 66,67 Although H + 2 and H + ions are produced, many H + 2 ions are produced from reaction (A). H + ions are produced from reactions (B)-(D), but the contributions of reactions (B) and (D) are small. Because the fast electrons are hardly detectable in the probe characteristics in the target plasma, most of the fast electron energy is consumed by reactions (A) and (C) close to the filaments. There is an upper limit to the discharge power that matches the water cooling capacity of the vacuum vessel and the discharge current determines the fast electron density. The fast electron density is low, H + 2 ions collide with H 2 molecules, and they are converted into H + 3 ions with the following reaction (E) while they fly to the mass spectrometer. As the typical gas pressure is 0.06 Pa, the abundance ratio of H + ions and molecular ions, i.e., the proton ratio, is low (∼ 0.18). [68][69][70] Al-PG and CG are shown as Fig. 2(a). The typical mass spectra of positive ions and negatively charged particles at V PG = V CG = +4 V are shown in Fig. 5, where the accelerating voltages of positive ions and negatively charged particles are Van = −150 V and +150 V, respectively. The driver plasma is not generated here. To clearly show the electron mass peak, the deflection magnetic field at CG is not applied here. V PG is close to the plasma potential, and the plasma electrons passing through Al-PG are shown as a peak at B d = −5 mT. The peak at B d ∼ −80 mT denotes H − ions. B d of the mass peak, B peak d , depends on mass-to-charge ratio and momentum of charged particles and the area of the deflection magnetic field.   Fig. 6. I − H becomes maximum at several electron volts, [71][72][73] and the negative ion yield is significantly reduced when the irradiation energy is 5 eV or more. Therefore, in the following, Al-PG is set at V PG = +2 V, which is the voltage condition for a high yield of negative ions.
The plasma potential ϕst of the target plasma is approximately +5 V and the driver plasma potential changes in proportion to V driver . Positive ions from the driver plasma are injected into the target plasma. The magnetic sector mass spectrometer shown in Fig. 2(a) confirms the presence of the superimposed positive ion beam. The mass spectra are shown in Fig. 7 at Van = -150 V. The dependence of B peak d on V driver is shown in Fig. 8. When no positive ion beam is superimposed (V driver = 0 V), the mass peaks of B peak d = 93 mT, 131 mT, and 162 mT indicate H + , H + 2 , and H + 3 ions in the target plasma, respectively. When the positive ion beam is superimposed, the mass peaks appear at B peak d > 150 mT and shift to a higher magnetic field in proportion to V driver . When V driver > +50 V, H + 2 and H + 3 ion mass peaks of the positive ion beam appear. The kinetic energy of the bulk positive ions in the magnetic sector is approximately 150 eV, and the energy of the positive ion beam is approximately 310 eV at V driver = +160 V. The B peak d of the bulk positive ion decreases in proportion to V driver . The energies of H + 2 and H + 3 ions at V driver = +160 V seem to be reduced by 46 eV and 60 eV from the decrease width of B peak d , respectively. However, it is unlikely that the kinetic energy of the bulk positive ions will decrease owing to superposition of the positive ion beam. It is qualitatively correct that the mass peaks of positive ions of the same species having different kinetic energy appear in different B peak d , but it is not appropriate to discuss the relationship between the kinetic energy and B peak d quantitatively. Figure 9 shows that the dependence of the positive ion current reached the collector on the sector electrode voltage V E at V driver > +50 V where the positive ion beam is clearly incident. Here  incident in the sector are denoted as K PG and K particle (eV), respectively. The condition for the orbit of positive ions to pass on the sector centerline is K particle = e|Van − V center E |/2 ln(router/r inner ), where the center of energy distribution is V center E . Considering the electrostatic potential at the entrance of the sector, the irradiation energy is K PG = K particle − e|Van| − eV PG . Because V center E is approximately −110 V at V driver = +60 V from Fig. 9, K PG is approximately 67 eV. Because V center E is approximately −150 V at V driver = +100 V, K PG is approximately 116 eV. That is, the central energy in the energy distribution of the positive ion beam is approximately 1.1eV driver (eV). The potential difference between the plasma potential and the Al-PG voltage is several volts, the irradiation energy is several electron volts. K particle are considered to contain the electrostatic potential of 30 eV. V center E for the bulk positive ions in the target plasma is -34 V, K PG is derived as 4.9 eV -30 eV -2 eV < 0 eV, which is obviously strange. The kinetic energy that can pass through the sector largely deviates from the calculated energy in the low energy range. Next, the particle orbital convergence is discussed. The convergence angle of the wellknown cylindrical symmetric electric sector is π/ √ 2 ∼ 127 ○ . [59][60][61][62][63] The orbital convergence leads to the energy convergence. The energy resolution at the convergence angle of 127 ○ is ΔK/K particle = Δr/r 0 = 2/12.5 = 0.16, where Δr is the slit width in front of the collector and r 0 is the curvature radius at the sector centerline. The energies of the positive ion beam incident in the sector are K particle ∼ 99 eV at V driver = +60 V and K particle ∼ 149 eV at V driver = +100 V. The calculated energy resolutions ΔK are 16 eV (V driver = +60 V) and 24 eV (V driver = +100 V). The full width at half maximum (FWHM) of the energy distribution is 51-64 eV from Fig. 9, which is approximately 3 times the energy resolution. Here, the deflection angle of 90 ○ , which is less than 127 ○ , is repeated twice, and the orbital convergence is insufficient. Along with the fact that the Einzel lens effect is insufficient, the energy width appears to have broadened. The FWHM ARTICLE scitation.org/journal/adv of the bulk positive ions is 14-18 eV, and the FWHM of the positive ion beam is wider. It appears that the positive ion beam has wide energy distribution at the time of irradiation on Al-PG. The positive ion beam from the driver plasma consists of energy scattered by collision with charged particles and hydrogen molecules while passing through the target plasma. The area of the positive ion beam component in the energy distribution is proportional to the beam current. The beam current increases as V driver is increased. Therefore, the energy and current of the beam cannot be controlled independently.
B. Generation of hydrogen ionic plasma As described above, Al-PG can be irradiated with positive ions in which the positive ion beam is superimposed. The role of the positive ion beam in the ionic plasma generation in the downstream region is described below. The density (bar height) and energy (arrow length) of positive ions and the z axial profile of plasma potential are schematically shown in Fig. 10. The plasma potential ϕ sd of the driver plasma is a positive potential in proportion to V driver . Positive ions in the driver plasma are accelerated and injected into the target plasma. The net acceleration energy of positive ions is e(ϕ sd − ϕst) (eV), and it is approximately 1.1eV driver (eV) from Fig. 9. The energy is scattered as the positive ion beam crosses the target plasma, but it has an energy of at least eV driver (eV). Based on Figs. 7 and 9, the density of the positive ion beam is lower than the density of the bulk positive ions by more than one order of magnitude, but the energy is higher by more than one order of magnitude. Based on Fig. 6, negative ions are produced when the positive ion energy is less than 10 eV. Hence, the positive ion beam does not contribute to the negative ion production. Negative ions produced by negative ionization of the bulk positive ions are promoted to be extracted to the downstream region when CG is positively biased. The bulk positive ions with low energy are decelerated near CG and electrostatically reflected. However, because the positive ion beam can sufficiently overcome the potential barrier of CG, it can reach the downstream region. If there is no positive ion beam, only negative ions are extracted to downstream, and a potential structure will spontaneously form to restrict the passage of negative ions and satisfy quasi-neutrality in the downstream region. Therefore, positive ions with energy that can pass through CG must be present for the density improvement of the ionic plasma in the downstream region.
The plasma profiles at the radial center (r = 0 cm) of the apertures can be measured from the target plasma of z = −4 cm to the downstream region of z = 40 cm by using a Langmuir probe (z probe) movable in the z direction. The region of −2 < z < +2.3 cm, in the apertures of Al-PG and CG, is important for the negative ion production and the ionic plasma generation, as shown in Fig. 1. Let Ip+ and Ip− be the positive and negative saturation currents at the probe voltages of Vp = −150 V and +150 V, respectively. Then the negative current is the sum of the negative ion current and the electron current. The negative current varies greatly, even if the plasma density is the same, depending on the presence or absence of electrons. On the other hand, if the energy distribution of positive ions is the same, the positive current is proportional to the plasma density. The Langmuir probe cannot separate and measure negative ions and electrons with the same charge, but it can qualitatively evaluate the presence of negative ions in the plasma. In a quasi-neutral hydrogen plasma including negative ions, the average densities and velocities of positive ions, negative ions, and electrons are defined as n+, n−, ne, v+, v−, and ve, respectively, where a quasi-neutral condition n+ = n− + ne is satisfied. The ratio of the negative to positive saturation currents can be approximated by |Ip−/Ip+| = (n−v− + neve)/n+v+. In a conventional electron plasma without negative ions, i.e., ne/n+ = 1, the current ratio becomes |Ip−/Ip+| = ve/v+ ∼ 30 at z = 4 cm. It can be approximated that negative ions and electrons have Maxwell energy distributions. In the downstream region of z > +3 cm, electrons, desorbed from negative ions, will have the same temperature as negative ions. The ratio of the negative ion to electron velocities is v−/ve = √ me/m− = 1/ √ 1836 = 0.023 and the fractional negative-ion velocity is derived as v−/v+ = 0.69. When the current ratio is |Ip−/Ip+| = ne/n+ ⋅ 30 + (1 − ne/n+) ⋅ 0.69 = 1, the residual fractional electron concentration is derived as ne/n+ = 1 × 10 −2 . Negative ion plasmas with m− ∼ 10m+ have been investigated and a residual fractional electron concentration was derived as ne/n+ < 10 −3 . 74,75 Figure 11 shows the z axial profiles of Ip+ and the ratio of the negative to positive saturation currents. Ip+ significantly decreases at z < +1 cm between Al-PG and CG. In addition, Ip+ decreases near z = +2.3 cm immediately after passing CG and becomes steady state at z > +6 cm. When V driver is increased and the positive ion beam is injected at V driver > +40 V, Ip+ in the downstream region significantly increases, and the current ratio becomes almost 1. According to the probe characteristics at z = −4 cm of the electron plasma, the plasma density is 1 ×10 11 cm −3 , and the positive saturation current is then Ip+ = 52 μA. Because the plasma potential at z = +4 cm is almost equal to the plasma potential at z = −4 cm, Ip+ is proportional to the plasma density. As Ip+ is 1 μA (V driver = +100 V) at z = +4 cm, the density of ionic plasma is 2 ×10 9 cm −3 . On the other hand, if there are few beam positive ions (V driver < +40 V), Ip+ is an order of magnitude less at z > +4 cm, but Ip− increases. The current ratio at z = +4 cm is approximately 14 at V driver = +40 V, where the positive ion beam injection is insufficient, and 100 or more at V driver = 0 V without the beam. In a quasi-neutral plasma, the high current ratio suggests that electrons are present in the downstream region.
The z axial profiles of the probe floating potential ϕ f in the vicinity of the extraction apertures of CG are shown in Fig. 12, where V PG = +2 V and V CG = +100 V. The floating potential is a potential at which the positive and negative currents flowing into the collector become equal, and it is not necessarily the same as the space potential ϕs. If the kinetic energies of electrons and positive ions are the same, the electron current is much higher than the positive ion current, because electrons are light. ϕ f negatively shifts more than ϕs; hence, the electron current becomes low and the potential shift is substantially proportional to the electron temperature. The positive and negative currents are nearly equal in the ionic plasma. The current ratio is close to 1 as described above, and ϕ f and ϕs are nearly equal. In the downstream region at V driver = 0 V and +40 V, the current ratio is high (Fig. 11) and ϕ f is a slightly negative potential (Fig. 12), so it can be said that electrons are present. In the situation where the positive ion beam is insufficient, a part of negative ions is expected to collapse and be replaced by desorption electrons. A plug potential structure in the z direction is formed at z ∼ 1.5 cm, where V driver = +100 V as the same as Fig. 11. The peak potential of the plug potential structure depends on V driver even though CG is constantly biased at V CG = +100 V. In the absence of the positive ion beam, V driver = 0 V, ϕ f at z = +1.5 cm is approximately +2 V, but the inner wall of the aperture is biased at +100 V. Hence, a well potential structure in the r direction is formed. Even in the presence of the positive ion beam, V driver = +100 V, the peak potential is approximately +24 V, and the well potential structure in the r direction is also formed. The positive ion beam passes near the center of the aperture where the space potential is low, and it flows to the downstream region. On the other hand, the energy scattered positive ions with relatively low energy are electrostatically reflected by the plug potential structure and cannot pass through. The plug potential structure controls the positive-ion flux passing through CG and plays a self-regulating role such that the plasma remains quasi-neutral in the downstream region. Therefore, the peak potential depends on the flux of beam positive ions.
The properties of the downstream plasma are determined by V driver and V CG . The current and energy of the positive ion beam are controlled by V driver . The extraction and acceleration of negative ions are controlled by V CG . The voltage range of V driver and V CG is investigated when the ionic plasma is maintained. The dependences of Ip+ on V CG and V driver at z = +4 cm and +10 cm are shown in Figs. 13(a) and (b), respectively. The dependences of the current ratio at z = +4 cm and +10 cm are shown in Figs. 14(a) and (b), respectively. At V driver > +50 V under conditions where the positive ion beam is sufficiently injected, Ip+, which is proportional to the plasma density, increases significantly in the downstream region, but V CG is constant at +100 V in Fig. 11. From Fig. 13(a), Ip+ increases as V driver increases, and the voltage which Ip+ is maximum depends on V CG . The voltage range of V CG in which the plasma density is high in a wide range of the positive ion beam energy is V CG = +130 to +150 V. Where the plasma state is stable, the voltage range is the same as at z = +4 cm, although Ip+ decreases by approximately 0.6 μA at z = +10 cm ( Fig. 13(b)). The current ratio is close to 1 at V driver > +50 V from Fig. 14 is insufficiently injected, the current ratio increases depending on V CG and the z position. As V CG increases at V driver < +20 V, the current ratio increases to several tens and the plasma changes into a plasma in which electrons are mixed. After passing through CG, negative ions start to collapse. The current ratio increases as V CG increases; however, from Fig. 14(b), it can be said that the collapse of negative ions is strongly dependent on V driver rather than V CG . The voltage conditions suitable for the ionic plasma generation with higher density are found to be V driver > +90 V and V CG = +130 to +150 V. Saleem 76 has pointed out that the existence of ion acoustic wave in the fullerene pair-ion plasma 24,25 indicates the presence of electrons in the system with significant density. Saleem presented a simple criterion to define a pure pair-ion plasma. 77 It was shown that the simplest requirement to have a pure pair-ion plasma in which electron dynamics can be ignored should be ωpe ≪ ωp+, where ωpe and ωp+ are plasma oscillation frequencies of electrons and positive ions, respectively. That means ne/n+ ≪ me/m+. The residual fractional electron concentration, indicated by Saleem's criterion, is ne/n+ ≪ me/m+ = 1/ √ 1836 = 5.4 × 10 −4 for a pure pair-ion plasma. The residual fractional electron concentration (ne/n+ ∼ 10 −2 ) in the downstream region of z > +3 cm does not satisfy the criterion. The criterion in the order of 10 −5 or less is out of range that can be discussed in Langmuir probe measurement. For accurate analysis, it is necessary to use a mass analyzer that can measure electrons and negative ions separately. An example of a mass spectrum is shown in Fig. 5, electrons and negative ions can be clearly separated and measured. Here, the electron peak is large because electrons are not intentionally removed. Since the noise current magnitude measured by the collector in the analyzer is of nano-ampere order, Ie that can be detected needs at least about 1 μA. Therefore, the positive ion current is required at hundreds micro-ampere. This means that the positive ion current has to be increased by two orders of magnitude. Three improvements, (1) increase the entrance aperture diameter to the analyzer, (2) increase the collector area, and (3) use a separation system with low loss, are factors that greatly reduce the mass resolution. However, low mass resolution is acceptable, as long as electrons and negative ions with large mass difference. By the development of the improved mass spectrometer, we think that the residual fractional electron concentration will be estimated in the order of 10 −5 .

IV. SUMMARY
Our goal at this stage is to realize a hydrogen ionic plasma. The plasma grid Al-PG made of aluminum is used to produce negative hydrogen ions from positive ions, and the control grid CG is used to remove plasma electrons and adjust the passage of positive and negative ions. Mass spectrometry of positive ions in the discharge plasmas indicated that many molecular positive ions are present depending on the hydrogen gas pressure. It has been clear that the proton ratio can be increased by increasing the fast electron current density, but the proton ratio is low in these plasmas owing to the upper limit of the water cooling capacity. An ionic plasma consisting of negative ions and positive ions including molecular ions is generated. Positive ions with several electron volts and a positive ion beam with 50 eV or more are irradiated on Al-PG. Negative ions are produced from positive ions with several electron volts, and the positive ion beam does ARTICLE scitation.org/journal/adv not contribute to the production. The kinetic energy distribution of positive ions is analyzed by electric sectors. The energy distribution of the positive ion beam is found to have broadened. The energy of the positive ion beam will be scattered owing to collision with particles while passing through the target plasma. The plasma profiles in the vicinity of CG are measured using a Langmuir probe movable in the z direction. A plug potential structure is self-formed in the CG apertures in the z direction. The peak potential of the plug potential does not significantly depend on the control grid bias voltage, but it depends on the positive-ion beam current. The structure is considered to have a role of maintaining the quasi-neutral condition in the downstream region by extracting negative ions and restricting of the positive ion flux. When the positive ion beam is absent or insufficient, the plasma density is low, the ratio of the negative to positive saturation currents becomes high, and many electrons are present in the plasma. Because electrons are removed near CG, negative ions seem to collapse after passing through CG. The coexistence of negative ions with positive ions is critical to the collapse, but the mechanism of collapse has not been elucidated. When the positive ion beam is injected, the ionic plasma with the residual fractional electron concentration of ne/n+ = 10 −2 beamis maintained in the downstream region, and its density increases. The suitable voltage condition for the ionic plasma generation with high-density is identified as the control grid bias voltage of V CG = +130 to +150 V with positive ion beam injection.