1. 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, Oroshi Toki Gifu , Japan
2The Graduate University for Advanced Studies, Shonan Village, Hayama, Kanagawa , Japan
3Doshisha University, Kyotanabe, Kyoto , Japan
4Division of Quantum Science and Engineering, Hokkaido University, Sapporo , Japan
5Plasma Engineering Group, Consorzio RFX - Corso Stati Uniti, 4, Padova, Italy
6Max-Planck-Institut für Plasmaphysik, Bereich ITER-Technologie & -Diagnostik / N-NBI
Boltzmannstr. 2, Garching, Germany
address:
The 16th International Conference on Ion Sources (ICIS 2015)
from 23rd to 28th August, 2015 at Manhattan NY, USA

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

3. Introduction

4. 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

5. 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

6. 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

7. 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

8. Source-plasma response to electric field

9. 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

10. 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.

11. 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, (2015)

12. Ho temperatures

13. 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: 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

14. 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, , (2015)

15. Flow of charged particles

16. 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

17. 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

18. H- flow and temperature

19. 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

20. 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.

21. Beam extraction

22. 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, (2015)
K. Ikeda et al., MonPE11 ICIS 2015, this conference

23. 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

24. 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

25. 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 mm
Red: PG-aperture 8.5 mm

26. 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.