Measurements Of Negative Ion Density Research Articles

Advancement of Langmuir probe-based laser photo-detachment technique for negative ion density measurement in a high-power helicon plasma source.

In the pursuit of precise diagnostics for measuring negative ion density in a helicon plasma source (HPS), a new approach utilizing a radio frequency (RF) broadband transformer-based Langmuir probe is developed specifically for laser photo-detachment (LPD) analysis. This inductively coupled LPD technique is useful for high power RF systems in which capacitive RF noise is in the same scale as the pulsed photo-detachment signal. The signal acquired by this transformer-based probe is compared against the conventional Langmuir probe-based LPD technique, revealing a remarkable enhancement in signal fidelity through an improved signal-to-noise ratio (SNR) achieved by the RF broadband transformer methodology. In addition, the localized hydrogen negative ion density measurements obtained through this probe are harmoniously aligned with the line-averaged negative ion density derived from the cavity ringdown spectroscopy (CRDS) technique. These concurrence measurements highlight the RF broadband transformer-based approach's accuracy in capturing localized negative ion density during helicon mode operation in an HPS setup. Furthermore, the correlation of negative ion density values with RF input exhibits a consistent trend in tandem with background plasma density. Notably, both CRDS and LPD measurements ascertain negative ion densities ranging from ∼5 to 6×1016m-3 under an RF power of 500-700W and a pressure of 8 × 10-3mbar, all under the influence of a 55G axial magnetic field. These specific parameters represent the optimal operational configuration for effective negative ion production with the present experimental HPS setup. Due to its better SNR, the RF broadband transformer-based Langmuir probe emerges as a useful tool for LPD diagnostics, particularly in the presence of pervasive RF noise.

Characterization of cesium and H−/D− density in the negative ion source SPIDER

The Heating Neutral Beam Injectors (HNBs) for ITER will have to deliver 16.7 MW beams of H/D particles at 1 MeV energy. The beams will be produced from H−/D− ions, generated by a radiofrequency plasma source coupled to an ion acceleration system. A prototype of the ITER HNB ion source is being tested in the SPIDER experiment, part of the ITER Neutral Beam Test Facility at Consorzio RFX. Reaching the design targets for beam current density and fraction of coextracted electrons is only possible by evaporating cesium in the source, in particular on the plasma facing grid (PG) of the acceleration system. In this way the work function of the surfaces decreases, significantly increasing the amount of surface reactions that convert neutrals and positive ions into H−/D−. It is then of paramount importance to monitor the density of negative ions and the density of Cs in the proximity of the PG. Monitoring the Cs spatial distribution along the PG is also essential to guarantee the uniformity of the beam current. In SPIDER, this is possible thanks to the Cavity Ringdown Spectroscopy (CRDS) and the Laser absorption Spectroscopy diagnostics (LAS), which provide line-integrated measurements of negative ion density and neutral, ground state Cs density, respectively. The paper discusses the CRDS and LAS measurements as a function of input power and of the magnetic and electric fields used to reduce the coextraction of electrons. Negative ion density data are in qualitative agreement with the analogous measurements in Cs-free conditions. In agreement with simulations, Cs density is peaked in the center of the source; a top/bottom non uniformity is also present. Several effects of plasma on Cs deposition and negative ion production are presented.

Characterization of Cs-free negative ion production in the ion source SPIDER by cavity ring-down spectroscopy

The neutral beam injectors of the ITER experiment will be based on negative ion sources for the generation of beams composed by 1 MeV H/D particles. The prototype of these sources is currently under testing in the SPIDER experiment, part of the Neutral Beam Test Facility of Consorzio RFX, Padua, Italy. Among the targets of the experimentation in SPIDER, it is of foremost importance to maximize the beam current density produced by the accelerator. The SPIDER operating conditions can be optimized thanks to a cavity ring-down spectroscopy diagnostic, which provides line-integrated measurements of negative ion density in proximity of the accelerator apertures. The specific implementation in SPIDER shows a drift in ring down time measurements, which develops in a time scale of few hours, thus possibly affecting the negative ion density estimates in plasma pulses of 1 h duration, as required by ITER. Possible causes and solutions are discussed. Regarding the source performance, this paper presents how negative ion density is influenced by the RF power used to sustain the plasma, and by the magnetic filter field present in SPIDER to limit the amount of co-extracted electrons. In this study, SPIDER was operated in hydrogen and deuterium, in Cs-free conditions.

Influence of high energy electrons on negative ion density in a hot cathode discharge

This study investigates the influence of high energy electrons on the negative ion density along with the effect of filament current, discharge voltage, pressure, and magnetic filter field strength by carrying out measurements of negative ion density in a hot cathode discharge system in the double plasma device using a Langmuir probe technique. In the volume production technique, negative ions are formed by the dissociative attachment of low energy electrons to metastable hydrogen molecules and among the negative ion loss mechanisms; the presence of high energy electrons in the plasma is one of the important factors. Although the magnetic filter could reduce the temperature of the electrons in the target region considerably, high energy electrons were still present in the diffusion region, which could reduce the negative ion density. Negative hydrogen ion density measured by using Langmuir probe in the target region is also compared with the particle balance model considering the set of reactions involved in the production and destruction of negative ions, and a reasonably good match between them has been observed.

Development and first operation of a cavity ring down spectroscopy diagnostic in the negative ion source SPIDER.

The neutral beam injectors of the ITER experiment will rely on negative ion sources to produce 16.7MW beams of H/D particles accelerated at 1MeV. The prototype of these sources was built and is currently operated in the SPIDER (Source for the Production of Ions of Deuterium Extracted from a Radio frequency plasma) experiment, part of the Neutral Beam Test Facility of Consorzio RFX, Padua. In the SPIDER, the H-/D- ion source is coupled to a three grid, 100 kV acceleration system. One of the main goals of the experimentation in SPIDER is to uniformly maximize the extracted current density; to achieve this, it is important to study the density of negative ions available in the proximity of the ion acceleration system. In SPIDER, line-integrated measurements of negative ion density are performed by a cavity ring down spectroscopy diagnostic. Its principle of operation is based on the absorption of the photons of a laser beam pulse by H-/D- photo-detachment; the absorption detection is enhanced by trapping the laser pulse in an optical cavity, containing the absorbing medium (i.e., negative ions). This paper presents and discusses the CRDS diagnostic setup in the SPIDER, including the first measurements of negative ion density, correlated with the main source parameters.

First result of CRDS system with cesium operation for radiofrequency negative ion source test facility at ASIPP

From September 2017, an advance study project of Negative ion-based Neutral Beam Injection (NNBI) prototype for the China Fusion Engineering Test Reactor (CFETR) was supported by the ministry of science and technology of China. The entire test facility will be integrated at Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). In order to meet requirement of the development of negative ion based neutral beam injection system for CFETR, a negative ion source test facility with radio frequency source is being built. At the same time, cavity ring-down spectroscopy (CRDS) is employed for measurement of negative ion density. CRDS is an ultrasensitive absorption diagnostic technique for density measurement. Based on photo-detachment process of hydrogen negative ions, CRDS can measure integrated line-of-sight density of hydrogen negative ions in high power ion source. In this paper, the principle of CRDS diagnostic system has been introduced. Based on previous tests, the system setup and improvement are mentioned. In this paper, typical ring-down signals and time evolution of density of hydrogen negative ions are reported. With the first time cesium seeding, density of hydrogen negative ions increased obviously. It increased from 6.3E15 m−3 to 1.8E16 m−3, about 2.8 times under the 40 kW RF power discharge. Several days condition up, the density of hydrogen negative ions is up to 3.4 E16 m−3 under the 50 kW RF power discharge.

Hydrogen negative ion density measurement by laser photodetachment and the effect of H recombination on the walls

A laser photodetachment (LPD) was applied to measure the absolute hydrogen negative ion (H−) density in an RF negative ion source. The ion source has been developed to study a Cs-free ion source utilizing a magnetic filter by permanent magnets to enhance the volume production of H− in the extraction region. Prior to a beam extraction experiment, the laser photodetachment was applied as an initial performance evaluation. The absolute H− densities were obtained by LPD with the variation of the RF power. The H− density reached 1.8×109 cm−3 at an RF power of 5 kW. The comparison with the calculation of a particle balance model showed the discrepancy between the model and the LPD results. The LPD result showed the rapid increase in H− density with respect to the RF power compared to the model. By assuming the wall temperature increase and adjusting the recombination coefficient (γrec) of hydrogen (H) atoms on the walls, the discrepancy could be resolved. The recombination coefficient (γrec) plays an important role in determining the H− density in view of the fact that it can affect both the generation and the destruction processes of H−.

Cavity ring-down spectroscopy system for the evaluation of negative hydrogen ion density at the ELISE test facility.

The large RF negative hydrogen (deuterium) ion source at the ELISE test facility (half of the ITER-NBI source size) has been equipped with a Cavity Ring-Down Spectroscopy (CRDS) system, in order to measure the negative hydrogen (deuterium) ion density in the region in front of the plasma grid (first grid of the extraction system). The challenge of this diagnostic for ELISE relies on the large size of the source and therefore on the plasma length across which the measurements are performed as well as the long pulses at RF power, which can affect the cavity mirror reliability. A dedicated experiment on the mirror reliability was performed, ensuring the feasibility of measurements for long pulses (several hundred seconds) at high RF power. Two horizontal lines of sight were dedicated to CRDS: the measured density was in the range between 4 × 1016 and 1 × 1017 m-3, with a slightly higher density for the bottom lines of sight, for both the isotope hydrogen and deuterium. Different temporal evolution was observed for the two isotopes, showing a higher instability for the deuterium case: this is in correlation with the extracted negative ion current density and inversely correlated with the coextracted electron current density. The CRDS system allowed performing the first measurements of negative ion density for a long pulse (1000 s) in a large source: the temporal behavior and the effect of the beam extraction will also be discussed.

Characterization of hydrogen plasma in a permanent ring magnet based helicon plasma source for negative ion source research

HELicon Experiment for Negative ion source (HELEN-I) with a single driver is developed with a focus on the production of negative hydrogen ions. In the Helicon wave heated plasmas, very high plasma densities (1018–1019 m−3) can be attained with electron temperatures as low as ∼1 eV in the downstream region. These conditions favor the production of negative hydrogen ions. In HELEN-I device at IPR, helicon plasma is produced using Hydrogen gas by applying a RF power (PRF) of 800–1000 W at 13.56 MHz frequency. A Nagoya-III antenna is used to excite m = 1 helicon mode in the plasma. A permanent ring magnet creates the required axial magnetic field. The plasma is confined by a multi-cusp field configuration in the expansion chamber. The transition from inductively coupled mode to Helicon mode is observed near PRF ∼ 700 W with plasma density ∼1018 m−3 and electron temperature ∼5 eV in the driver and ∼1 eV in the expansion volume. Negative hydrogen ion density, averaged over the line of sight, is measured in the expansion chamber by employing an optical emission spectroscopy diagnostic technique using Hα/Hβ ratio and a laser photodetachment based cavity ring down spectroscopic diagnostic technique. The measured value of negative hydrogen ion density is in the order of 1016 m−3 at 6 m Torr pressure and does not vary significantly with power, pressure and downstream axial magnetic field variation in the helicon mode. The negative ion density measurements are compared with theoretically estimated values calculated using a particle balance method considering different reaction rates responsible for negative hydrogen ion creation and destruction. It is to be noted that, at present, cesium is not injected in the plasma discharge to enhance H− ion density.

Negative hydrogen ion production in a helicon plasma source

In order to develop very high energy (>1 MeV) neutral beam injection systems for applications, such as plasma heating in fusion devices, it is necessary first to develop high throughput negative ion sources. For the ITER reference source, this will be realised using caesiated inductively coupled plasma devices, containing either hydrogen or deuterium discharges, operated with high rf input powers (up to 90 kW per driver). It has been suggested that due to their high power coupling efficiency, helicon devices may be able to reduce power requirements and potentially obviate the need for caesiation due to the high plasma densities achievable. Here, we present measurements of negative ion densities in a hydrogen discharge produced by a helicon device, with externally applied DC magnetic fields ranging from 0 to 8.5 mT at 5 and 10 mTorr fill pressures. These measurements were taken in the magnetised plasma interaction experiment at the Australian National University and were performed using the probe-based laser photodetachment technique, modified for the use in the afterglow of the plasma discharge. A peak in the electron density is observed at ∼3 mT and is correlated with changes in the rf power transfer efficiency. With increasing magnetic field, an increase in the negative ion fraction from 0.04 to 0.10 and negative ion densities from 8 × 1014 m−3 to 7 × 1015 m−3 is observed. It is also shown that the negative ion densities can be increased by a factor of 8 with the application of an external DC magnetic field.

Laser measurement of H– ions in a field-effect-transistor based radio frequency ion source

Hydrogen negative ion density measurements are required to clarify the characteristics of negative ion production and ion source performance. Both of laser photodetachment and cavity ring down (CRD) measurements have been implemented to a field-effect-transistor based radio-frequency ion source. The density ratio of negative hydrogen ions to electrons was successfully measured by laser photodetachment and effect of magnetic filter field on negative ion density was confirmed. The calculated CRD signal showed that CRD mirrors with >99.990% reflectivity are required and loss of reflectivity due to cesium contamination should be minimized.

Negative ion density measurements in an inductively driven hydrogen discharge

The study presents experiments on the determination of the electronegativity and its axial variation in an inductively driven hydrogen discharge, performed by using the laser photodetachment technique. The results showing high electronegativity with nonmonotonic axial variations and a maximum next to the rf power deposition region provide an experimental indication that a design of an efficient source of negative hydrogen ions based on a single-chamber discharge might be possible.

Negative ion density measurements in a reactive dc magnetron using the eclipse photodetachment method

The density of negative oxygen ions in the bulk plasma of a reactive dc magnetron has been determined for the first time using a combination of laser photodetachment and Langmuir probing. Experimental results are obtained for various O2/Ar gas mixtures (0–100%), applied powers (50–600 W) and total discharge pressures (2–25 mTorr). The measurements reveal that the O− ion dominates over with the latter less than 2% of the total observed. Variation of the operating parameters showed clear trends in the negative ion densities with maxima observed at particular powers (200 W) and oxygen partial pressures (10% O2). The negative ion density was found to increase with the chamber pressure and the main loss reaction for O− was determined to be ion–ion recombination with O+, and Ar+.In this study, the maximum negative ion density obtained was found to be 7.7 × 1015 m−3 at 200 W applied power, 25 mTorr total pressure and 50% oxygen partial pressure, giving the ratio of the negative ion to electron density, α = 1.4, indicating that the plasma is moderately electronegative. These new results show that significant concentrations of negative ions are present in the bulk magnetron plasma when operated in Ar/O2 gas mixtures during dc sputtering. The influence of these ions on thin film growth is briefly discussed.

Plasma Electrode Bias Effect on the $\hbox{H}^{-}$ Negative-Ion Density in an Electron Cyclotron Resonance Volume Source

The role of the plasma electrode (PE) bias in the extraction process in a large-volume hybrid multicusp negative-ion source, which is driven by 2.45-GHz microwaves, is studied. Spatially resolved negative-ion and electron density measurements close to the extractor aperture were performed under various pressures (1-4 mtorr) by means of the electrostatic probe and photodetachment technique. As the low positive voltage applied to the PE is slightly increased (from 4 to 7 V), the electron temperature passes through a minimum (0.2-0.6 eV), while simultaneously, both the negative-ion density and the H extracted current reach a maximum (~1-2 times 109 cm-3 and ~0.5 mA/cm2, respectively). Optimum pressure values for the extracted negative-ion current and the negative-ion density are found between ~1.5 and 3 mtorr. It is deduced that the negative-ion density measured in the center of the source cannot be directly correlated with the ion extracted current. The electron density and the associated extracted electron current linearly decrease as a function of the PE bias. The physical mechanisms explaining the experimental results are discussed.

Fluorine negative ion density measurement in a dual frequency capacitive plasma etch reactor by cavity ring-down spectroscopy

F − negative ions were detected by direct observation of the weak photodetachment absorption continuum below 364.5nm by cavity ring-down spectroscopy. The negative ions were generated in a modified industrial dielectric plasma etch reactor, with 2+27MHz dual frequency capacitive excitation in Ar∕CF4∕O2 and Ar∕C4F8∕O2 gas mixtures. The F− signal was superimposed on an unidentified absorption continuum, which was diminished by O2 addition. The F− densities were in the range of (0.5–3)×1011cm−3, and were not significantly different for single (27MHz) or dual (2+27MHz) frequency excitation, not confirming recent modeling predictions.

Development of Laser Photodetachment Technique Using Heated Probe to Eliminate the Effect of Probe Surface Ablation Phenomena

The characteristics of the phenomena caused by laser irradiation to an electrostatic probe in plasmas are studied to avoid the disturbance of the laser photodetachment signals for negative ion density measurement. In helium–hydrogen and hydrogen–methane plasmas, a probe surface ablation phenomenon was observed as an anomalous excess electron current in response to the laser irradiation to the electrostatic probe, while the phenomenon was not observed in pure hydrogen plasmas. Contaminations of the probe surface appear to be the mechanism causing the ablation phenomena. In order to clean the probe surface, a filament-type heated probe, which is the same type of conventional emissive probe, is applied to the laser photodetachment technique. When the surface is cleaned by heating the probe, the ablation phenomenon disappears, and the negative ion density can be evaluated at a sufficiently high laser pulse energy to saturate the photodetachment rate of negative ions. The method developed in this paper is useful for the measurement of negative ion density in plasmas where the probe surface is easily contaminated.

Effect of Probe Surface Ablation on Laser Photodetachment Signals

AbstractAn electric probe exposed to laser irradiation during negative ion density measurement by applying the laser photodetachment technique may suffer ablation even though the laser power is not significantly high. In this paper, the ablation effects on laser photodetachment signals were examined in a linear plasma device using plasmas of both helium and a mixture of helium and hydrogen. It was found that there was a specific threshold laser power necessary to cause the ablation, and that it depended on the electron density. The ablation signals were also detected using a negatively biased probe. A method for monitoring the ablation phenomena is then presented. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparative studies of the laser Thomson scattering and Langmuir probe methods for measurements of negative ion density in a glow discharge plasma

The newly developed method of the negative ion density measurement in a plasma by laser Thomson scattering (LTS) was checked by comparing the obtained results against an independent technique, namely the Langmuir probe method. Both measurements were performed at the same position of the same inductively coupled plasma. The results agree quite well with each other and this has given confidence in the LTS method of negative ion density measurement. At the same time, both methods are complementary to each other, because the Langmuir probe measurement requires knowledge of the positive ion mass number.

Two-dimensional pulsed-plasma simulation of a chlorine discharge

A two-dimensional (r,z) continuum model was developed to study the spatiotemporal dynamics of a pulsed power (square-wave modulated) chlorine discharge sustained in an inductively coupled plasma (ICP) reactor with a planar coil. The self-consistent model included Maxwell’s equations for the power deposition profiles coupled to the electron energy equation and the species mass balances. Simulation results showed separation of the plasma into an electronegative core and an electropositive edge during the active glow (power on) and the formation of an ion–ion plasma ∼15 μs into the afterglow (power off). During the early active glow, the negative ion flux was convection dominated near the quartz window of the ICP reactor due to the formation of large electrostatic fields, leading to a self-sharpening front propagating into the plasma. The negative ion density profiles were found to have a strong spatial dependence underlying the importance of spatial resolution in negative ion density measurements. The time dependent ion and radical flux uniformity was also studied. Simulation results were compared with experimental data and reasonable agreement was observed.

Laser Thomson scattering measurements of negative ion density in a glow discharge plasma

Laser Thomson scattering (LTS) is shown to provide a useful means for measurements of negative ion density in a glow discharge plasma. Because the saturation energy density is so low that complete photo-detachment occurs at a very early stage of the probing laser pulse for the LTS measurement, the main part of the laser pulse is Thomson scattered by the photo-detached electrons as well as those already generated in the discharge. The energies of photo-detached electrons are below the energy difference between the photon energy and the electron affinity of the negative ions, and Thomson scattering from these appears below the difference energy. The principle has been tested using the second harmonic of a YAG laser light (532 nm) against an inductively coupled plasma operated in a mixture of Ar(80%)/O2(20%) at a pressure of 20 mTorr, and yielded the negative ion density of 1.5×1017 m-3, which amounted to 37% of the electron density of 4.1×1017 m-3 or 27% of the negative charges (electrons and negative ions).