R. Riedl

Overview of the RF source development programme at IPP Garching

The development of a large-area RF source for negative hydrogen ions, an official EFDA task agreement, is aiming at demonstrating ITER-relevant ion source parameters. This implies a current density of 200?A?m?2 accelerated D? ions at a source filling pressure of ?0.3?Pa and an electron-to-ion ratio of ?1 from an extraction area similar to the positive-ion based sources at JET and ASDEX Upgrade and for pulse lengths of up to 1?h. The work is progressing along three lines in parallel: (i) optimization of current densities at low pressure and electron/ion ratio, utilizing small extraction areas (<0.01?m2) and short pulses (<6?s), in this parameter range the ITER requirements are met or even exceeded; (ii) investigation on extended extraction areas (<0.03?m2) and pulse lengths of up to 3600?s and (iii) investigation of a size-scaling on a half-size ITER plasma source. Three different test beds are being used to carry out these investigations in parallel. An extensive diagnostic and modelling programme accompanies the activities. The paper discusses the recent achievements and the status in these three areas of development.

Spectroscopy—a powerful diagnostic tool in source development

The development of negative hydrogen ion sources for neutral beam systems is closely linked with an optimization of negative ion formation in hydrogen plasmas, which requires knowledge of the plasma parameters. Emission spectroscopy is introduced as a non-invasive and in situ diagnostic tool for line of sight averaged plasma parameters. Diagnostic lines and simplified analysis methods for a variety of plasma parameters, such as electron density and electron temperature, gas temperature, atomic and molecular hydrogen density, caesium densities (atoms and ions), and negative ion densities are identified and prepared for direct application. Emphasis is laid on results obtained in RF generated negative ion sources. Correlations of plasma parameters with extracted negative ion current densities are discussed. Stripping losses in the extraction system are quantified by using beam emission spectroscopy.

Physical performance analysis and progress of the development of the negative ion RF source for the ITER NBI system

For heating and current drive the neutral beam injection (NBI) system for ITER requires a 1 MeV deuterium beam for up to 1 h pulse length. In order to inject the required 17 MW the large area source (1.9 m × 0.9 m) has to deliver 40 A of negative ion current at the specified source pressure of 0.3 Pa. In 2007, the IPP RF driven negative hydrogen ion source was chosen by the ITER board as the new reference source for the ITER NBI system due to, in principle, its maintenance free operation and the progress in the RF source development. The performance analysis of the IPP RF sources is strongly supported by an extensive diagnostic program and modelling of the source and beam extraction. The control of the plasma chemistry and the processes in the plasma region near the extraction system are the most critical topics for source optimization both for long pulse operation as well as for the source homogeneity. The long pulse stability has been demonstrated at the test facility MANITU which is now operating routinely at stable pulses of up to 10 min with parameters near the ITER requirements. A quite uniform plasma illumination of a large area source (0.8 m × 0.8 m) has been demonstrated at the ion source test facility RADI. The new test facility ELISE presently planned at IPP is being designed for long pulse plasma operation and short pulse, but large-scale extraction from a half-size ITER source which is an important intermediate step towards ITER NBI.

The development of the radio frequency driven negative ion source for neutral beam injectors (invited).

Large and powerful negative hydrogen ion sources are required for the neutral beam injection (NBI) systems of future fusion devices. Simplicity and maintenance-free operation favors RF sources, which are developed intensively at the Max-Planck-Institut für Plasmaphysik (IPP) since many years. The negative hydrogen ions are generated by caesium-enhanced surface conversion of atoms and positive ions on the plasma grid surface. With a small scale prototype the required high ion current density and the low fraction of co-extracted electrons at low pressure as well as stable pulses up to 1 h could be demonstrated. The modular design allows extension to large source dimensions. This has led to the decision to choose RF sources for the NBI of the international fusion reactor, ITER. As an intermediate step towards the full size ITER source at IPP, the development will be continued with a half-size source on the new ELISE testbed. This will enable to gain experience for the first time with negative hydrogen ion beams from RF sources of these dimensions.

Rf ion sources for fusion applications: design, development and performance

Abstract The use of rf ion sources in neutral beam injection (NBI) systems offers reduced maintenance and cost due to the absence of filaments. For the first time high-power rf positive ion sources have been put in operation recently in a real NBI system on a tokamak. The performance of the new system is equivalent to that of a conventional one with arc discharge sources. Due to the relevance for ITER, rf sources are also being investigated in context with negative ions: 8.5 mA/cm2 H− have been achieved so far in a non-optimised version.

Low Pressure and High Power RF Sources for Negative Hydrogen Ions for Fusion Applications (ITER neutral beam injection) (invited)

The international fusion experiment ITER requires for the plasma heating and current drive a neutral beam injection system based on negative hydrogen ion sources at 0.3 Pa. The ion source must deliver a current of 40 A D(-) for up to 1 h with an accelerated current density of 200 Am/(2) and a ratio of coextracted electrons to ions below 1. The extraction area is 0.2 m(2) from an aperture array with an envelope of 1.5 x 0.6 m(2). A high power rf-driven negative ion source has been successfully developed at the Max-Planck Institute for Plasma Physics (IPP) at three test facilities in parallel. Current densities of 330 and 230 Am/(2) have been achieved for hydrogen and deuterium, respectively, at a pressure of 0.3 Pa and an electron/ion ratio below 1 for a small extraction area (0.007 m(2)) and short pulses (<4 s). In the long pulse experiment, equipped with an extraction area of 0.02 m(2), the pulse length has been extended to 3600 s. A large rf source, with the width and half the height of the ITER source but without extraction system, is intended to demonstrate the size scaling and plasma homogeneity of rf ion sources. The source operates routinely now. First results on plasma homogeneity obtained from optical emission spectroscopy and Langmuir probes are very promising. Based on the success of the IPP development program, the high power rf-driven negative ion source has been chosen recently for the ITER beam systems in the ITER design review process.

Long pulse large area beam extraction with a rf driven H−∕D− sourcea)

IPP Garching is heavily involved in the development of the rf driven H(-)/D(-) ion source for the ITER NBI. After the successful demonstration of the required physical parameters, the experimental conditions have been extended to long pulses and large area beam extraction. This paper contains descriptions of the source and power supply modifications necessitated for long pulses as well as the latest results including the first 1 h pulse. Suppression of the coextracted electron current is a key issue. Experiments with potential control, different magnetic filter fields, and cesium handling to suppress the electrons and stabilize the currents are also reported.

Progress of the ELISE test facility: results of caesium operation with low RF power

The Max-Planck-Institut f?r Plasmaphysik test facility ELISE is an important intermediate step towards the in-time realization of the ITER neutral beam injection system (NBI). ELISE is equipped with a large radio-frequency (RF) driven negative hydrogen ion source (1???0.9?m2) of half the size of the ITER NBI source. The paper reports on the main results of the very first operation of the source with caesium, but with low RF power, both for hydrogen and deuterium, with pulse lengths of up to 500?s. The results are rather encouraging for the achievement of the required ITER NBI parameters, especially in hydrogen, where large current densities with respect to the low RF power could be achieved at a ratio of co-extracted electrons to extracted ions of 0.5?0.6 at the relevant source pressure of 0.3?Pa. The required magnetic filter field was significantly lower than expected from the experience with the prototype RF source. Similar large extracted ion currents could be achieved also in deuterium, but with larger amounts of co-extracted electrons. Here, the required ratio of co-extracted electrons to extracted ions of one could be achieved only in short pulses.

Towards 20 A negative hydrogen ion beams for up to 1 h: Achievements of the ELISE test facility (invited)

The large-scale RF-driven ion source of the test facility extraction from a large ion source experiment is aimed to deliver an accelerated ion current of 20 A D(-) (23 A H(-)) with an extracted electron-to-ion ratio below one for up to 1 h. Since the first plasma pulses for 20 s in volume operation in early 2013, followed by caesiation of the ion source, substantial progress has been achieved in extending the pulse length and the RF power. The record pulses in hydrogen are stable 400 s pulses with an extracted ion current of 18.3 A at 180 kW total RF power and 9.3 A at 80 kW stable for 1 h. For deuterium pulse, length and RF power are limited by the amount of co-extracted electrons.

Progress in the development of a large RF negative ion source for fusion

Abstract A large area RF source for negative ion-based injection systems has been developed and for the first time H − -current densities have been extracted which are comparable to those produced by conventional arc sources. This paper describes the design of the source and gives the results of H − -extraction experiments. A feature of this RF source is an increase of extracted ion current by up to a factor of four if noble gases are added to the hydrogen discharge. By this effect, a current density of 9 mA/cm 2 at 0.65 Pa has been achieved in volume production. With caesium seeding up to 20 mA/cm 2 could be extracted at 1 Pa source pressure with the addition of argon and a hot plasma grid. Recently, the RF source has been modified for long pulse operation by the installation of a Faraday screen.