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NBI,).%!

NIFS NBI (16QUEST&' *+-#" &' $&'3 and 4 October 2019

Outline• Negative-Ion Based NBI for LHD• Plasma and Beam Uniformity• High Performance Accelerator• Beamlet Monitoring• Physics Study of Source Plasmas and Beams• Injection from Hydrogen to Deuterium Beams• SummaryNegative-Ion Based NBI for LHD

LHD NBI
1. Profile measurement of Ti and Er (CXRS)• The beam energy of tangential-NBI was
too high for CXRS meas.
2. Ion heating with NBI• Electron heating is dominant with
tangential high-energy NB injectors.
Ion heating rate at Te=4 keV:
ü Injection energy of 180 keV à 40%
ü Injection energy of 40 keV à 80%
3. D beam injection• BL4: max 40 keV (for H)[max 60 keV (for D)]• BL5: max 60 keV (for H)
max 80 keV (for D)

BL5(positive NBI)

BL2
(negative NBI)

BL3
4. Confinement study of perpendicular injected
high-energy particles(negative NBI)
• Normalized Larmour-radius of 40keV
Hydorgenic ions at 1.5T LHD-configurationis almost comparable to those of aparticles in ITER and/or FFHR-2m2

BL1(negative NBI)

LHD

BL4
(positive NBI)

Structure of N-NBI in LHD• Hydrogen beam injection
with 180 keV-5 MW / injector(2 ion sources).
• Length of gas-neutralizing
cell : 5 m• Focal length of the ion sourceis 13 m, and pivot point of
two ion sources locates 15.4
m in the horizontal direction.
• Injection port is ~3 m long,and the narrowest part is~0.5 m in diameter with thelength of ~0.7 m.

Hydrogen/Deuterium Negative Ion Source
Cs Lines

Cs LineFilaments

Arc Chamber(H- generator)

Accelerator
Beam Direction
Cross-sectional view of ion sourcein the direction of the short side

Cross-sectional view of ion sourcein the direction of the long side

• Multi-cusp source with external magnetic filter• Plasma production : Filament arc discharges• Cs vapor is seeded to enhance H- production• Inner dimensions : 1400 mm (H) x 350 mm (W) x 230 mm (D)• H- beam : Single stage acceleration• Beam extraction area : 1250 mm (H) x 250 (W)Issues to be achieved
Stable beam injection

Sufficient H-/D- production• High H-/D- current to discharge power→ stability for long duration• Plasma uniformity• Effective and stable Cs operation

Effective beam acceleration• Optimization for beam optics of H-/D- beams→ less accelerator breakdown• Beam uniformity• High performance accelerator→ High port through rate with high power beams

Physics study of ion-source
plasma and beam

• Physics of negative ion plasma• Negative ion production• Extraction mechanism of negative ions
• Meniscus formation

Plasma and Beam Uniformity

Improvement of Arc Efficiency• Better electron confinement à higher atom and proton densities• The distribution of primary electrons is better for H- production in the
optimized configuration of cusp magnets.
• The arc efficiency: 0.13 A/kW à 0.23 A/kW

National Institute for Fusion Science

Electron Drift inside Arc Chamber
Arc plasma is shift on the one long side of due to the one directional
electron drift caused by the magnetic field inside the arc chamber.

National Institute for Fusion Science

Plasma Uniformity• In order to adjust the plasma shift, the filament and arc power supplies are
separated into 12 modules, and each modules are connected a pair of filaments at)
ion source.
m• Using the separated power supplies, distribution m(of arc-plasma density can becontrolled considerably uniform.
Beamline 3
Filament P.S.

Arc Chamber

Filament Position [mm]Filament

n
o
i
t
i
800
s
o 600
P

400
l
a 200
c
i
0
t
r -200
e
V-400
-600

Arc P.S.

-800

0

2
4
6
8
Arc Power (kW)10

National Institute for Fusion Science

Beam Uniformity 1•)mExtracted beam distribution is controlled by adjusting the arc plasma
distribution.
m(• The beam profile is monitored at 2.2 m downstream of the ion source by the Hanvideo camera and at 8.5 m downstream
by the calorimeter array.
)
Beamline 3

.
u
.
a
(

Ion Source Chamber
0

Vertical Position [mm]0.2 0.4 0.6 0.8

1

1.2

Measured
Calculated

Vertical Position [mm]

n-100
o
i
t -200
i
s
o-300
P

o
i
t
0
i 400
s
o 300
P
200
l
a 100
c
i
0
t
r -100
e
V-200

-400

-300
-500
0

CCD view of Ha intensity 2.2m
downstream

20 40 60 80 100
Mod-intensity (a.u.)

Distribution of Ha intensity
2.2m downstream-400
0

0.2 0.4 0.6 0.8 1
Intensity (a.u.)

1.2

Beam profile on calorimeter 8.5m
downstream

High Performance Accelerator

Highest beam heat load onto grounded grid
Cooling channel

SG

PG

HBeam

EG

Ho

Cooling channel

Beam timinge-e-Multi-Slit GG

Beam heat load onto grids
Comparison of beam heat load onto EG and GG(Green mark denotes beam timing and duration)
TC(in)TC(out)Tgg(out)

Tgg(in)ΔTgg = Tgg(out) - Tgg(in)Cooling Channel in Multi-Slit GG

Water calorimetric to measure grid heat load

• Beam heat loads onto EG and GG were
measured by means of water calorimetrictechnique.
• The measurement shows that heat load
onto GG is much larger than that onto EG.

Multi-hole to multi-slot grounded grid
am
Be

am
Be

Plasma Grid (PG)
Extraction Grid (EG)Extraction Grid (EG)Electron Deflection
Magnet (EDM)

Multi-Hole Grounded Grid
(MHGG)

Electron Deflection
Magnet (EDM)

Multi-Slot Grounded Grid
(MSGG)

Steering Grid (SG)Beam accelerator
with multi-slot grounded grid

Beam accelerator
with multi-hole grounded grid
• Plasma Grid
: made of Mo• Extraction Grid : including electron
deflection magnets• Grounded Grid : Multi-aperture grid

Plasma Grid (PG)

•
•
•
•

Plasma Grid
: made of Mo
Extraction Grid : electron deflection
magnets are inside
Steering Grid : made of Mo
Grounded Grid: Multi-slot grid

Multi-hole and multi-slot grounded gridMulti-Hole Grounded Grid (MHGG)

Multi-Slot Grounded Grid (MSGG)• Beam heat load onto the grounded grid is larger than that onto the others.
• A single slot corresponds to a single row of multi-hole.
• Beam transparency of the MSGG is twice as large as that of MHGG.
• Steering grid is added to adjust the beamlet trajectories.

Beam Heat Load on Multi-Holeand Multi-Slot Grounded Grids

Temperature rise of multi-holeand multi-slot GGs
with respect to injection heat

Heat load onto multi-holeand multi-slot GGs
with respect to acceleration heat

• The MSGG has the heat acceptance corresponding to injection heat of ~16 MJ• The ratios of heat load to acceleration heat onto MHGG and MSGG are 14.3 %and 7.3 %, respectively.
• The ratio on MSGG is a half value to that on MHGG. The ratio of 7.3 to 14.3 %is inverse proportional to the grid transparency.

Progress of injection power (1998 - 2004)

The maximum injection powers
and energies obtained with MSGG
from 2002 to 2004.

•

Progress of the maximum
injection power by year.

The injection power and energy increased by using theaccelerator with MSGG.
FY2002 :FY2003 :FY2004 :4.4 MW / 180 keV / 2.0 sec
5.7 MW / 186 keV / 1.6 sec
5.0 MW / 189 keV / 2.0 sec
5.7 MW / 175 keV / 2.0 sec

Trouble due to Beam Elongation
Ion Sources

Neutralization Cell

Beam Dump
Injection PortH+

Ho
Melted Mo tiles

HIon Source
Chamber

Beam Dump
Chamber

LHD

Elongated beam in one
direction concentrates on someplaces in the beamline.
Negative beam dump andsome amours protecting
injection port were melted dueto the beam concentration.

Bi-focal Characteristic of Multi-Slot Grid
PG
Vext

EG

SG
Vacc
MSGG

• Mismatch of symmetry– The SG apertures: Axial symmetric.
– MSGG slot:
Mirror symmetric.
• Defocusing effect is different along the GG-slot long and short sides.
• Consequently, the focal condition separates in both directions ofparallel and perpendicular to the slot long side.

Improvement for Beam Optics

f12
SG
13

10

Parallel to slot long side

• The MSGG works one directional defocus lens. Pre-compression
in the direction along the slot short side can compensate the bifocal beam characteristic.
• Racetrack apertures are adopted to the SG, and the bi-focal
condition was reduced.

Beamlet MonitoringBeamlet Monitor

Fig. 1. Opening apertures at PG(solid red circles)

•
•
•
•

Fig. 2. NBI-beamline and beamlet monitor

Beamlet monitor plates made of graphite is installed in the beamline, and beam-carrying
heat is observed as temperature rise
Temperature distribution is monitored using an infrared camera through ZnSe viewing
window.
Beam extraction area at plasma grids (PGs) are covered by the masks, which limit theaperture pitches of 3 x 3 as shown in Fig. 1.
Using the limiter mask, whole the beamlet profile is possible to observe without overlapping
the beamlet tail area.

Infrared Images of Beamlets
Plasma Grid (PG)
Extraction Grid
(EG)

A: Rv=13.5

Vext

Vacc

Steering Grid(SG)

B: Rv=14.5

Multi-Slot
Grounded Grid
(MSGG)
Connection of voltage power supplies
to each electrode grid
Vext : extraction voltage
Vacc : acceleration voltage•••

Voltage ratio (Rv=Vacc/Vext)
and global beam profile

C: Rv=15.5

Infrared beamlet images with different Rv

The extraction voltage is applied between PG and EG, and the acceleration voltage isapplied between EG and MSGG. EG and SG are electrically connected.
The beam width obtained at calorimeter changes as a function of voltage ratio of acceleration
voltage to extraction voltage (Rv = Vacc/Vext).
Infrared beamlet images are shown on the right hand side of above figures at the voltage
ratios of 13.5, 14.5 and 15.5.

Analysis of Infrared Beamlet Images
parallel to slot long side
(Horizontal)
V1
V2
V3
V4
position index

H1

H2

H3

H4

H5

Infrared beamlet images.
Horizontal direction is parallel
to slot long side of grounded grid.

•
•
•
•

Digitization of IR image and curve
fitting with Gaussians

Horizontal and vertical beamlet widths
In terms of voltage ratio (Rv=Vacc/Vext)

2-dimensional infrared beamlet images are converted numeric matrix data.
The numeric data are fit with Gaussian functions, and 2-dimensional e-folding half
widths are obtained from the data.
Horizontal and vertical beamlet divergent angles change as a function of voltage ratioof acceleration voltage to extraction voltage (Rv = Vacc/Vext).
Minimal beamlet divergences:
Horizontal direction: 4.1 mrad,
Vertical direction: 6.1 mrad

Status of negative-ion-based LHD NBI (H- case)• Each negative-ion-based NBI achieved the nominal injection power of 5 MW.

• Total Injection power withall the negative injectors
reaches 16 MW in 2007
and has been kept above
15 MW since 2007.

Physics Study of Source Plasmas and Beams

Production and Transport Mechanism of H-/D1. Improvement for ion sources• Enhancement of H- / D- current in high power injection.
• Improvement for the beam uniformity on whole the electrode.
Wide-area perveance matching and reduction of co-extracted
electrons.

2. Mechanism of H- / D- production in ion sources• What is the parent particle of H- / D-? H+ / Ho (D+ / Do)• Density distribution of the positive and negative ions, electron
and neutral particles in the beam extraction region.

3. Transport and extraction mechanism of H- / D• Does meniscus exist in the Cs seeding condition?
• Potential structure and distribution of charged particles in the
beam extraction region.

Ion source plasma is controlled with the structureand strength of magnetic fields

X

230 mmView fromY direction

Cs vapor

350 mmDriver region

Extraction region

Y
Magnet configuration

Magnetic field induced by filter magnets

View from• Driver region: Filament-arc plasma (hot electrons) X direction• Driver region and beam extraction region:

Plasma grid (PG)

Dense plasma is confined with cusp magnetic field.
• Extraction region: Diffused plasma
Magnetic filter field reduces the plasma density andelectron temperature < 1eV.
• H- ions are produced on the plasma grid (PG)
• Electron deflection field plays an important rolein negative ion sources.

57 (19x3) mm
Magnetic field induced byelectron-deflection magnets

29/22

Plasma diagnostics in a NIFS negative ion source• Filament-arc source with a pair of filter magnets.
• Size of arc chamber: 350 (W) x 230 (D) x 700 (H) in mm.
• Diagnostic modules are installed on the bias insulator with 52 mm thick.
350 mmNegative ion
generator
Plasma Grid (PG)

H- accelerator

H- beam

Cross-section of NIFS-RNIS

Arrangement of diagnostic system
viewing from back-plate side

30/22

Formation of Negative-Ion plasma
in Cs seeded plasma
Decrease of electron due to Cs seeding

Changes in probe V-I curve

Cs deeding

H2 discharge• before Cs seeding
positive ion : electron ≈ 1 : 10 (at 9 mm apart from PG)• after Cs seeding- symmetric V-I characteristic- quite low electron density- (positive ion) : (negative ion) ≈ 1 : 1• “electron” saturation current à “negative” saturation current, I-.
• “ion” saturation current à “positive” saturation current, I+.

positive-ion negative-ion plasma

Negative saturation current increases
by applying beam extraction field
Beam extraction
Pre-arc period

Pre-arc period

I-: negative saturation
current
I+: positive saturation
current(I- - I+): subtracted current
Beam extraction

Response of Negative-Ion Plasma
to External Electric Field
Electron compensates for
extracted H- ion density

Waveform of H- density(Photodetachment signal)

Without Ext. Field

H- density

DriverregionExtraction
region

e-

H+

H0 H+

H-

With Ext. Field

e-

H-

e-

H+

H+
H-

Waveform of negative saturation current(Langmuir-probe signal)

Negative saturation
Current (electron)

H-

H+

e-

H-

H+
H-

• Without extraction field, the charge neutrality is
kept by H- and positive ions in the beam
extraction region.
• By applying extraction field, H- density
decreases in extraction region.
• To conserve the charge neutrality, electrons
move from electron-rich driver region.

Formation of negative-ion plasma
Electron, Positive ion(Ambipolar diffusion)

H+

e

H-

e

H+
H-

Electrons have smaller gyro radii.
à They are absorbed partially in the metal
part of plasma grid.
à Electrons are filtered by the outer loopsof electron deflection magnets.

6/16

ElectronplasmaAbove metal
part of PG,
negative-ion plasma
includes more electrons.

On the other hand, very
low density of electron
observed in the negative
ion plasmaplasma
above PG
Negative-ion
apertures.
Positive ions (H+, H2+, H3+) have larger gyroradii than electrons.
à They cannot penetrate inside plasma grid.
à Converted H- ions have larger gyro radiithen electrons, and possible form
negative-ion plasma inside EDM loops.
S. Geng et al., Plasma Fusion Res. 10, 3405016 (2015)

Where are the negative ions extracted?
Schematic view ofthe setup of Ha-filtered CCD

Array of PG
apertures

View taken w/o plasma

Subtracted CCD image,during and before extraction

Decrease
Ha intensity(H- density)
Increase

Distribution of extracted H- ions localized mainly in theelectron deflection field penetrated inside source plasma.
K. Ikeda et al., New J. Phys. 15 (2013) 103026

35/22

Directional Photodetachment Langmuir Probe (DPLP)
Directional Photodetachment Langmuir Probe (DPLP)
Rotatable aroundstem axisvAvC

H- flow
tip-B
tip-C

ceramic stem

Photodetachment signal, DI

tip-A

tip-D

Laser PulserL
H- flow

recovery time, t

vflow = (vA - vc) / 2

Using a shade of center stem, flow velocity of
H- ions is enable to estimate.
The velocity at each probe tip is evaluated bytime
rL / t, where rL and t are radius of laser column
S. Geng et al., Rev. Sci. Instrum. 87 (2016) 02B103 and recovery time, respectively.

36/22

Negative ion flow
S. Geng et al., Fusion Eng. Des.,
http://doi.org/10.1016/j.fusengdes.2017.02.041.

ExB

H- flow (before Extraction)

E
B

H- flow (during Extraction)

37/22

Injection from Hydrogen to Deuterium Beams

From Hydrogen to Deuterium Beams
H- acceleration

Totally 15 MW (H)

N-NBI

H

Ie / IH-

EnergyInj. power

0.27

190 keV

5.6 MWBL2

0.30

178 keV

4.6 MWBL3

0.23

185 keV

4.8 MW

H BL1
H- accelerationDD- acceleration

Beam injection Power with respect to
input arc power in H- and D- operations

1/16

D- acceleration
D

Ie / ID-

EnergyInj. power

BL1

0.49

190 keV

2.1 MWBL2

0.54

171 keV

1.9 MWBL3

0.39

178 keV

2.3 MW

K. Ikeda et al., Nucl. Fusion 59 (2018) 076009.

Measurement of the source plasmas (H and D)
Densities of negative ions, positive ions,electron and plasma potential were measured
in NIFS NBI teststand.
Discharge gas was changed from H2 to D2.

discharge power(constant input)
negative ion density(Cavity Ring-Down)

X 1.3

Density of D- ions increased 1.3 times lager
than that of H-.

positive ion density(Langmuir probe)

Density of electrons increased much higher,~3 times, in the D2 discharge.

Electron density(interferometer)

Higher transport of deuterium plasma leadto the higher electron current
H. Nakano et al., OR17AM-C04 (this conference)
M. Kisaki et al., PO18AM-025 (this conference)

2/16

X 2.2

X 2.8
plasma potential

0.4 ⟶ 0.6 [V]What is the key point?

3/16

At moving the acceleration from H- to D- beams, the following changes
appeared;
1. Increase of co-extracted electron current.
2. Decrease of negative ion current.
3. Increase of Cs consumption.
Higher electron current possible to damage the accelerator electrodes.
àCannot increase the discharge power to increase D- ions and toincrease the beam power.
Electron density increases simultaneously.

Electron suppression inside the ion-source plasma is essential !

Changes in the acceleration from H- to D- Beams
Extractiongap

By decreasing the extraction gap, electron
deflection field penetrates deeper inside
ion source plasma and becomes stronger.
extraction gap: 7 mm
electron/negative ion ratio,(Iext - Iacc) / Iacc

electron/negative ion ratio,(Iext - Iacc) / Iacc

extraction gap: 8 mm

arc power, Parc [kW]electron deflection field
penetrate more inside plasmaarc power, Parc [kW]This result suggests that enhancement of the magnetic field near plasma grid is
42/22
effective to reduce the density of electrons.

Result with Egg Box Cell

H- current

ele. current

8/16

Electron Fence (EF)
Cross-sectional view of LHD ion source

Arc Chamber
Filaments
Filter magnets
Plasma Grid (PG)

Electron Fence (EF) installed on PG

9/16
Electron Fence is madeof parallel φ5 Mo rods

Magnetic field lines are cut with Electron Fence

X

Y
Magnet configuration

10/16

Ee: 1eVComparison of time evolutions of H- andco-extracted electron currents
Trend of original Hcurrent, IH-(org), andco-extracted electron
current, Ico-ext.

with EF(2019)13/16

without EF(2018)

IH-(org) : Iacc
Ico-ext : Iext - Iacc
Trend of the ratio of coextracted electron
current, Ico-ext, to original
H- current, IH-(org).
[ Re : (Iext – Iacc) / Iacc ]Ratio of co-extracted electron current to H- current is reduced about a half value with EF.

Summary• Optimized magnetic configuration, which leads to longer electron life time, isnecessary to obtain larger production rate of H-/D- ions
• Multi-Slot Grounded Grid (MSGG) with high beam transparency is adopted forhigh power beam injection.
• Horizontal and vertical focal condition of MSGG has been resolved by applying“racetrack aperture” to steering grid.
• Detailed beam characteristics were investigated with beamlet monitor.
• Total injection power with three n-NBI systems achieved 16 MW by accumulating
the results obtained with above investigations.
• Diagnostics study of ion-source plasmas and beams have started to understandthe physics of H-/D- production, negative-ion transport, meniscus and beamtransport.
• Formation of “negative ion plasma” with very low electron density was found at
beam extraction region of Cs seeded negative ion source.
• Flow pattern of H- ions has been obtained using directional photo-detachment
probe.
• To solve the issues of D- beams, optimization for D- beam has started.

Neutral Beam Injection

Production of hydrogen ions in ion sources
Electrostatic acceleration of the ions (Beam formation)
Neutralization of the ion beams
Removal of the residual ions using the ion deflection magnet
Injection of neutral beam
Ionisation
Collisional slowing-down

Inside target plasmaWhy negative ion beams are necessary?
• High neutralization efficiency in the beam energy morethan 100 keV.
• Beam divergence becomes about 5 mrad at 180 keV by
seeding caesium (Cs).
180 keVneutralization efficiency [%]

100

100

Photodetachment
Plasma Neutralizer

80

Negative ions:60 Higher neutralization
efficiency

~60 %

D-H

40

60
40

20
0
10

80

~25 %
20

30

50 70 100
200 300
beam energy [keV]

D3++
H
3
D22++
H
D++
H

20

0
500 700 1000

50/22

Hydrogen negative ion••

Hydrogen negative ion (H-) is a twoelectron system.
One electron occupies the state close
to 1S orbital, and another occupies theorbital with it maximum electronprobability at 3.8 times of Bohr radius.

•

Additional electron of 1S’ orbital trappedat stable “affinity level, εA”, whose energy
level is -0.754 eV from the vacuum level.
* The vacuum level is the electron of 0
eV without the external field.

1S’ additional electron

1S electron(RS’ = 3.8 rB)(RS = rB : Bohr radius)

Electron Affinities of Some Elements
He: 0.0, Li : 0.62, Be: 0.0, B : 0.28, C : 1.60/1.27
N : 0.07, O : 1.46, F : 3.41, Ne: 0.0 [eV]εvac
E.A. (εA )I.P. (εI)

0.754 eV13.6 eVProcess of H- production (volume process)•Volume Production :Two-stage electron collisions
1. H2 + e (≥ 20eV)→ H2*(excited molecular) + e(fast)
Vibrational excitation
2. H2*(excited molecular) + e (~ 1eV)
→ H- + H
Dissociative attachment

Surface Production
Electron donation on Cs/Metal surfaceof low workfunction
1a. H2 + e(>20 eV) → 2H(~2 eV) + e
Flank-Condon process
1b. H+ + e (on metal surface) → H
Auger neutralization
2. H + e (on metal surface) →Resonant transition

H-

7

35

6

30

5

25

4

20

3

15
ΦCs : 2.3 eV

102Φmin : 1.45 eV

1
0

0

5
10
15
Number of Cs Depositions

5
0
20

Conversion Efficiency [%]•Workfunction [eV]Process of H- production (surface process)Electron transition on metal surface
Atomic Field
Δφ
φ

Cs

ε vac
εf
Cs/Metal

Cs-0.754eV

H - StateHεa

As a hydrogen atom approaches to metal
surface • • •1)The affinity level of hydrogen is lowered by
the image potential.
2)The affinity level interacts to electron levels
in metal surface and bulk, and the level
becomes broad with the width of D in below
equation.

D(x) = 4π/h∑r(e k)|Vak|2••

In very vicinity of metal surface, bulk electron near Fermi level transits to thebroadened affinity level due to the “resonant charge transition”, and H- ion is formed.
When H- escapes from the surface, the affinity level rises above Fermi level. The
electron at affinity level transits back to metal with a probability of following equation:

P(t) = exp(-4pDoeggv
•With adsorption of Cs, energy difference of the affinity and Fermi levels decreases.
In such situation, the transition probability is enhanced, and the back-donating is
reduced. Consequently, the production rate of H- ions increases.

Issues to be achieved
1.

Stable H- production High arc efficiency
 Longer filament life time Stable cesium operation
2. Plasma and beam uniformity
 Better beam perveance ® Less voltage breakdowns at beam
accelerator
 Higher total H- current
3. High performance beam accelerator• Less breakdown in high energy beam acceleration• Less beam damages on the accelerator electrode
4. Optimization for beam optics
 Higher beam transmission
 Less beam induced damages inside beamline