RF-driven Negative Hydrogen Ion Source Research Articles

Over 7200 hours commissioning of RF-driven negative hydrogen ion source developed at CSNS

The China Spallation Neutron Source project Phase-II (CSNS-II) aims to deliver a proton beam of 500 kW on the tungsten target. To accomplish this goal, an RF-driven negative hydrogen ion source was developed to replace the penning ion source used in CSNS-I. The RF-driven ion source has been put into commissioning on CSNS accelerator since September 8th, 2021. And it was shut down on July 26th, 2022, together with the whole accelerator for the annual maintenance in summer. In this run cycle it has accumulated service time of over 7200 hours without major maintenance. The availability of the ion source is above 99.99%, except for one or two sparks per day of the 50 kV high voltage platform, each spark causing 1 second trip of the accelerator. The RF-driven ion source has an external antenna winding around a silicon nitride plasma chamber, which is quite robust in high duty-factor operation. In this paper, we present the structure of the ion source, the improvements over other ion sources, and the issues met in the commissioning.

Optimization of transverse emittance for RF-driven negative hydrogen ion source developed at China Spallation Neutron Source.

The China Spallation Neutron Source project Phase-II aims to deliver 500 kW beam power to the spallation target. To meet the beam power requirement, an RF-driven negative hydrogen ion source with an external-antenna has been developed. In order to optimize the beam transmission through the radio frequency quadrupole and the downstream linac, the low energy beam transport line needs to be carefully studied and the transverse emittance is focused in this paper. With computational simulation and experimental verification, the emittance growth caused by nonlinear magnetic fields of the solenoid and the residual magnetic fields at the measuring position has been carefully analyzed. The measurement uncertainty of the double-slit scanner has also been quantitatively estimated. Using the same plasma-beam boundary setting, the beam extraction system is also optimized with particle tracking simulation in CST PARTICLE STUDIO.

Plasma impedance measurement of uncesiated RF-driven negative hydrogen ion source in CSNS

We developed and optimized an RF-driven H- source at the China Spallation Neutron Source (CSNS) to achieve the requirement of project phase-II. At the beginning of 2019, the first uncesiated H- beam was produced, but 15% RF power was reflected when the forward power was 20 kW. To decrease the reflection of the RF power, the RF power matching network was optimized based on the measurement of the plasma equivalent impedance, and the reflected power was decreased to less than 200W at the same forward power. The plasma equivalent impedance is studied with different conditions (RF power, hydrogen flow rate, and frequency). The results show that the plasma equivalent resistance increases at first and then decreases as the RF power increase. Then the transformer model is adopted to derive the plasma impedance.

Beam formation and transport in the BATMAN Upgrade test facility

In the BATMAN Upgrade test facility, an RF-driven negative hydrogen ion source is developed in support of the ITER NBI. Its 5 × 14 apertures represent approximately one ITER beamlet group. Diagnostics include a tungsten wire calorimeter, a copper calorimeter, and two arrays of beam emission spectroscopy lines of sight.When the RF-power is increased at fixed extraction potential, the extracted current density saturates. This is due to scraping of the beamlet by the extraction grid; a method is developed to characterize the injected current density. This current density can vary locally, e.g. due to the E × B drift induced by the filter field. A procedure is introduced to infer the vertical injected current density profile from the behaviour of the different beam emission spectroscopy lines of sight during an RF-power scan.The injected current density profile is used to calculate aperture specific beamlet properties with IBSimu. The beamlet properties are projected towards the calorimeters with the ABC3D code, and compared with measurements to benchmark the current density profile and IBSimu calculations. Beamlet-to-beamlet variations of power and width in the tungsten wire calorimeter are well reproduced. It is crucial to include heat transfer in the copper calorimeter to reproduce the measured profiles.

Operation of RF-driven negative hydrogen ion source in China Spallation Neutron Source.

The China Spallation Neutron Source started delivering neutron beams to users in March 2018. To upgrade the beam power to 500 kW and improve the performance of the ion source, an RF-driven negative hydrogen (H-) ion source is under development. The source has a silicon nitride ceramic plasma chamber surrounded by a 4.5-turn antenna. The plasma is ignited by a pulsed DC spark gap and then driven by a 2 MHz solid-state amplifier with a repetition rate of 25 Hz. The commissioning of the source started in January 2019. When uncesiated, it produced about 20 mA beam at an RF power of 32 kW and pulse width of 450 μs. This paper describes the configuration of the ion source, several peculiar technologies used in it, and the first negative hydrogen (H-) ion beam extraction.

Experimental and theoretical study on the formation of hybrid discharge structure in a compact rf-driven negative hydrogen ion source

This study reports on an experimental and theoretical investigation of the formation of a hybrid discharge structure in a compact rf-driven negative hydrogen ion source. This structure suggests deposition of dc power in the expanding plasma region of the source, leading to a continuous axial increase of the plasma potential which is needed to drive the volume-produced negative hydrogen ions towards the extraction system. The dc power deposition is established by means of applying a bias that is higher than the plasma bulk potential at the first electrode of the extraction system. The 2D fluid-plasma model employed reveals the spatial distribution of the main plasma parameters of electron density, electron temperature and plasma potential, and it describes the mechanism of dc power deposition when the plasma electrode is biased. Optical emission spectroscopy is applied for diagnostics of the three regions of the source: rf power deposition, plasma expansion/dc power deposition, and extraction. The experimental results are not only for the electron ensemble (electron density and temperature), but also for that of the positive ions (ion densities) and neutrals (atomic and molecular temperatures and densities). The theory and experiment show good agreement, confirming the presence of the hybrid discharge structure.

Status of the RF-driven H− ion source for J-PARC linac

For the upgrade of the Japan Proton Accelerator Research Complex linac beam current, a cesiated RF-driven negative hydrogen ion source was installed during the 2014 summer shutdown period, with subsequent operations commencing on September 29, 2014. The ion source has been successfully operating with a beam current and duty factor of 33 mA and 1.25% (500 μs and 25 Hz), respectively. The result of recent beam operation has demonstrated that the ion source is capable of continuous operation for approximately 1100 h. The spark rate at the beam extractor was observed to be at a frequency of less than once a day, which is an acceptable level for user operation. Although an antenna failure occurred during operation on October 26, 2014, no subsequent serious issues have occurred since then.

On the electron extraction in a large RF-driven negative hydrogen ion source for the ITER NBI system

The test facility ELISE, equipped with a large RF-driven ion source (1 × 0.9 m2) of half the size of the ion source for the ITER neutral beam injection (NBI) system, has been constructed over the last three years at the Max-Planck-Institut für Plasmaphysik (IPP), Garching, and is now operational. The first measurements of the dependence of the co-extracted electron currents on various operational parameters have been performed. ELISE has the unique feature that the electron currents can be measured individually on both extraction grid segments, leading to vertical spatial resolution. Although performed in volume operation, where the negative hydrogen ions are created in the plasma volume solely, the results are very encouraging for operation with caesium, this being necessary in order to achieve the relevant negative ion currents for the ITER NBI injectors. The amount of co-extracted electrons could be suppressed sufficiently with moderate magnetic filter fields and by plasma grid bias. Furthermore, the electron extraction is more or less decoupled from the main plasma, as the observed vertical asymmetry of electron extraction is not correlated at all with the plasma asymmetry, which is anyway rather small. Both effects are superior to the experience from the small IPP prototype source; the reason for these encouraging results is most probably the larger size of the source as well as the new geometry of the source having unbiased areas in its centre. The reasons, however, for the observed asymmetry of the extracted electron currents and their dependencies on various operational parameters are not well understood.

On the proton flux toward the plasma grid in a RF-driven negative hydrogen ion source for ITER NBI

The transport of protons in a RF-driven negative hydrogen ion source for ITER NBI was investigated by means of a 3D Monte Carlo transport code. As input for the code a consistent axial profile of the electrostatic potential for the complete ion source was constructed. The potential difference between the plasma generation volume and the plasma grid is in the range 10–20 V and depends on the RF power, the filling pressure and the plasma grid bias voltage. The mean free path for proton collisions with the background gas is in the range of several centimeters which means that the protons are noticeably decelerated and scattered. The velocity distribution function of the protons impinging the plasma grid resembles much more a low-temperature Maxwell distribution function than the high energetic proton beam which would be expected by looking only at the maximum value of the potential in the plasma generation volume. This finding is of high relevance for further analysis of the negative hydrogen ion production on the plasma grid surface as well as the ion transport and beam formation.

Performance of multi-aperture grid extraction systems for an ITER-relevant RF-driven negative hydrogen ion source

The ITER neutral beam system requires a negative hydrogen ion beam of 48 A with an energy of 0.87 MeV, and a negative deuterium beam of 40 A with an energy of 1 MeV. The beam is extracted from a large ion source of dimension 1.9 × 0.9 m2 by an acceleration system consisting of seven grids with 1280 apertures each. Currently, apertures with a diameter of 14 mm in the first grid are foreseen.In 2007, the IPP RF source was chosen as the ITER reference source due to its reduced maintenance compared with arc-driven sources and the successful development at the BATMAN test facility of being equipped with the small IPP prototype RF source ( of the area of the ITER NBI source). These results, however, were obtained with an extraction system with 8 mm diameter apertures.This paper reports on the comparison of the source performance at BATMAN of an ITER-relevant extraction system equipped with chamfered apertures with a 14 mm diameter and 8 mm diameter aperture extraction system. The most important result is that there is almost no difference in the achieved current density—being consistent with ion trajectory calculations—and the amount of co-extracted electrons. Furthermore, some aspects of the beam optics of both extraction systems are discussed.

Laser photodetachment on a high power, low pressure rf-driven negative hydrogen ion source

Powerful, low pressure negative hydrogen ion sources are a basic component of future neutral beam heating systems for fusion devices. The required high ion currents (>40 A) are obtained via the surface production process, which requires negative ion densities in the range of in the plasma region close to the extraction system. For spatially resolved diagnostics of the negative hydrogen ion densities, the laser photodetachment method has been applied to a high power, low pressure, rf-driven ion source (150 kW, 0.3 Pa) for the first time. The diagnostic setup and the data evaluation had to cope with the rf field (1 MHz), the high source potential during extraction (−25 kV) and the presence of magnetic fields (<10 mT). Horizontal profiles of negative ion densities and electron densities along 15 cm with a typical step length of 1 cm and a probe tip of 5 mm length show a broad maximum in the centre of the extraction region. The variation of a bias voltage applied to the plasma grid with respect to the source body yields a correlation between the detachment signals for the negative ion density and the electron density with the extracted ion and electron currents, respectively. The density ratio of negative hydrogen ions to electrons is in the range of , demonstrating that the negative ions are the dominant negatively charged species in these types of ion sources. Absolute negative ion densities are in good agreement with line-of-sight integrated results of cavity ring-down spectroscopy and optical emission spectroscopy.

Plasma Characteristics of a Large RF-Driven Negative Hydrogen Ion Source

A large rf-driven negative hydrogen ion source has been developed for a neutral beam injection (NBI) system in future experimental fusion devices. Dimensions of the rf plasma generator are 30×30×20 cm3. An induction coil antenna is fully immersed into the plasma generator. The rf-driven plasma with an electron density of 1.5×1012 cm-3 was generated at 28 kW rf power at a frequency of 2 MHz. The electron temperature in the extraction region decreases as the line-integrated magnetic filter field strength increases. An electron temperature in the extraction region of 1 eV, which is suitable for generating negative ions, was obtained at a line-integrated filter field strength of 1030 G cm. Negative hydrogen ions are extracted from a single aperture of 13 mm in diameter in a pure-volume operation. An extracted negative ion current of 5.5 mA was obtained at 13.1 mTorr gas pressure and 15 kW rf power. This negativeion current corresponds to a power efficiency of 5.0 ( mA/cm2)/( W/cm3), which is superior to that in filament-arc negative ion sources.