Back-streaming Positive Ions Research Articles

Experimental and simulation study on the backstreaming positive ions on the quarter-size negative ion source for CRAFT NNBI test facility

As an effective methods of plasma heating, neutral beam injection (NBI) systems based on negative hydrogen ion sources will be utilized in future magnetic-confinement nuclear fusion experiments. Because of the collisions between the fast negative ions and the neutral background gas, the positive ions are inevitable created in the acceleration region in the negative NBI system. These positive ions are accelerated back into the ion source and become high energy backstreaming ions. In order to explore the characters of backstreaming ions, the track and power deposition of backstreaming H+ beam is estimated using the experimental and simulation methods at NNBI test facility. Results show that the flux of backstreaming positive ions is 1.93 % of that of negative ion extraction from ion source, and the magnet filed in the beam source has an effect on the backstreaming positive ions propagation.

Characteristics of the pressure profile in the accelerator on the RF negative ion source at ASIPP

Neutral beam injection (NBI) systems based on negative hydrogen ion sources—rather than the positive ion sources that have typically been used to date—will be used in the future magnetically confined nuclear fusion experiments to heat the plasma. The collisions between the fast negative ions and neutral background gas result in a significant number of high-energy positive ions being produced in the acceleration area, and for the high-power long-pulse operation of NBI systems, this acceleration of positive ions back to the ion source creates heat load and material sputtering on the source backplate. This difficulty cannot be ignored, with the neutral gas density in the acceleration region having a significant impact on the flux density of the backstreaming positive ions. In the work reported here, the pressure gradient in the acceleration region was estimated using an ionization gauge and a straightforward 1D computation, and it was found that once gas traveled through the acceleration region, the pressure dropped by nearly one order of magnitude, with the largest pressure drop occurring at the plasma grid. The computation also revealed that the pressure drop in the grid gaps was substantially smaller than that in the grid apertures.

Investigations on Caesium Dispersion and Molybdenum Coating on SPIDER Components

SPIDER is the 100 keV full-size Negative Ion Source prototype of the ITER Neutral Beam Injector, operating at Consorzio RFX in Padova, Italy. The largest Negative Ion Source in the world, SPIDER generates an RF driven plasma from which Deuterium or Hydrogen negative ions are produced and extracted. At the end of 2021, a scheduled long-term shutdown started to introduce major modifications and improvements aiming to solve issues and drawbacks identified during the first three years of SPIDER operations. The first action of the shutdown period was the disassembly and characterization of the SPIDER beam source after removal from the vacuum vessel and its placement inside the clean room. Each component was carefully assessed and catalogued, following a documented procedure. Some source components, i.e., the Plasma Grid, Extraction Grid and Bias Plate, revealed the presence of different and non-uniform red, white and green coatings that might be correlated to back-streaming positive ions impinging on grid surfaces, electrical discharges and caesium evaporation. Thus, several analyses have been carried out to understand the nature of such coatings, with the study still ongoing. The evidence of caesium evaporation and deposition on molybdenum-coated SPIDER components, such as the formation of oxides and hydroxides, is demonstrated through surface characterization analyses with the use of the Scanning Electron Microscope (SEM), X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS).

Preparation of the ELISE test facility for long-pulse extraction of negative ion beams

The test facility ELISE (Extraction from a Large Ion Source Experiment) at IPP Garching, Germany, aims to demonstrate ITER-relevant negative ion beam parameters which are required for the NBI system of ITER. ELISE is equipped with a Radio Frequency driven source and an ITER‐like extraction system with half the ITER size. An H− or D− beam can be extracted for 10s every 3min from the continuously operating plasma source. The duration of the beam pulses is currently limited by the power supplies available at IPP. Although up to now record-setting 1h plasmas have been produced in H− as well as in D−, long plasma pulse operation with multiple beam blips showed a key issue: the co-extracted electron current during the extraction phase is strongly dynamic and temporally instable, particularly in D−. These instabilities are likely caused by back-streaming positive ions and Cs dynamics in the source. In order to investigate the source physics in long beam pulses, an upgrade of ELISE using a steady state high voltage power supply is envisaged. This upgrade requires a new steady state diagnostic calorimeter for which a few concepts are being investigated, which make use of several thermocouples, IR thermography and water calorimetry to measure beam intensity, divergence, profile and homogeneity. In addition the suitability of a tungsten wire calorimeter to characterize the steady state beam is being examined. Shielding of delicate components in the beam line, e.g. a large DN 1250mm gate valve, by means of suitable protection scrapers, is being considered. A selection of these technical solutions is presented and discussed in the paper.

R&D activities for the design of the MITICA Plasma Driver Plate manufacturing process via explosion bonding technique

The back plate of the MITICA plasma source, named Plasma Driver Plate (PDP), will be protected from the impact of the highly energetic back-streaming positive ions (BSI+), generated inside the accelerator, by a 1.0mm thick molybdenum layer that will be joined by Explosion Bonding (EB) technique to the copper heat sink.This technology has been investigated and used for manufacturing prototypes, demonstrating very high strength of the obtained molybdenum–copper interface.The production of the shaped edge profile of the driver-hole, after the EB, is an open point. In order to demonstrate the possibility to produce the PDP by explosion bonding, the manufacturing of a full scale prototype of the area just around one of the PDP driver-holes was identified as the road to address most of the manufacturing issues.Elasto-plastic finite element analyses have been carried out to improve the hole rim geometry and the process parameters of all the manufacturing steps. A full scale prototype of the PDP driver-hole has been manufactured and tested.This contribution gives an overview of the R&D activities carried out to address the main open issues, to define the PDP component detailed geometry and its manufacturing processes, via EB technique.

A study on backstreaming positive ions on a high power negative ion source for fusion

Future magnetic-confinement nuclear fusion experiments will be using neutral beam injection (NBI) systems for plasma heating that are based on sources for negative hydrogen ions as opposed to the positive ions mostly used to date. An unavoidable effect in negative NBI (NNBI) systems is the creation of positive ions in the acceleration region due to collisions between the fast negative ions and the neutral background gas. These positive ions are accelerated back into the ion source. At the high extracted ion current densities from NNBI sources and the high acceleration voltages—1 MeV in the case of ITER—the resulting heat load on the backplate of the source and the sputtering rates of the backplate material can be substantial. In this work, sputtering probes and a simple 1D calculation were employed to estimate the flux density of the backstreaming ions in the rear part of the ion source at the long pulse NNBI testbed MANITU (20 kV extraction voltage). It was found that the flux of backstreaming ions is approximately between 0.8 and 2.5% of the flux of negative ions extracted from the source. Experiment and theory are in fair agreement.

Measurements and effects of backstreaming ions produced at bremsstrahlung converter target in Dragon-I linear induction accelerator

Positive ions released from x-ray converter target impacted by electron beam of millimeter spot size can be trapped and accelerated in the incident beam's potential well. As the ions move upstream, the beam will be pinched first and then defocused at the target. Four Faraday cups are used to collect backstreaming ions produced at the bremsstrahlung converter target in Dragon-I linear induction accelerator (LIA). Experimental and theoretical results show that the backstreaming positive ions density and velocity are about 10(21)/m(3) and 2-3 mm/micros, respectively. The theoretical and experimental results of electron beam envelope with ions and without ions are also presented. The discussions show that the backstreaming positive ions will not affect the electron beam focusing and envelope radius in Dragon-I LIA.

Thermo-mechanical design of the Plasma Driver Plate for the MITICA ion source

In the framework of the activities for the development of the Neutral Beam Injector (NBI) for ITER, the detailed design of the Radio-Frequency (RF) negative ion source has been carried out. One of the most heated components of the RF source is the rear vertical plate, named Plasma Driver Plate (PDP), where the Back-Streaming positive Ions (BSI+) generated from stripping losses in the accelerator and back scattered on the plasma source impinge on. The heat loads that result are huge and concentrated, with first estimate of the power densities up to 60 MW/m 2. The breakdowns that occur into the accelerator cause such heat loads to act cyclically, so that the PDP is thermo-mechanically fatigue loaded. Moreover, the surface of the PDP facing the plasma is functionally required to be temperature controlled and to be molybdenum or tungsten coated. The thermo-hydraulic design of the plate has been carried out considering active cooling with ultra-pure water. Different heat sink materials, hydraulic circuit layout and manufacturing processes have been considered. The heat exhaust has been optimized by changing the channels geometry, the path of the heat flux in the heat sink, the thickness of the plate and maximizing the Heat Transfer Coefficient. Such optimization has been carried out by utilizing 3D Finite Element (FE) models. Afterwards all the suitable mechanical (aging, structural monotonic and cyclic) verifications have been carried out post-processing the results of the thermo-mechanical 3D FE analyses in accordance to specific procedures for nuclear components exposed to high temperature. The effect of sputtering phenomenon due to the high energy BSI+ impinging on the plate has been considered and combined with fatigue damage for the mechanical verification of the PDP. Alternative solutions having molybdenum (or tungsten coatings) facing the plasma, aiming to reduce the sputtering rate and the consequent plasma pollution, have been evaluated and related 3D FE models have been developed, analyzed and compared. In this paper, the design will be presented in detail, as well as the analyses carried out for the thermo-mechanical verification of the component and an outlook to open issues.

神龙一号轫致辐射靶靶区回流离子测量

The Dragon-Ⅰlinear induction accelerator(LIA) uses mm level diameter pulsed electron beam to strike the bremsstrahlung converter target to generate X-ray, which generates the backstreaming ions as well. Four Faraday cups are combined to collect backstreaming ions produced at the bremsstrahlung converter target of the Dragon-ⅠLIA. Experiment and calculation results show that the backstreaming positive ions density and velocity are about 1021/m3 and 2～3 km/s,respectively.The calculated results show that the intensity of the backstreaming positive ion beam is only 0.27% of that of the targeting electron beam, thus the backstreaming ions would not affect the focus of the electron beam.

Detailed design of the RF source for the 1 MV neutral beam test facility

In the framework of the EU activities for the development of the Neutral Beam Injector for ITER, the detailed design of the Radio Frequency (RF) driven negative ion source to be installed in the 1 MV ITER Neutral Beam Test Facility (NBTF) has been carried out. Results coming from ongoing R&D on IPP test beds [A. Stäbler et al., Development of a RF-Driven Ion Source for the ITER NBI System, this conference] and the design of the new ELISE facility [B. Heinemann et al., Design of the Half-Size ITER Neutral Beam Source Test Facility ELISE, this conference] brought several modifications to the solution based on the previous design. An assessment was carried out regarding the Back-Streaming positive Ions (BSI+) that impinge on the back plates of the ion source and cause high and localized heat loads. This led to the redesign of most heated components to increase cooling, and to different choices for the plasma facing materials to reduce the effects of sputtering. The design of the electric circuit, gas supply and the other auxiliary systems has been optimized. Integration with other components of the beam source has been revised, with regards to the interfaces with the supporting structure, the plasma grid and the flexible connections. In the paper the design will be presented in detail, as well as the results of the analyses performed for the thermo-mechanical verification of the components.

Spectroscopic observations of beam and source plasma light and testing Cs-deposition monitor in the large area negative ion source for LHD-NBI

In the large area negative ion source for the LHD negative-ion-(H(-))-based neutral beam system, (I) we used the spectrometer to measure caesium lines in the source plasma during beam shots. (II) With Doppler-shifted measurements, the H(alpha) line at three different locations along the beam as well as the spectrum profile for cases of different plasma grid areas. (III) Caesium deposition monitor with a high speed shutter was tested to measure the weight of the deposited Cs layer. In the observation, cleaner spectra of Doppler-shifted H(alpha) line with only a small level of background light were obtained at a new observation port which viewed the blueshifted light in the drift region after the accelerator of a LHD ion source. Both the amounts of Cs I (852 nm, neutral Cs(0)) and Cs II (522 nm, Cs(+)) in the source plasma light rose sharply when beam acceleration began, and continued rising during a 10 s pulse. It was thought that this was because the cesium was evaporated/sputtered from the source back plate by the back-streaming positive ions. Cs deposition rate to the crystal sensor measured by adjusting the shutter open time was evaluated to be 2.9 nanograms/s cm(2) for preliminary testing. More neutral Cs tended to be evolved in the source after arc discharge. Much Cs could be consumed in a high rate-pulsed operation (such as LHD source).