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The conceptual design of a neutral injector for ITER(EDA) presented here is based on a combination of established technology and the European negative ion development programme. The injector is based on negative ions as the energy required is more than 250 keV, above which the production of neutral beams from positive ions becomes unrealistic on grounds of efficiency. It is proposed to have a negative ion source at a high negative potential, with the ions accelerated up to the ground potential of the neutralizer via a SINGAP electrostatic accelerator. In the initial design a gas neutralizer is proposed, but a future upgrade using a plasma neutralizer is clearly possible. The charged fraction of the beam emerging from the neutralizer will be deflected electrostatically onto conventional water-cooled dump plates, and the neutral beam will fall onto a long pulse copper calorimeter downstream of the residual ion dump during beamline commissioning. The necessary low gas pressure in the beamline is maintained by cryo condensation pumps, which will require overnight regeneration.

The European negative ion based neutral beam development programme consists presently of experiments on high energy electrostatic accelerators and high current D − sources, and of neutral beam physics and system design studies. At CEA-Cadarache the source test bed MANTIS started operation last June, and the 1 MV, 0.1 A, D − acceleration project SINGAP will be operational by the end of 1994. Additionally, negative ion source physics-oriented experiments will be conducted at Dublin City University. A Pagoda source, developed for the SINGAP experiment, recently produced more than 1 A D − beams and j (D − ) of 10 mA cm −2 , with Cs seeding, at 0.6 Pa operating pressure. 1.3 A H − and 0.5 A D − beams have also been obtained without the use of Cs during the first experiments on MANTIS. Studies on neutral beam heating (NBH) for a next step machine have been conducted during 1993, mainly at JET and CEA-Cadarache. NBH physics studies based on the RLW model indicate that ignition on ITER-EDA should be reached with about 40 MW of 0.4–1 MeV D 0 beams. The outlines of an NBH system for ITER-EDA, based on SINGAP, have been studied.

A 1 MV, 0.1 A, 10 mA cm−2 current density, multi-second D− ion beam acceleration project called SINGAP is scheduled to start at the end of 1994 at Cadarache. This experiment will be equipped with a PAGODA ion source for which experimental results are presented here. The negative ion beam was accelerated with a three-electrode system to up to 65 keV and measured calorimetrically on a target located 3 m from the ion source. With the full extraction area (240 cm2) and with caesium seeding in the source, a total D− transmitted current of 1 A was obtained. By masking some parts of the plasma grid consisting of five segments, various ion beam extraction areas were utilized, and higher current densities were obtained from smaller extraction surfaces. With 1 5 th of the extraction area (central segment), a transmitted D− mean current density of 10 mA cm−2 was measured at a filling pressure of 0.65 Pa with caesium seeding in the ion source, which meets the SINGAP requirements. Spectroscopic and caesium deposition measurements were also performed. The energetic electron leaks were measured and compared with the predictions of a Monte Carlo code.

Energy recovery experiments have been conducted for the first time with a negative ion based neutral beam injector. A new type of energy recovery system was utilized, which: (i) separates electrostatically the neutrals and charged components of the beam at the exit of the neutraliser; (ii) decelerates the residual unneutralised negative ions; (iii) prevents secondary emission of electrons from the negative ion collector by means of electrostatic trapping. With 102 keV, 1.2 A D − beams, the residual negative ions have been decelerated with high efficiency down to 8 keV: even when the additional power consumption of the electrostatic deflector is taken into account, the efficiency of recovery of the residual negative ion power was as good as 81%. The energy recovery system operation has been shown to be rather insensitive to the background gas pressure in a range of values relevant for the injector operation.

A collaborative programme has been agreed between the Association Euratom-Commissariat a l'Energie Atomique (CEA) and the Japan Atomic Energy Research Institute (JAERI). The object was to perform experiments on the neutral beam injector with energy recovery developed by CEA at Cadarache, using a plasma generator developed by JAERI, which supplies more than 90% atomic ion rate. Energy recovery experiments have been performed up to 3 MW extracted ion beam power. A 25% improvement of the neutral beam injector power efficiency was obtained by energy recovery, with a 100 keV deuterium beam operated at a neutralizer target thickness of 6 × 10 15 mol/cm 2 . The operating characteristics of the system are presented and analyzed.

Most of the European activities in the field of negative ion based neutral beam development are conducted at Cadarache. This paper describes the 1 MV 0.1 A D/sup -/ multi-second beam acceleration project, which is scheduled to start at the end of 1994, results from the PAGODA ion source and preliminary results from the large volume source on the MANTIS testbed. The 1 MV experiment will test the simplified concept of beam accelerator SINGAP at relevant ion current densities and address several High Voltage issues. This will use a PAGODA source which has already produced, with caesium seeding, 1 A-60 keV transmitted D/sup -/ beam. At present, this source reliably produces 10 mA/cm/sup 2/ of D/sup -/ beam at 0.6 Pa as required for the 1 MV experiment. The main task of the MANTIS test bed is the development of a large area, multi-ampere D/sup -/ source which satisfies all the requirements of an injector, e.g. for ITER, MANTIS, which started operation in June this year has so far produced beams of approximately 1 A of H/sup -/ and 0.4 A of D/sup $/.

A 1 MeV 0.1 A D - beam acceleration experiment is under preparation at Cadarache. It will study a simplified concept of negative ion beam electrostatic accelerator called SINGAP. Some critical HV components, like a large MV bushing or protection systems against the HV vacuum breakdowns will also be tested. The experiments reported here have been conducted during the preparatory phase of this MeV project. A negative ion source was developed and produced 1 A D - beams and j(D - ) = 12 mA/cm 2 beams. A new type of extraction grid was designed which traps almost100% of the stray electrons extracted from the source plasma. A new emittance diagnostic has been sucessfully tested with 60–100 keV D - beams. A 1 MV 120 mA Haefely power supply has been moved from KfK, adapted and operated on a resistive load at the nominal operating characteristics.

A large negative ion source producing multi–ampere D−ion beams in long pulses has been developed for the joint JAERI / EURATOM–CEA Experiment on energy recovery for neutral beam injectors. D−ion beams of more than 2.2 A were formed in a cesium−seeded volume ion source and accelerated at 100 keV by a multi−aperture extraction system, which has 240 holes of 11.3 mm in diam. within an area of 120 cm × 6 cm. The corresponding current density (JD– ˜ 10 mA/cm2) is close to the requirements of the negative−ion−based NB injectors for JT-60U and ITER. The operating gas pressure and the extracted electron current were reduced significantly by the cesium seeding.

The neutral, positive, and negative beam components have been measured at the exit of the gas neutralizer of a powerful D−/H− neutral beam injector. Their relative fractions have been studied as a function of the neutralizer gas target thickness. These experiments used a diagnostic consisting of a large electrostatic deflector and three calorimetric targets, one for each beam component. These measurements have been performed for the first time with high power D− and H− beams (up to 200 kW, multisecond pulses). The negative ion beam energy was varied from 25 to 100 keV/nucleon. The results are in agreement with the theoretical predictions derived from the cross‐sections data. They confirmed in particular, that negative ion neutralization efficiencies of about 60% can be obtained at 100 keV/nucleon.

Cesium effects have been investigated in a semi‐cylindrical negative ion source whose inner dimensions are 34 cm in diameter and 129 cm in length. D− and H− ion beams were extracted and accelerated by a multiaperture extraction system, which has 240 holes of 11.3 mm in diameter within an area of 6 cm×120 cm. The cesium effects, i.e., enhancement of the negative ion current and reduction of the optimum pressure and electron current, have been observed both in deuterium and in hydrogen operations. The enhancement factor of the D− ion beam current by cesium was higher than that of the H− ion beam current. Though the D− current had a strong tendency of saturation with arc discharge power in pure volume operation, the D− ion beam current was increased linearly with arc discharge power in cesium seeded operation. The ration of the D− ion beam current to the H− ion beam current was 0.8 in cesium seeded operation while the ratio in pure volume operation was 0.5. The D− ion beams more than 2.2 A with an average cu...

The acceleration of D− and H− beams has been studied at energies of up to 105 keV, with currents of the order of 2 A, in multi‐second pulses. The optimum extraction voltage has been studied as a function of the beam energy and current. The perveance of the negative ion accelerator has been measured over a wide range of operating conditions with H− and D− beams. The experimental results have been compared to beam optics calculations performed with the SLACAD negative ion code developed at Cadarache: a good agreement has been found. Strong perturbations of negative ion beamlet trajectories have been evidenced in the case of large gap, large aspect ratio accelerators. A comparison between merged beamlets and multi‐hole accelerators has been performed. The observed linear increase of the stray accelerated electron current with the extraction voltage has been explained by simulation with a Monte‐Carlo code developed at Cadarache.

A neutral beam injector, based on the concept of energy recovery by electrostatic deceleration of the un-neutralized beam fraction, has been designed and operated. A new type of electrode, the STEM, has been developed for the energy recovery system in view of electron suppression and trapping. The results of a series of experiments with H, D and He beams, up to 70 keV, 16 A of extracted beam, 0.5 s duration, are presented. The recovery of the un-neutralized part of the beam is clearly demonstrated. At least 90% of the full energy residual ions are decelerated and collected at a potential as low as 5% of the accelerating beam potential. Seventy per cent of the secondary electrons emitted by ionic impact are trapped in the STEM electrode. An extrapolation of the energy recovery system to higher energies shows that this new scheme should make the operation of a positive ion based 200 keV deuterium injector with an acceptable efficiency possible.

A 160‐keV neutral deuterium beam injector has been developed for plasma heating in JET (Joint European Torus) fusion experiment. This injector has been reliably operated at 5.9‐MW extracted deuterium ion beam power (37‐A, 160‐keV ion beam) and 5‐s pulse duration. The neutral beam power was 1.75 MW corresponding to an overall neutral injector power efficiency of 30%. The beam divergence was 0.6°. The ion beam was accelerated with a three‐grid multiaperture system (aperture diameter φ=11 mm). The accelerating gap length was 24 mm. The extracted deuterium ion current density amounted to 150 mA/cm2. Grid and source body power loadings were acceptable for quasi‐steady‐state operation.

The Tore-Supra neutral beam injectors require an ion source with an extraction area of 113 × 6.6 cm 2 . 40 A in deuterium must be available during up to 30 s. Fitted to these requirements, a plasma source with a multipolar magnetic configuration has been constructed and operated at FAR, firstly in H 2 , on a test stand and then, in D 2 , on the prototype injector. The nominal ion current density of 150 mA/cm 2 has been achieved with an uniformity better than 10 %, in pulses of 1 s. Two discharge regimes, depending on different magnetic topologies, have been studied with regard to the discharge efficiency, deuterons yield and plasma uniformity. Proton contents up to 85 % at 100 mA/cm 2 have been measured.

The first results of the Tore-Supra neutral injector prototype line are presented. The beam was extracted from a 6.6 × 113 cm 2 surface, the plasma source being grounded. The residual fast ion component was electrostatically deflected out of the beam at the exit of the neutralizer. In the first phase of experiment we operated the injector up to 70 kV and 17 A of extracted deuterium beam. The measured neutral beam power fraction transmitted at 6.4 meter on a 24 × 40 cm 2 target was about 45 % of the total extracted power, for a beam divergence of 0.7°.