Negative Ion Production and Beam Extraction Processes in a Large Ion Source K. Tsumori1,2, K. Ikeda1, H. Nakano1,2, M. Kisaki1, S. Geng2, M. Wada3, K. Sasaki4, S. Nishiyama4,G. Serianni5, C. Wimmer6, P. Agosttineti5, E. Sartori5, M. Brombin5, K. Nagaoka1, M. Osakabe1,2, Y. Takeiri1,2,O. Kaneko1,2 and NIFS NBI group 1National Institute for Fusion Science, 322-6 Oroshi Toki Gifu 509-5292, Japan 2The Graduate University for Advanced Studies, Shonan Village, Hayama, Kanagawa 240-0193, Japan 3Doshisha University, Kyotanabe, Kyoto 610-0321, Japan 4Division of Quantum Science and Engineering, Hokkaido University, Sapporo 060-8628, Japan 5Plasma Engineering Group, Consorzio RFX - Corso Stati Uniti, 4, 35127 Padova, Italy 6Max-Planck-Institut für Plasmaphysik, Bereich ITER-Technologie & -Diagnostik / N-NBI Boltzmannstr. 2, 85748 Garching, Germany e-mail address: tsumori@nifs.ac.jp The 16th International Conference on Ion Sources (ICIS 2015) from 23rd to 28th August, 2015 at Manhattan NY, USA

Outline Introduction Source-plasmas response to electric field Measurement of Ho and H- temperatures Flow measurement of charged particles Beam extraction Summary

Introduction

Direction of our research Negative-ion (H-) rich plasmas are generated widely in beam extraction region in Cs seeded plasma. They are contaminated with electron diffused from driver region by applying beam extraction field. To investigate charged-particles dynamics further, we start to measure followings: Ho and H- temperatures Detailed distribution of charged particles and its response to electric field. flow of electron, positive and H- ions Before beam extraction (Cs seeded plasma) During beam extraction Negative-ion (H-) rich plasmas are generated widely in beam extraction region in Cs seeded plasma. e- e- H0 H+ e- H+ e- H- H- H+ H+ H+ H- H+ H- H- H- H- density ele. density

NIFS-R&D Negative Ion Source 350 mm A side B 230 mm A Magnetic field induced in NIFS R&D source Magnet setup B side Multi-cusp source with a pair of filter magnets. Inner size: 700 mm (Height) x 350 mm (Width) x 230 mm (Depth). Beam extraction region from PG to filter magnets are the target of this research. Filter and electron deflection fields combines in the region. Magnetic field in very vicinity of plasma grid

Diagnostic devices Particle transport in extraction region Cs pressure and H-, ele densities Cs LAS and Cs-IG Normal and ionic plasmas Langmuir probe and CRD Magnetic structure Diffusion of ele and pos. ion Ambipolar diffusion and drift Distribution of plasma potential Langmuir probe Bias and plasma potentials Langmuir probe Sheath formation and extraction electron and ion flows Directional Langmuir probe H- flow Directional PD probe Extracted H- distribution Ha CCD imaging H- temperature Directional PD probe Saturation CRD Meniscus formation Distribution of extracted H- Beamlet interaction Aperture mask, mini-STRIKE Beam transport H- beam divergence Beamlet monitor Space charge compensation Meniscus oscillation

experimental setup NIFS-R&D negative ion source has been applied for the investigation. Plasmas in extraction region are measured with multiple diagnostic system. Driver region Extraction region

Source-plasma response to electric field

Distribution of plasma potential (bias dependence in H2 plasma) PG Electric field, slope of potential distribution, does not change in pure H2 plasma  metal like character The field strength is 20~30 V/m Sheath gap at PG is proportional to bias voltage

Distribution of plasma potential (bias dep. in Cs seeded plasma) Slope of potential distribution changes the by increasing the bias voltage in Cs seeded plasma  Simi-conductor like character This indicates the applied field penetrates inside Cs seeded plasma. Sheath gap is less sensitive to bias voltage than the case of H2 plasma  Applied field is relaxed less with H--rich sheath.

Change of saturation currents before and during extraction Change of negative and positive saturation currents near PG with beam extraction. 2D mapped negative saturation currents near PG with beam extraction. ratios of saturation currents with to w/o beam extraction. PG PG 2D distribution of negative saturation current shows the electron concentrates along the magnetic field of electron deflection magnet S. Geng et al., Plasma Fusion Res. 10, 3405016 (2015)

Ho temperatures

Ho(H+/H2+) temperature (OES) Collimated window of HR- OES (High Resolution Optical Emission Spectroscopy) is installed at the top of bias insulator. Wavelength precision: 1 pm Resolution: 10 pm Blue wing of Ha has strong bias dependence. THo ≥ 1 eV (5 eV at Max). Parent particles are H2+ or H+ in this measurement. H2+ + e  H(n=3) + H(n=1) H+ + H-  H(n=3) + H(n=1) Collimator Fiber (to HR-OES) VB: -10 to 0 V Higher bias VB: 0 to 10 V Higher bias M. Wada et al., TuePE35: ICIS 2015, this conference

Ho temperature (Ha LAS) Ha laser absorption spectroscopy (Ha LAS) is applied to measure Ho temperature, THo. The temperature is almost proportional to input arc power. THo ~ 0.3 eV at 50 kW of input. THo decrease by increasing operational pressure. parent particle is H(n=2) in this diagnostic. Arc dependence Parc: 50 kW Pressure dependence H. Nakano et al., AIP Conference Proceedings 1655, 020018, (2015)

Flow of charged particles

Flow of electron and positive ions (four-pin Langmuir probe) (directional Langmuir probe) Rotatable around stem axis C-tip B-tip D-tip Available to scan in 3D A-tip Electrons  Positive Ions Flow directions of electron and positive ion are similar due to ambipolar diffusion

Flow of electron and positive ions (estimation) Estimation of flow velocity at a position 26 mm apart from PG. Bias voltage: Perpendicular direction to filter field Depth direction (z direction) Estimation using saturation current Estimation with cross-field drift Estimation using flow direction Estimation using diffusion = 3.8 x 103 m/s = 4.2 x 103 m/s vz = 4.7 x 102 m/s vz = 2.4 x 102 m/s

H- flow and temperature

H- temperature and flow measured with 4-pin PD vth = (vA + vc) / 2 vflow = (vA - vc) / 2 H- flow B-tip Laser pulse A-tip C-tip Flow direction is opposite to electron and positive ions. vA = vth - vflow vC = vth + vflow C-tip Using the decay time of photodetachment signal, H- temperature is estimated as 0.12 ± 0.03 eV. vth: thermal velocity vflow: flow velocity

Saturated CRD Mirror Mirror high power laser low H- temperature PD Requirement for density measurement Density is underestimated Using higher power of YAG, over-neutralization occurs in cavity path. H- temperature of 0.1 eV is obtained by comparing the saturation.

Beam extraction

Ha CCD Imaging Ha CCD imaging, which subtracts image with beam off from beam on phases, shows the H- decrement due to the extraction. By increasing the bias voltage, the reduction rate of H- decreases as shown in the left-side figure. Next question is how the decreasing H- is extracted from PG apertures. K. Ikeda et al., AIP Conference Proceedings 1655, 040005 (2015) K. Ikeda et al., MonPE11 ICIS 2015, this conference

Beamlet eclipse: Setup PG aperture was shaded with movable ceramic cylinder (eclipse shade) to investigate influences of an obstacle for (1) H- production and (2) beamlet interaction. H- beamlets extracted from ion source was monitored with beamlet monitor called mini-STRIKE. Movable in 3D directions Eclipse shade PG mask PG apertures Idea of Dr. H. Nakano and M. Kisaki P. Veltri et al., TuePS34 ICIS 2015, this conference

Beamlet eclipse: IR images Beamlet-eclipse shade changes the beamlet pattern drastically as shown below. Nearest neighbor beamlets shift due to beamlet interaction. Normal extraction with beamlet eclipse shade

Beamlet eclipse: 2D profiles By setting the beamlet-eclipse shade at 8.5 mm apart from PG, shaded beamlet decreases the intensity down to ~30 %. This decrease is mainly caused by shading the parent particle of H-. Beamlet-eclipse method has possibilities to provide more information on beam extraction and formation. Green: w/o PG-aperture plug Blue: PG-aperture plug@18.5 mm Red: PG-aperture plug@ 8.5 mm

Summary In H2 plasma, potential slope is not affected with bias voltage, and electrons sheath adjusts the slope. In negative-ion-rich plasma, sheath cannot adjust the bias voltage completely, and potential gradient changes the polarity due to the bias voltage. Near PG surface, electron density is rich on the cusp line of electron deflection magnets. Ho temperature has been measured with HR-OES and Ha LAS. Origins of Ha are considered H+/H2+ and H(n=2), respectively. Flow velocity of electron, positive ion and H- are 10 times lower than their thermal velocities of 0.1 to 0.5 eV. Beamlet-eclipse method could provide some new information on beam extraction and formation.