1. Fast Ion Measurements in JT-60U
10th Meeting of the ITPA Topical Group on Diagnostics
Moscow, 10 – 14, April, 2006
JT-60U
Fast Ion Measurements in JT-60U
M. Ishikawa, Y. Kusama1, T. Nishitani1, M. Takechi1, K. Shinohara1, M, Baba, T, Itoga, M. Sasao, K. Nomura, V. A. Krasilnikov2, Yu. Kashuck2 ,
I am Masao Ishikawa. Today, I will be talking about fast ion measurements in JT-60U.
Tohoku University, JAPAN
1 Japan Atomic Energy Agency (JAEA), JAPAN
2TRINITI, Russia
JT-60U

2. Contents of talk Introduction Diagnositcs for fast ion measruements
JT-60U
Introduction
Diagnositcs for fast ion measruements
Highlight Results and issues
of fast ion measurements
Development of Digital Signal Processor
of neutron detectors using Flash ADC
- Summary
In this presentation, first I am gonna talk about introduction of fast ion experiments.
I will then explain Diagnostics for fast ion measurements briefly.
Then, I will show you some of the highlight data taken from fast ion experiments and issue.
Next, I’ll explain development of digital signal processor of neutron detectors using Flash ADC
Finally I will make a brief summary (and talk about the future plan.)

3. Introduction
JT-60U
As fast ion study in JT-60U, recently, Alfvén Eigenmodes (AEs) have been extensively studied.
AE experiments have been performed using
Co-injected Negative-ion-based Neutral Beam (NNB)
(ENNB : 340 ~ 400keV, PNNB :3 ~ 5MW)
in several kinds of magnetic shear configurations
in Reversed shear (Weak Shear) plasma,
• Reversed-Shear induced Alfvén Eigenmodes (RSAEs)
　　 (Alfvén Cascades (ACs)),
• Transition from RSAEs (ACs) to TAEs
(M. Takechi, et al, POP 12(2005), )
in Weak shear plasma with high h
• Abrupt Large-amplitude Events (ALEs)
(K. Shinohara, et al.,Nucl. Fusion 41(2001) p603)
Yome!

4. Fast Ion Measurements in JT-60U
Diagnostics for investigation of fast ion behavior
• total neutron emission rate
• neutron emission profile
Stilbene neutron detector
• charge-exchange neutral particle flux
Natural Diamond detector
6 channel Neutron monitor
CX-Neutral Particle Analyzer
Neutral Particle
Analyzer (NDD)
Next I explain diagnostics for fast ion measurements
In order to investigate of fast ion behavior,
We measure total neutron emission rate using Fission Chamber
And neutron emission profile using neutron profile minitor. Detector is stilbene neutron detector.
And we measure charge-exchange neutral particle flux using natural diamond detector.
Neutron Profile
Monitor

5. Highlight Results and issue of fast ion measurements
JT-60U
Highlight Results and issue of fast ion measurements

6. fast Ion Transport due to ALE in WS plasmas
JT-60U
When bursting modes called Abrupt Large-amplitude Events (ALEs) were exited,
(M. Ishikawa, et al. Nucl. Fus. 45 (2005) 1474)
Peripheral signals increase and center signals decrease
Averaged change rate of neutron emission profile measurement
Only ions in limited energy are affected.
=>Agrees with AE resonant condition
r/a
Energetic ion profile
(1018 m-3)
before ALE
after ALE
<energetic ion profile>
We show here the highlight results of ALE experiment.
When ALE occurred, change in neutron emission profile and fast neutral particle spectrum was observed. What you see here averaged change rate of neutron emission profile measurement. peripheral signals increase and center signals decrease
and this figure shows the energy distribution of neutral particles. It was found this result only ions in limited energy are affected. This agrees with AE resonant condition.
Then, what you can see is energetic ion profile inferred from these measurement BEFORE and AFTER ALE. This figure indicated ALEs expel a significant energetic ion population from core to the outer region, that is redistribution and loss.
This is the highlight result of previous studies
energy distribution of neutral particle
ALEs expel a significant energetic ion population from core to the outer region (redistribution and loss )

7. Observation of two phases of AEs (RSAEs, TAEs) and w/o AEs
JT-60U
In (weak) reversed shear plasma, Reversed-Shear-induced AE (RSAE) and transition from RSAE to TAE was observed during NNBI. After that, mode was stabilized.
• During RSAEs, TAEs (t ~ 4.5 – 5.5 s)
TAE
An increase of total neutron emission rate (Sn) was suppressed
RSAE
• After TAEs are stabilized (t ~ 5.5 s)
The rate of the increase of Sn is enhanced rapidly.
suggests confinement degradation of fast ions due to AEs
Next, we show highlight results of another AE experiments
What you see here frequency spectrum of magnetic fluctuation and mode amplitude and total neutron emission rate.
First, during AE, time is from 4.5s to 5.5s, an increase of total neutron emission rate seems to be suppressed. Also fast charge-exchange neutral particle flux increase.
Next, after Aes disappeared, from t = 5.5s,
The rate of the increase of Sn is enhanced rapidly. On the other hand, CX neutral prticle flux decrease.
Further, after another AE appear, from time about 5.9s
Sn decrease and CX neutral particle flux increase again.
These result may suggest confinement degradation of energetic ions.
However, we must check changes of bulk plasma due to Aes.
These instability behavior had been predicted by full wave code TASK WM [2].
Further, Task WM had predicted AE in transition phase is most unstable.
[2] A. Fukuyama et al, in proceeding of 6TH IAEA Technical Committee Meeting on Energetic Particles in Magnetic Confinement Systems (12~14 October 1999, Naka)

8. Confinement degradation of fast ions due to AEs was observed
JT-60U
Neutron emission rate is calculated with OFMC (Orbit Following Monte Carlo) code assuming as follows
• Energetic ion profile in the calculation are classical
• Neutron emission component is beam-thermal reaction
(beam-thermal neutron rate accounts for ~ 90% of total neutron rate)
RSAE
TAE
no-AE
Zeff ~ 2.4
Measured neutron emission rate is smaller than calculated one (classical) during RSAE and TAE.
After TAE was destabilized, measured neutron rate is close to calculated one.
In order to estimate a change in fast ion due to AEs, Neutron emission rate is calculated with OFMC code assuming as follows.
• Energetic particles in the calculation with OFMC are classical.
• neutron emission component is beam-thermal reaction.
This figure shows just a comparison between the measured neutron rate and calculation with OFMC code. Led line Is time trace of measured neutron emission rate , which I just showed you in my previous viewgraph, and circle is calculated neuron rate at the several time.
From this figure, it is found, calculated neutron emission rate is larger than measured one during RSAE and TAE.
And, after TAE disappeared, measured neutron rate is close to calculated one.
Then, from these estimation, confinement degradation of energetic ions due to RSAE and TAE is clearly observed for the first time.
However, question remains open whether these assumptions is valid. This is going to be our next experiment after this meeting.
Confinement degradation of fast ions was observed.
It was found the reduction rate is largest in the transition phase
(Sn/Sn)Max ~ 45 %

9. Issue of neutron profile measurements
JT-60U
Our interest is how the fast ion profile change.
Neutron emission profile
TAE
RSAE
Change in neutron signals during and after modes was observed.
However, Neutron signal of some channels were saturated after the modes were stabilized
Next is issue of neuron profile measurements.
So, our next interest is how the fast ion profile change,
What you see here is time trace of frequency spectrum of magnetic flucutuation and total neutron emission and signal of neutron emission profile measurements
Change in neutron signals during and after AEs was observed.
However, neutron signal of some channels were saturated after AEs were stabilized.
Issue is that maximum count rate of the present system is relatively row, less than about ten to fifth counts per second
Issue
Saturation
of counts
Maximum count rate of the present system is < ~ 1x105 cps.
Dynamic range is narrow.
(Statistics error is large)

10. JT-60U
Development of Digital Signal Processor using Flash ADC and Stilbene Detector
JT-60U

11. Fast data acquisition system using Flash ADC
JT-60U
Maximum count rate of Stilbene Neutron Detector : < ~ 105 counts/s
Pulse Shape discrimination via an analog electric circuit
Development of digital signal processor using Flash ADC
Objective : count rate > 106 counts/s
Anode signal(~400ns pulse) is directly fed to Flash ADC
Flash ADC : Acqiris DC282
10bits, 8 G [samples/s]
DC282 PC
Memory
ADC
HDD
Anode signal n γ
As I mentioned Maximum count rate of Stilbene neutron detector is less than about ten to fifth counts per seconds. The reason why is pulse shape discrimination between neutrons and gamma-rays are performed by analog electric circuits
In order to measure neutron higher counts rate, we are developing digital signal processor using Flash ADC. Then, our object is measurements under the counts rate over ten to sixth counts per seconds.
In this system, Anode signal from neutron detector is directly fed to Flash ADC.
Further after the signal was stored in PC, pulse shape discrimination is carry out.

12. Pulse Shape Discrimination by integrated charge with two different integration time
JT-60U
Optimization of integration time
and sampling time
neutron
gamma 1 τ2
Reduction of data amount
decrease number of sampling
Avoidance of Pile-up
reduce discrimination time
In our system, pulse shape discrimination is carried out using two integrated charge with different integration time.
In order to reduce of data amount and avoid pile up, optimization of integration time and sampling time is needed.
Then, present condition of our system is sampling time is 2ns and two integration time is 25ns and 60ns, respectively.
Present condition of our system
Sampling time : 2ns
Integration time : 25ns(fast)
60ns (slow)

13. First measurement in JT-60U experiment
The DSP system have been installed in understairs of JT-60U torus hall
The sight line passes through the center region of the plasma
Measurement of neutron signals was started and PSD was successfully performed.
So, The DSP system have been installed in understairs of JT-60U tours hall to measure neutron signal in DD experiments.
The sight line passes through the center region of the plasma.
Measurements of neutron and gamma ray signal were started and pulse shape discrimination was successfully performed.
Stilbene PC
Flash-ADC

14. Summary
JT-60U
In JT-60U, AE experiments in ITER relevant domain have been extensively studied.
In order to investigate behavior of fast ions during AEs,
• total neutron emission rate
• neutron emission profile
( • charge-exchange neutral particle flux )
have been measured.
Transport and confinement degradation of fast ions deu to AEs were observed.
Maximum counts rate of neutron profile measurements is < ~105 cps because of the analog PSD circuit.
Digital signal processor (DSP) using Flash ADC and Stilbene detector is developing in order to measure neutron signals with higher counting rate (> 106 counts/s)
The DSP system have been installed in understairs of JT-60U torus hall. Then, Measurements of neutron signals was started and PSD was successfully performed.

15. Future Plan DSP using Flash ADC and Stilbene detector
JT-60U
DSP using Flash ADC and Stilbene detector
Optimization of method of PSD (integration time)
Development of software for PSD
Installation of the DSP
to the present neutron emission profile monitor
Charge-exchange neutral particle flux with the NDD
Application of DSP system
[ Collaboration with TRINITI Lab.]

16. RSAEs (Alfvén cascades) and its transition to TAEs
JT-60U
(M. Takechi, et al, POP 12(2005), )
• Case of reversed-shear configuration with qmin ~ 3.0 -> 2.0
3.0 -> qmin -> 2.5
Transition
2.5 -> qmin -> 2.0
TAE
RSAE
Frequency
qmin
2.5 3 2

17. TASK/WM predicted AE transition from RSAE to TAE is most unstable
JT-60U
the full wave code (TASK/WM) [1]
- eigenfrequency, damping rate, eigenfunction
RSAE frequency changes rapidly as qmin changes.
RSAE more unstable than TAE.
AE in transition from RSAE to TAE is most unstable.
Eigenfrequency
Damping rate
RSAE
stable
time
[1] A. Fukuyama et al, in proceeding of 6TH IAEA Technical Committee Meeting on Energetic Particles in Magnetic Confinement Systems (12~14 October 1999, Naka)

18. AE Experiments in ITER relevant domain
JT-60U
AE instabilities in ITER relevant domain in terms of <βh> v.s. vb///vA are investigated in JT-60U by using N-NB of ~ 360keV
vb// : parallel beam ion velocity
vA : Alfven velocity
h / a is also comaprable,~ 0.05 for JT-60U and ~0.025 for ITER
h : Hot ion Larmor radius
a : Minor radius of plasma
Fast ion profile produced by NNB in JT-60U is more peaked than that of alpha particle in ITER