Tetrode Accelerator Research Articles

Beam Sweeping for Long-Pulse Operation of an Ion Source in Neutral Beam Injectors

Long-pulse operation has been initially and successfully demonstrated during a 100-s stable beam extraction in the neutral beam test stand (NBTS) system of the Korea Atomic Energy Research Institute (KAERI) for the positive ion source (IS) of the JT-60SA neutral beam injector. The NBTS system was constructed at KAERI to develop 300-s deuterium beam extractions of 100 kV/50 A as an auxiliary heating system of the Korea Superconducting Tokamak Advanced Research (KSTAR). The IS of the JT-60SA neutral beam injector is composed of a plasma generator and a set of tetrode accelerators. The beamline components include an optical multichannel analyzer duct, a neutralizer, a bending magnet (BM), a calorimeter, and a vacuum pump system. The beam power deposition of the IS and the beamline components along the NBTS have been measured by water flow calorimetry (WFC), and a total of 99.7% of the extracted beam power (Vacc∙Iacc) was counted for a hydrogen beam of 82 kV/25 A (2.05 MW) during 100-s beam extraction. To reduce the localized heat load on the calorimeter plate, a method of small-angle deflection for the ion beam particles was applied using a small alternate current of 8 A, 0.5 Hz for the BM coil.

Performance of positive ion based high power ion source of EAST neutral beam injector.

The positive ion based source with a hot cathode based arc chamber and a tetrode accelerator was employed for a neutral beam injector on the experimental advanced superconducting tokamak (EAST). Four ion sources were developed and each ion source has produced 4 MW @ 80 keV hydrogen beam on the test bed. 100 s long pulse operation with modulated beam has also been tested on the test bed. The accelerator was upgraded from circular shaped to diamond shaped in the latest two ion sources. In the latest campaign of EAST experiment, four ion sources injected more than 4 MW deuterium beam with beam energy of 60 keV into EAST.

Analysis of ion beam optics of tetrode accelerator for neutral beam injector on the experimental advanced superconducting Tokamak

The ion optics is very important for an accelerator. The tetrode accelerator with slot apertures was employed on the neutral beam injector (NBI) for the Experimental Advanced Superconducting Tokamak (EAST). The ion optics of ion source accelerator was simulated using the IGUN code. The results have shown that the accelerator with diamond cross section plasma grid can get lower divergence angle comparing with the one with circular cross section plasma girds. In this paper, the voltage applied on the gradient grid (80%, 83% and 86% of plasma grid voltage) and the gap between plasma grid and gradient gird, gradient grid and suppression grid were also changed to estimate the ion extraction efficiency and ion beam trajectory. The results are helpful for the development and optimization of EAST-NBI ion source.

Design and Development of a Power Supply System for NBI Test Stand of EAST

A neutral beam injector (NBI) test stand was constructed to develop a multi-megawatt prototype ion source as an auxiliary heating system on experimental advanced superconducting tokamak. A power supply system for the NBI test stand components such as a set of dc power supplies for plasma generator, a dc high voltage power supply of a tetrode accelerator, a transmission line and a surge energy suppressor. Stable arc discharges of the plasma generator with hydrogen gases for 100 s long pulse have been produced by six Langmuir probes feedback loop regulation mode to control the arc power supply. The 4 MW hydrogen ion beam of 1 s is extracted with beam energy of 80 keV and the beam current of 52 A. The dc high voltage power supply for the plasma grid of the prototype ion source was designed to contribute maximum voltage of 100 kV and current of 100 A. The high voltage power output is continuously adjustable to satisfy with plasma physics experiment in operation frequency of 10 Hz. To prevent damage of the beam source at high voltage breakdown, core snubber using deltamax soft magnetic materials have been adopted to satisfy the input energy into the accelerator from the power supply can be reduced to about 5 J in the case of breakdown at 80 kV. For the transmission line, a disc shape multi cable coaxial configuration was adopted and which the dimension of the diameter is 140 mm at the core snubber. The major issues of discharge characteristics with long pulse and beam extraction with high power for the prototype ion source were investigated on the NBI test stand.

The R&D progress of 4 MW EAST-NBI high current ion source

A high current ion source, which consists of the multi-cusp bucket plasma generator and tetrode accelerator with multi-slot apertures, is developed and tested for the Experimental Advanced Superconducting Tokamak neutral beam injector. Three ion sources are tested on the test bed with arc power of 80 kW, beam voltage of 80 keV, and beam power of 4 MW. The arc regulation technology with Langmuir probes is employed for the long pulse operation of ion source, and the long pulse beam of 50 keV @ 15.5 A @ 100 s and 80 keV @ 52A @ 1s are extracted, respectively.

Development and conditioning of the high current ion source for the EAST neutral beam injector

A 4MW high current ion source is developed for the Experimental Advanced Superconducting Tokamak (EAST) neutral beam injector (NBI). The EAST-NBI ion source is conditioned and the primary results and analysis are reported in this paper. The plasma current of plasma generator reaches 1100A and maximum arc power reaches 90kW. Plasma density uniformity is measured by Langmuir probes and is estimated as 90%. The hydrogen ion beam extracted from a tetrode multi-slot accelerator with beam energy of 80keV. The mean arc efficiency is 0.5A/kW and beam pervereance is 2.5μp. The experimental results show that, the high current ion source is developed and conditioned successfully.

Experimental Results of Ion Beam Extraction of High-Current Ion Source for EAST Neutral Beam Injector

The first campaign of ion beam extraction tests are completed for EAST NBI high current ion source. The hydrogen ion beam with beam voltage of 80 keV is extracted from tetrode accelerator system. During the experiment, the characteristics of arc discharge and ion beam extraction are studied. The arc power reaches 120 kW and the extracted ion beam power reaches 3 MW. The relationship of some key parameters of arc and beam are investigated, and the details are described in this paper.

Estimation of Ion Beam Trajectories for a Slot Aperture Accelerator of Long-Pulse Ion Source in Neutral Beam Injector

A neutral beam injection (NBI) system has been developed and is being tested for an Experimental Advanced Superconducting Tokamak (EAST) device. The NBI system needs to be employed for an auxiliary heating and current drive of EAST plasmas. The first long pulse ion source (LPIS-1) has been installed in the neutral beam test bed (NBTB) system, and the performance is being tested in the NBTB. The LPIS-1 consists of a magnetic bucket plasma generator with multipole cusp-fields and a set of tetrode accelerators with slit-type apertures (a transparency of 60%). The ion beam trajectories of the accelerator column are estimated for the LPIS-1, including an original structure, with the change of slit aperture distance, plasma grid shape, grid gap distance, and voltage ratio between a plasma grid and a gradient grid using the IGUN code. This kind of calculation for the ion beam trajectory may be useful for the estimation of beam extraction characteristics and the direction of accelerator upgrade or modification, prior to the experiments of ion beam extraction.

Performance of 300 s-beam extraction in the KSTAR neutral beam injector

The first neutral beam injector (NBI-1) has been developed for the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak. The first long pulse ion source (LPIS-1) has been installed in the NBI-1 for an auxiliary heating and current drive of KSTAR plasmas. The performance of 300 s ion beam extraction in the LPIS-1 was investigated on the KSTAR NBI-1 system, prior to the neutral beam injection for long pulse operation. The ion source consists of a magnetic bucket plasma generator with multi-pole cusp fields and a set of prototype tetrode accelerators with circular-type apertures. The inner volume of the plasma generator and accelerator column in the LPIS-1 is approximately 123 L. The nominal operation requirements for the ion source (IS) were a 100 kV/50 A deuterium beam and a 300 s pulse length. The extraction of ion beams was initiated by the formation of arc plasmas in the LPIS-1, called an arc-beam extraction method. A stable ion beam extraction of the LPIS-1 was achieved with 80 kV/27 A and a beam perveance of 1.19 microperv for a 300 s pulse length. Beam power deposition along the NBI-1 has been measured using water-flow calorimetry (WFC), and the sum of the deposited power on the ion source and beamline components was about 93% of the drained acceleration power (Vacc•Iacc). The beam power deposition was compared to the calculated results of the beam transport with re-ionization (BTR) code.

Results of Beam Extraction Performance for the KSTAR Neutral Beam Injector

The first neutral beam injector (NBI-1) has been developed for the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak. The first long pulse ion source (LPIS-1) has been installed on the NBI-1 for an auxiliary heating and current drive of KSTAR plasmas. The performance of ion and neutral beam extractions in the LPIS-1 was investigated initially on the KSTAR NBI-1 system, prior to the neutral beam injection into the main plasmas. The ion source consists of a magnetic bucket plasma generator with multipole cusp fields and a set of prototype tetrode accelerators with circular apertures. The inner volume of the plasma generator and accelerator column in the LPIS-1 is approximately 123 L. Design requirements for the ion source were a 120 kV/65 A deuterium beam and a 300 s pulse length. The extraction of ion beams was initiated by the formation of arc plasmas in the LPIS-1, called the arc-beam extraction method. A stable ion beam extraction of the LPIS-1 was achieved up to 85 kV/32 A for a 5 s pulse length and 80 kV/25 A for a 14 s pulse length. An optimum beam perveance of 1.15 µperv was observed at an acceleration voltage of 60 kV. Neutralization efficiency was measured by a water-flow calorimetry (WFC) method using a calorimeter and the operation of a bending magnet. The full-energy species of ion beams were detected by using the diagnostic method of optical multichannel analyzer (OMA). An arc efficiency of the LPIS was 0.6–1.1 A/kW depending on the operating conditions of arc discharge. A neutral beam power of ∼1.0 MW must be sufficiently injected into the KSTAR plasmas from the LPIS-1 at a beam energy of 80 keV.

Design of neutral beam injection system for KSTAR tokamak

The neutral beam injection (NBI-1) system has been designed for providing a 300 s deuterium beam of 120 kV/65 A as an auxiliary heating and current drive system of the KSTAR (Korea Superconducting Tokamak Advanced Research) tokamak. The deuterium beam is produced from a long pulse ion source composed of a bucket-type plasma generator and a multi-aperture tetrode accelerator with the help of discharge power supplies and high voltage (HV) power supplies. The beamline components (BLCs) include a neutralizer with an optical multi-channel analyzer (OMA) section, a bending magnet (BM), an ion dump assembly, a movable calorimeter, beam scrapers, and a cryo-sorption pump system in a rectangular vacuum tank. A beam duct equipped with bellows and a voltage break is placed between the NBI vacuum tank and the KSTAR vacuum vessel. All data and parameters of the NBI system are controlled by a control and data acquisition (CODAQ) system through the EPICS based Ethernet interface.

Energy Distribution of a Prototype KSTAR Neutral Beam Ion Source for 300 s Arc Discharge

A neutral beam test-stand (NBTS) system has been developed for the extraction of a 300 s deuterium beam of 120 kV/65 A as an auxiliary heating system of Korea Superconducting Tokamak Advanced Research (KSTAR). The prototype long pulse ion source (LPIS) consists of a plasma generator and a set of tetrode accelerators. Beam extraction for 300 s was achieved at a maximum hydrogen beam power of 1.6 MW (70 kV/23 A) with an arc discharge power of 63 kW. The energy distribution of the ion source was analyzed by water-flow calorimetry (WFC) by monitoring the cooling-water temperature during the arc discharge. The power dissipation rate on the accelerator column was 0.97% of the total extracted ion beam power with a power loss of 0.2% caused by the collision of back stream electrons with the electron dump plate of the plasma generator. 74.2% of the total energy of was estimated to be distributed in the plasma generator and the accelerator for an arc discharge of 300 s. Also, 75.6% of the total energy was distributed in the ion source for an arc discharge of 2 s. The remaining energy was lost through the structures around the water-cooling path.

Arc discharge efficiency of a multi-megawatt long pulse ion source for the KSTAR neutral beam injector

A multi-megawatt long pulse ion source (LPIS) was developed for the Korea Superconducting Tokamak Advanced Research (KSTAR) neutral beam (NB) injector. Arc discharge characteristics of the ion source were investigated on the NB test stand. The ion source consists of a magnetic bucket plasma generator with multi-pole cusp fields and a set of tetrode accelerators with circular apertures. The inner volume of the ion source, including the accelerator column, is approximately 150 litres. Design requirements for the ion source were a 120 kV/65 A deuterium beam and a 300 s pulse length and initially an 80 kV/48 A hydrogen beam for a 20 s pulse length. Arc discharges of the plasma generator with hydrogen gases have been controlled by the emission-limited mode, operated by the applied heating voltage of the cathode filaments. Stable and efficient arc plasmas, with a maximum arc power of 110 kW, have been produced by a constant voltage (CV) mode operation of an arc power supply. The CV mode operation of the arc powers was more efficient than the constant power (CP) mode operation in the LPIS. A maximum ion density of 9.1 × 1011 cm−3 was measured by using electrostatic probes. The plasma non-uniformity of the ion source was less than 8% and was under the design limit. An optimum arc efficiency, defined by the ratio of extractible ion beam current to arc input power, of 0.46 A kW−1 was estimated for the CV mode operation and 0.44 A kW−1 for the CP mode operation. This arc efficiency is enough to extract the expected hydrogen beam of 48 A.

Development of a Long-Pulse (30-s), High-Energy (120-keV) Ion Source for Neutral Beam Applications

Multimegawatt neutral beams of hydrogen or deuterium atoms are needed for fusion machine applications such as MFTB-B, TFTR-U, DIII-U, and FED (INTOR or ETR). For these applications, a duoPIGatron ion source is being developed to produce high-brightness deuterium beams at a beam energy of approx. 120 keV for pulse lengths up to 30 s. A long-pulse plasma generator with active water cooling has been operated at an arc level of 1200 A with 30-s pulse durations. The plasma density and uniformity are sufficient for supplying a 60-A beam of hydrogen ions to a 13- by 43-cm accelerator. A 10- by 25-cm tetrode accelerator has been operated to form 120-keV hydrogen ion beams. Using the two-dimensional (2-D) ion extraction code developed at Oak Ridge National Laboratory (ORNL), a 13- by 43-cm tetrode accelerator has been designed and is being fabricated. The aperture shapes of accelerator grids are optimized for 120-keV beam energy.

High energy, high current neutral beam injector operation with single stage and two-stage multi-aperture extraction systems

Neutral beam development for JET injections at FAR laboratory has led to the study of properties of a single stage (triode) and a two-stage (tetrode) multi-aperture extraction system at ion beam powers exceeding the megawatt level and up to 80 keV beam energy. The results of the experimental measurements and of a numerical study of the beam optical qualities and grid power loadings of these systems are presented. Grid power loading levels of less than 1% of the high-voltage drain power were measured in both the triode and the tetrode accelerators. This would allow long pulse operation (10 s with water-cooling) as required for JET. The beam divergence angle (α ≅ 0.7°) and the transmission characteristics were almostidentical. At the same energy, higher current densities, at optimum perveance, were obtained with the triode at a lower electric field stress on the high-voltage gap. The triode offers the additional advantage of being simpler from the mechanical and electrical points of view. Operation of the injection line with an electrostatic beam dump associated with a grounded source is also demonstrated for a 25 ion beam up to 60 keV.