development of a new beamlet monitor system: time
TRANSCRIPT
Development of a new beamlet monitor system: time resolution and phase space structure
Yasuaki Haba1)
K. Nagaoka2,1), K. Tsumori2,3), M. Kisaki2), H. Nakano2,3), K. Ikeda2), Y. Fujiwara2), S. Kamio2), S. Yoshimura2) and M. Osakabe2,3)
The 6th International Symposium on Negative Ions, Beams and Sources 3rd-7th September 2018, Novosibirsk, Russia
1) Nagoya University2) National Institute for Fusion Science3) SOKENDAI (The Graduate University for Advanced Studies)
2 / 30Outline
1. Introduction
2. Diagnostics■ A new beamlet monitor system
■ Pepper-pot-type Phase Space Analyzer (PPSA)
■ Fast Beamlet Monitor (FBM)
3. Results■ FBM
■ PPSA
4. Discussion
5. Summary
3 / 301. Introduction
NBI
DNB
Negative ion source is inevitable in nuclear fusion
The optimization of beam optics is one of the most important subjects.
ITER neutral beam system requirements:
■ 16.7 MW, 3600 s,
■ 230 A/m2
(achieved 330 A/m2, P. Franzen 2011)
■ 1 MeV
(achieved 0.98 MeV, M. Taniguchi 2012)
■ < 7 mrad:
(experimentally unknown)
4 / 301. Introduction Beam optics in LHD-NBI
We should study beam characteristics in more detail.
LHD negative-ion-based NBI typical parameters
(Filament-arc source)
P = 6.4 MW, V = 190 kV
divergence: θh=4.1 mrad, θv=6.1 mrad
(K. Tsumori 2010)
Divergence for RF sources → ?
Image of negative ion beamlets obtained with IR camera
(Filament-arc source)
5 / 301. Introduction Our viewpoint
We propose here that
(1) the beamlet stability
(2) the phase space structure of negative ion beamlet
should be measured in more detail.
6 / 301. Introduction (1) Beam stability
We focus on the time evolution of the “beamlet width” and “beamlet axis”.
Vext Vacc
• Stability of power supplies (Vacc/Vext)
• Stability of source plasma
• Stability of meniscus formation
Beam stability was affected by
Why should the beamlet stability be studied ?
PG EG SG GG
Because the beamlet stability may degrade the beam optics.Oscillations of
Beamlet width,
Bemalet axis
7 / 301. Introduction (2) Phase space structure
We should evaluate the phase space structure of negative ion beamletin experiment.
(J. Hiratsuka 2015)
Numerical study Experimental study
Equipotential lines in the extractor
Phase space structure at the acceleration grid
Evaluation of beam emittance
0.59 mm mrad (S. K. Guharay 1996)
0.54 mm mrad (B. Klump 2014)
0.82 mm mrad (Ueno, Shinto 2018)
Phase space structure is not discussed in detail.
vy / vz
y
Why should the phase structure be studied ?
Because, the phase space has much information on beam optics
8 / 301. Introduction Our approach
Purpose of a new beamlet monitor system
(1) Measurement of the time evolution of the beamlet width and beamlet axis
(2) Measurement of the phase space structure of negative ion beamlet
We aim to understand the beam optics in more detail. In particular, we want to elucidate the difference of the beam optical properties between rf source and filament arc source.
Outline
1. Introduction
2. Diagnostics■ A new beamlet monitor system
■ Pepper-pot-type Phase Space Analyzer (PPSA)
■ Fast Beamlet Monitor (FBM)
3. Results■ FBM
■ PPSA
4. Discussion
5. Summary
10 / 302. Diagnostics A new beamlet monitor system
Fast Beamlet Monitor (FBM)
Pepper-pot-type Phase Space Analyzer (PPSA)
A new beamlet monitor system is a hexagonal box mounting multi-diagnostics.
The diagnostics can be switched for shots interval by rotating or vertical motion.
Multi-diagnostics
Rotatable
Vertically moved in vacuum chamber
FBM
PPSA
Outline
1. Introduction
2. Diagnostics■ A new beamlet monitor system
■ Pepper-pot-type Phase Space Analyzer (PPSA)
■ Fast Beamlet Monitor (FBM)
3. Results■ FBM
■ PPSA
4. Discussion
5. Summary
12 / 302. Diagnostics Pepper-pot-type Phase Space Analyzer
Several sets of PPSAs were mounted on the hexagonal box.
kapton foilpinhole array kapton foil pinhole array
: Diagnostic for measuring the phase space structure of a beamlet
PPSA consists of a pinhole array and kapton foil.
PPSA
13 / 302. Diagnostics Concept of measurement using PPSA
The horizontal and vertical axes show the measuring point y and normalized velocity 𝒗𝒚/𝒗𝒛, respectively.
We obtain the distribution function: 𝑓(𝑦, 𝑣𝑦).
Phase space structure
O
Many partial beamlets passing through each pinhole reach the surface of the kapton foil, and make burn pattern.
The deviation of footprint from the pinhole position, Δ𝑦, corresponds to the vertical velocity at the pinhole position.
𝑣𝑦
𝑣𝑧=𝛥𝑦
𝐿
-10 -5 0 5 10-10
-5
0
5
10
y
v y /
vz
0
2
4
6
8
10
12
intensity [a.u.]
𝑦pinhole
Δ𝑦
𝐿
PPSA
Outline
1. Introduction
2. Diagnostics■ A new beamlet monitor system
■ Pepper-pot-type Phase Space Analyzer (PPSA)
■ Fast Beamlet Monitor (FBM)
3. Results■ FBM
■ PPSA
4. Discussion
5. Summary
15 / 302. Diagnostics Fast Beamlet Monitor (FBM)
Fast Beamlet Monitor:
Faraday-cup-type 32-channel
electrode array for beam
current profile monitor
・Two-grid system to suppress secondary
electrons
・Wide frequency range of DC~40 MHz
・The spatial resolutions are 4.1mm and 4.9 mm~40 MHz
~50 MS/s
FBM
16 / 302. Diagnostics Experimental configuration
The beam accelerator is identical to those of the working beam injector in LHD plasma experiment.
910 mm
The measurement of negative ion beamlet was carried out
Neutral beam test stand at NIFS (NIFS-NBTS)
Filament arc source: R&D negative ion source (NIFS-RNIS)
Outline
1. Introduction
2. Diagnostics■ A new beamlet monitor system
■ Pepper-pot-type Phase Space Analyzer (PPSA)
■ Fast Beamlet Monitor (FBM)
3. Results■ FBM
■ PPSA
4. Discussion
5. Summary
18 / 303. Results Suppression of secondary electron current
The absolute value of the negative ion current was obtained.
e
H-
Mo
Mo ceramic
ceramic
Cu
Two-grid system of FBM
e
• VFBM: Collection voltage for secondary electrons
• The saturation of negative ion current was
observed when the suppression voltage is
positively higher than 40V.
-80 -40 0 40 80-10
0
10
20
curr
ent
den
sity
[A
/m2]
VFBM [V]
enhancement
suppression
I-V characteristicsFBM
19 / 303. Results Two-dimensional beam profile
Two-dimensional beam profile was obtained by FBM.
Observation was carried out on shot by shot basis.
Isolated beamlet due to masking of the neighboring apertures on PG
0 10 20
-10
0
10
20
30
NIFS-NBTS #147,768-779
2018.05.16
nH-=(2.76±0.12)×1017m-3,
Parc=51.2±3.0 kW
curr
ent
den
sity
[A
/m2]
y [m
m]
x [mm]
3
6
8
11
14
16
19
21
24
𝑦
𝑧, beam
Masking on PGFBM
half 1/e width
20 / 303. Results Time evolution of the beamlet width and axis
The beamlet width and beamlet axis are strongly depended on Vacc/Vext.
FBM has the ability to observe the time evolution of the beamlet width and beamlet axis.
0.3 0.4 0.5 0.6
4
7
10
11
12
13
20
30
40
0
5
10
1
3
5
Vac
c[kV
]
Vac
c/V
ext
Vex
t[kV
]
J 0 [
A/m
2]
x 0 [
mm
]
10
60
sx
[mm
]
time [s]
𝐽 = 𝐽0 exp −𝑥 − 𝒙𝟎𝝈𝒙
2
+ 𝐶
・Vacc: acceleration voltage
・Vext: extraction voltage・J: current density written by
・x0: beamlet axis
・σx: beamlet width (half 1/e)
・J0: current amplipute
・C: background level
FBM
Outline
1. Introduction
2. Diagnostics■ A new beamlet monitor system
■ Pepper-pot-type Phase Space Analyzer (PPSA)
■ Fast Beamlet Monitor (FBM)
3. Results■ FBM
■ PPSA
4. Discussion
5. Summary
22 / 303. Results Black level detected by PPSA
The burn pattern was digitized by a scanner with a gray scale depth of 8 bit and
spatial resolution of 0.042 mm/pixel.
In this analysis, it is assumed that the increase of black level (∆𝜂) of the kapton
foil is proportional to the time integral of the beam current density.
nH-=2.78±0.15×1017 m-3
Parc=54.2±4.5 kW
Beam burn pattern
on the kapton foil
∆𝜂 ∝ 𝐽 𝑑𝑡
-20 -10 0 10 200
20
40
60
80 experiment
y [mm]
PPSA
23 / 303. Results Procedure to distinguish two beamlets
-20 -10 0 10 200
20
40
60
80
bla
ck l
evel
: D
h [
a.u
.]
experiment
y [mm]
In order to distinguish the target beamlet (A) from the neighboring one (B),
we put two conditions as follows:
1. The footprint width (ߪ) is constant for the same beamlet.
2. The interval of footprints (𝛿𝑦) is constant for the same beamlet.
∆𝜂 =
𝑖=−6
4
𝛼A𝑖 exp −𝑦A − 𝑦A0 − 𝑖𝛿𝑦A
2
𝜎𝑦A2 +
𝑗=0
4
𝛼B𝑗 exp −𝑦B − 𝑦B0 − 𝑗𝛿𝑦B
2
𝜎𝑦B2
-20 -10 0 10 200
20
40
60
80 experiment
target
neighbor
y [mm]
4
3
2
1
0
3
2
10
4-
3-
2-
1-
5-6-
4
Model eq.
raw data
PPSA
24 / 303. Results Phase space structure measured by PPSA
-20 -10 0 10 200
20
40
60
80
bla
ck l
evel
: D
h [
a.u
.]
experiment
envelope
bla
ck l
evel
: D
h [
a.u.]
y [mm]
-20 -10 0 10 20
-20
-10
0
10
20
y [mm]
v y /
vz
[mra
d]
2
19
36
53
70
The phase space structure of the negative ion
beamlet can be successfully measured by PPSA.
The negative ion beamlet profile in the vertical
direction was obtained.
The envelope of the target beamlet profile
shows a good agreement with Gaussian
distribution.
-20 -10 0 10 200
20
40
60
80
bla
ck l
evel
: D
h [
a.u
.]
experiment
y [mm]-20 -10 0 10 20
0
20
40
60
80 experiment
y [mm]
Raw dataFitting of the
neighboring beamlet
=−
vertical (y) direction
PPSA
25 / 303. Results Black level is proportional to the charge density
We changed single shot beam duration,
but total beam duration is constant.
Linearity
The black level is proportional to the charge density, and this linear relationship is
applicable to the evaluation of the phase space structure for the fixed beam energy.
-20 -10 0 10 200
20
40
60
80 experiment
envelope
bla
ck l
evel
: D
h [
a.u
.]
y [mm]
0
100
200
char
ge
den
sity
: òJ
dt
[As/
m2]
∆𝜂 ∝ 𝐽 𝑑𝑡
We discuss about the applicability of our assumption:
0 100 2000
30
60
90
experiment: 0.6s×20shots
experiment: 0.2s×60shots
bla
ck l
evel
: D
h [
a.u.]
mea
sure
d b
y P
PS
A
charge density òJdt [As/m2]
measured by FBM
PPSA FBM
26 / 304. Discussion Discussion about the strategy for future study
1. Introduction
2. Diagnostics■ Pepper-pot-type Phase Space Analyzer (PPSA)
■ Fast Beamlet Monitor (FBM)
3. Results■ FBM
■ PPSA
4. Discussion
5. Summary
In the previous session,
We have successfully demonstrated that
and work well.
In this session, I will show you
two important observations.
I discuss about the strategy for future study related to
Phase space structure
Beamlet stability
PPSAFBM
27 / 304. Discussion Discussion-I (future plan-I)
-20 -10 0 10 200
20
40
60
80
bla
ck l
evel
[a.
u.]
well focused
over focused I
over focused II
bla
ck l
evel
[a.
u.]
y [mm]
-20 -10 0 10 20
-20
-10
0
10
20
x [mm]
v x /
vz
[mra
d]
2
16
29
43
56
70
x-direction
In this case, no overlap of neighboring beamlet was
observed.
The phase space structure in the horizontal direction is
more complicated than that in the vertical direction.
The black level profile can be decomposed to three
components by the same model based on ߪ and 𝛿𝑦.
PPSA
28 / 304. Discussion
-20 -10 0 10 20
-20
-10
0
10
20
x [mm]
v x /
vz
[mra
d]
2
16
29
43
56
70
Discussion-I (future plan-I)
The phase space structure of negative
ion beamlet will be studied for further
understanding of beam optics.
horizontal (x) direction
Multi-velocity components were observed
in x-direction.
We will try to
● reveal the perveance dependence
● compare with numerical model
PPSA
29 / 304. Discussion Discussion-Ⅱ (future plan-Ⅱ)
Further study of the beamlet stability is required for understanding of beam optics.
0.00 0.01 0.02 0.03 0.04 0.05 0.063
4
5
horizontal beam width
bea
mle
t w
idth
: s
x [m
m]
time [s]
bea
mle
t ax
is:
x 0 [
mm
]
1
2
3
position of beam axisThe oscillations of the beamlet width and
beamlet axis due to source plasma
fluctuation were observed.
We try to investigate the linkage between
the oscillations and the stability of
● source plasma
● meniscus formation
→ ??
FBM
30 / 305. Summary Summary
A new beamlet monitor system with the combination of FBM and PPSAs has been
developed for measurements of negative ion beamlet.
Using we have measured
the absolute value of negative hydrogen ion beam
two-dimensional current profile
the time evolution of the beamlet width and beamlet axis
Using we have demonstrated
subtraction of the overlapped components coming from the neighboring beamlet
construction of the phase space structure
confirmation of the linearity between black level and the charge density
In future, we will study
beamlet stability and the relation to the source plasma and meniscus stabilities
phase space structure due to perveance dependence and comparison with
numerical model
PPSA
FBM
Thank you for your attention.
Negative ion beam production
Large-scale nuclear fusion experiments
Nuclear physicsMedical applications
ITERLHD
Common requirements of these negative ion beam productions are to produce the high current negative ion beam with good optics.
In nuclear fusion, negative-ion-based neutral beam injectors (NNBI) are an effective method to heat magnetically confined plasmas.
Beam width depended on the H- density
0 10 20 300
10
20
30cu
rren
t d
ensi
ty [
A/m
2]
x [mm]
1 2 3 40
5
10
15
sx, FBM
sx, CFC
sx
[mm
]
H- density [×1017 m-3]
Time evolution of σx×J0
0.42 0.43 0.44 0.45 0.46 0.47120
130
140
150
sx×
J 0
time [sec]
0.3 0.4 0.5 0.6
1.0
1.5
2.0
C
time [sec]
Current amplifier of FBM
Charge exchange of negative ions
Charge exchange of negative ions is easier than that of positive ions.
+ーー +ー
Neutral particleNegative ion
+ +ー
Neutral particlePositive ion
Easy!
Difficult
Neutralizer (gas, plasma, laser)
Negative ions are useful for neutral beam injections.
Neutralization efficiency of negative ions
Negative-ion-based neutral beam injection (negative-NBI) is an effective method to heat magnetically-confined plasmas because of high fraction of beam neutralization in high energy regime (180~190 keV for LHD, 1 MeV for ITER).
Maximum neutralization efficiency depended on the beam energy in the case of using the gas neutralizer
Higher than 60% even the
beam energy reaches 1 MeV!
Difficulty of negative ion beam production and extraction
■ Negative ion beam production itself is not so easy.
■ Co-extracted electrons are useless for neutral beam, so it is required to remove the electrons from the extracted particles using a series of electron deflection magnets (EDM).
𝑦
𝑧, beam
𝑦
𝑧, beam
A steering grid (SG) is also installed for correction of the beam axis z.
H- ions are slightly changed their orbits due to EDM and SG.
Extracted particles including
electrons are derived from here.
Previous study (idea of the device development)
Proceedings of LINAC 2014, Geneva, Switzerland
B. Klump, U. Ratzinger, W. Schweizer, and K. Volk
The ion beam penetrates from the left side through a
pinhole array, which made of wolfram-copper. Partial
beams hit the aluminum screen.
This collector was build and installed in a diagnostic
chamber, which also contains a Faraday cup, the
vacuum system and a viewing window.
The system can be moved into the beam by a motor
drive. For the investigations, the ion beam of the
FRANZ ion source was used.
Collector based on the pepper-pot method for
evaluation of phase space structure of ion beams
Faraday cup for measuring beam current profiles
What can we do for the combination of these two diagnostics?
J. Hiratsuka et al. PFR (2015)