External Magnetic Filter Research Articles

Design of a Two-Region Arc Plasma ion source for Cs-free H-

The Korea Atomic Energy Research Institute (KAERI) is developing a new concept of cesium-free negative hydrogen (deuterium) ion source based on a Two-Region Arc Plasma (TRAP) ion source for a neutral beam injection (NBI) system in a fusion tokamak. The plasma generator of the ion source consists of magnetic cusp field bucket as an anode and filaments as a cathode. The main feature of the TRAP ion source is that the generated plasma has two regions (a high-energy electron region and a low-energy electron region) without an external magnetic filter field. This plasma is generated by optimization of the filament electron emission and magnetic cusp field. The plasma generator of the TRAP ion source has been designed to study the effects of two-region plasma generation and the resulting negative ion generation. An extractor (also called the pre-accelerator) was also designed. The extractor consists of four grids (G1, G2, G3, G4) and permanent magnets are employed to suppress co-extracted electron currents. The design of the TRAP ion source is finished and fabrications are currently in progress. This study discusses the main considerations and simulation results for the design of the TRAP ion source.

Conceptual design of magnetic filter for the prototype negative ion source at ASIPP

In order to accumulate the research experience and technical reserves for future negative ion sources of nuclear fusion reactors, a prototype negative ion source is being designed and constructed at ASIPP with a reduced scale extraction area (166cm2 on the opening of 12×48cm2). Based on the existing negative ion sources having the similar width, an external magnetic filter with permanent magnets is applied to the prototype negative ion source during the initial experiments. For the specific experiments of magnetic field dependence, an adjustable magnetic filter is required. Hence, the plasma grid current filter for short-pulse or long-pulse source and a combination filter using permanent magnets and plasma grid current are designed. The main design purpose is to produce a homogenous and sufficient filter field in front of the plasma grid, with the supply current as low as possible. The filter field distributions of these different magnetic filters are studied in detail by finite element method, especially on the plane paralleling with the plasma grid.

Uniform H− ion beam extraction in a large negative ion source with a tent-shaped magnetic filter

Based on previous studies on the spatial uniformity of the negative ion beam, the external magnetic filter was replaced to a novel tent-shaped magnetic filter in the JAEA 10 A negative ion source. The line-cusp field configuration on the source chamber was also changed to form a symmetric magnetic field like many of positive ion sources aiming at high proton yield. This magnetic field configuration allows fast electrons emitted from filament cathodes to rotate azimuthally inside the source chamber. The source configuration thus prevents localization of fast electrons due to their B x grad B drift in the filter field. As a result, the H(-) ion beam profile extracted from a wide region of 340 x 170 mm(2) showed reduction of standard deviation from 16% in the original to 7.9% with the tent filter. The negative ion source with the tent filter satisfied the requirement of the beam uniformity for a large negative ion source in the ITER neutral beam injection.

Effects of external magnetic filter on behaviors of fast primary electrons in a volume negative ion source

The relationship between enhancement of hydrogen negative ion H(-), volume production, and behaviors of fast primary electrons is studied theoretically. Trajectories of fast primary electrons are calculated, including collisional effects with hydrogen molecules. Active region of fast electrons depends strongly on the field intensity of the magnetic filter (MF), B(MF), and filament position. Therefore, spatial distributions of ionization and vibrational excitation collision points are also affected with changing B(MF). Although number of vibrational excitation collision points is much smaller than that of ionization collision points due to different mean free path, the relative intensity is varied by changing energy of fast electrons. According to the results and discussions including transport of produced vibrationally excited molecules, for enhancement of H(-) production, it is desired that fast electrons should approach the MF position and move into the downstream region as deep as possible without destruction of H(-) ions. To realize these conditions, good combination between filament position and the intensity of the MF should be required.

D − ∕ H − negative ion production versus plasma parameters in a volume negative ion source

Pure volume production of D(-) negative ions is studied in rectangular negative ion source equipped with an external magnetic filter (MF). Plasma parameters (n(e) and T(e)) in D(2) plasmas are varied mainly in the downstream region by changing the magnetic field intensity of the MF (i.e., B(MF)). Production and control of D(2) plasma is nearly the same as that of H(2) plasmas, although the values of n(e) and T(e) in D(2) plasma are slightly higher than that of H(2) plasmas in both the source and the extraction regions. On D(-)/H(-) production, however, it appears some different points. By varying the B(MF), H(-) production in the extraction region is remarkably changed corresponding to the variation of n(e) and T(e) in the extraction region. On the other hand, D(-) production is not so varied under the same discharge conditions in H(2) plasmas. This difference in D(-) production is not well explained only variation of n(e) and T(e) in D(2) plasmas. Optimum B(MF) and gas pressure for D(-) production is slightly higher than that for H(-) production.

Ion-beam extraction with single hole extractor from multiantenna rf ion source in NIFS

A new multiantenna system for large area radio-frequency H− ion source using a filamentless ion source was tested to extract ions from dense plasmas with high rf input power. A 1∕6 scale large helical device ion source was equipped inside a plasma chamber with four linear antenna rods. Positive-ion extraction with a single hole extractor was studied in detail, while a negative one only showed preliminary results. The positive-ion current density of 34mA∕cm2 to the collector plate was achieved with a new multiantenna after optimization of the antenna number and source gas pressure, where the number of the antennae was 2 with ∼36kW of rf input power and with a gas pressure of ∼0.1Pa with H2. If the external magnetic filter optimized for negative-ion production in the multifilament discharge source is taken off, the current density is estimated to reach about 120mA∕cm2. This corresponded closely to the value measured with a Langmuir probe near the center. Two antennae appear to be required to generate uniform plasmas as well as dense plasmas in the present size of the 35×35cm2 source. A negative-ion current density of 0.3mA∕cm2 to the collector plate was detected with hydrogen without optimization. This multiantenna rf ion source is expected to be adaptable advantageously to large area positive- and negative-ion sources, owing to not only a filamentless source but also potentially due to uniform plasmas as well as probable low-electron-temperature plasmas, compared to that with a one-turn coil system.

Negative hydrogen ion source development for large helical device neutral beam injector (invited)

Large-scaled hydrogen negative ion source development is reviewed for a negative ion based neutral beam injector (NBI) in the large helical device (LHD) fusion machine. The target performance of the ion source is characterized by a high current of 30–40 A with a relatively low energy of 120–180 keV. A series of negative ion source development is conducted with a one-dimensionally reduced size of ion sources which still have a large beam area of 25 cm×26 cm or 50 cm with multi apertures. We employed a cesium-seeded volume production source with an external magnetic filter for the source development. Improvement of the arc plasma confinement is effective to produce a high-current negative ion of 16 A with a current density of 31 mA/cm2 at a low operational gas pressure below 0.4 Pa. Suppression of the accelerated electrons is achieved both by strengthening the magnetic field at the extraction grid and by shaping the inside of the extraction grid aperture to shield the secondary electrons against the acceleration electric field. Multi beamlets delivered from a large area are finely focused with the aperture displacement technique applied to the grounded grid. Based on these results, the LHD-NBI negative ion source was designed and fabricated with a beam area of 25 cm×125 cm. The LHD-NBI source produced 25 A of negative ions with an energy of 104 keV at a low gas pressure of 0.3 Pa. A long-pulse negative ion beam of 81 keV–1.3 MW was produced for 10 s. Four sources were installed to the LHD-NBI system, and around 4 MW of neutral beams were injected into the LHD plasmas with an energy of 100–110 keV in the first period for the NBI experiments. The LHD-NBI ion source is still being developed to improve its performance, and the key issues for the improvement are discussed.

Effects of filament positions on the arc discharge characteristics of a negative hydrogen ion source for neutral beam injector

The influence of filament positions on the arc discharge characteristics of a negative hydrogen ion source for a neutral beam injector were studied by simulations and experiments. We have made a simulation code named “PRIMELOC2,” which calculates the orbits of primary electrons emitted from cathode filaments, including collisions with H2 gas molecules filled in the source, to obtain the spatial distribution of ionization points. We applied this code to a large negative hydrogen ion source having an external magnetic filter for LHD-NBI No. 1 (neutral beam injector 1 for a large helical device). As a result of the calculations, we have found that the moving area of emitted primary electrons differs markedly according to the filament positions. Calculation results agreed well with the experimental results of high-power arc discharge and the spatial distribution of plasma parameters obtained by Langmuir probe measurements. By applying PRIMELOC2 to filament–arc ion sources, we can choose appropriate areas in arc chambers for filament positioning.

High-current negative-ion beam acceleration with a large negative-ion source for large helical device-neutral beam injection system

A large hydrogen negative-ion source was constructed as a prototype one for the negative-ion-based neutral beam injection (NBI) system in large helical device. The ion source is designed to produce 180 keV-40 A of the negative ion beam with a current density of 40 mA/cm2, and is a cesium-seeded volume production source characterized by the external magnetic filter. The arc chamber of multicusp bucket is rectangular of 35 cm×145 cm in cross section and 21 cm in depth. The accelerator is a three-grid single-stage one, and the total grid area is 25 cm×125 cm, which is divided into five sections. The ion source was installed on the negative-ion-based NBI test-stand, and using one section of the grid 5.5 A of the negative-ion beam was produced at a gas pressure of 1.8 mTorr corresponding to a current density of 27.5 mA/cm2. The extracted electron current is low of 50% of the negative ion current, and the acceleration efficiency, defined as the negative ion current divided by the acceleration drain current, is more than 75%. The ion source operation with the full grid area has just started, and, so far, 21 A of the negative ion beam was obtained. The large area beam has been successfully focused by both the geometrical arrangement of five grid sections and the aperture displacement technique of the grounded grid.

Energetic negative ion source for fusion at NIFS (invited)

Large high current hydrogen negative ion sources have been developed for the negative ion based neutral beam injector of the Large Helical Device (LHD) at NIFS. The prototype of the negative ion source is required to deliver a negative ion beam of 45 A at the beam energy of 125 keV. The optimization of 1/3 scale ion sources which are multicusp ion source with a rod and an external magnetic filter, respectively, has been investigated for the operation parameter of the plasma source. A total H− current of 16 A is extracted at an operating pressure of 0.9–0.45 Pa with Cs seeding operation. Negative hydrogen ion current is proportional to the input arc power and a beam current density of 45 mA/cm2 is attained. The beam extraction and acceleration characteristics are studied for a single-stage and a two-stage acceleration electrode. A beam divergence angle of 5 mrad is obtained. The results of research and development of a hydrogen negative ion source at NIFS will be reviewed.

High-energy and high-current hydrogen negative-ion beam production with an external-filter-type large negative-ion source

A large hydrogen negative-ion source with an external magnetic filter has been developed for a neutral beam injection (NBI) system in the Large Helical Device (LHD), and a high-energy and high-current H− ion beam has been produced. The ion source is operated at a high arc efficiency of 0.1 A/kW at an operational gas pressure of less than 3.5 mTorr, and produces 47 keV–16.2 A of a H− ion beam from a grid area of 25 cm×50 cm. With two-stage acceleration, 13.6 A of a H− ion beam has been successfully accelerated to 125 keV. Multibeamlet focusing by the aperture displacement technique has been achieved 11.2 m downstream with a gross divergence angle of 9 mrad. The alternate beamlet deflection by the magnetic field at the extraction grid, which results in beam broadening in the deflection direction, was well compensated also by the aperture displacement technique. These results satisfy the specification of the negative-ion-based LHD–NBI system.

Cesium injection into a large rf-driven hydrogen negative-ion source

A large rf-driven hydrogen negative-ion source has been developed for a neutral beam injection (NBI) system in the fusion application. The rf plasma generator is a metal-walled multicusp bucket source, the dimensions of which are 30 cm×30 cm in cross section and 20 cm in depth. An induction coil antenna is fully immersed in the rf plasma generator, and is used for coupling the rf power into plasma. A strong external magnetic filter is generated in front of the plasma electrode for negative-ion production, and a filter field strength of more than 1000 G cm is required to reduce the electron temperature in the extraction region as low as 1 eV. Precisely 5.5 mA of a negative-ion current was obtained from a single aperture, corresponding to 4.1 mA/cm2 of current density. A small amount of cesium was supplied to the rf plasma generator from a cesium oven. As a result, a lowering of the operational gas pressure and improvement of the negative-ion production efficiency were observed. The optimum bias voltage was reduced by the cesium injection, which may be related to the reduction of the plasma potential.

Development of an intense negative hydrogen ion source with an external magnetic filter

An intense negative hydrogen ion source has been developed, which has a strong external magnetic filter field in the wide area of 35 cm×62 cm produced by a pair of permanent magnet rows located at 35.4 cm separation. The filter strength is 70 G in the center and the line-integrated filter strength is 850 G cm, which keeps the low electron temperature in the extraction region. Strong cusp magnetic field, 1.8 kG on the chamber surface, is generated for improvement of the plasma confinement. These resulted in the high arc efficiency at the low operational gas pressure. 16.2 A of H− ion current with the energy of 47 keV was obtained at the arc efficiency of 0.1 A/kW at the gas pressure of 3.8 mTorr in the cesium-mode operation. The magnetic field in the extraction gap is also strong, 450 G, for the electron suppression. The ratio of the extraction current to the negative ion current was less than 2.2 at the gas pressure of 3 mTorr. The two-stage acceleration was tested, and 13.6 A of H− ion beam was accelerated to 125 keV.