Initial Results from the Scintillator Fast Lost Ion Probe
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Transcript of Initial Results from the Scintillator Fast Lost Ion Probe
Initial Results from the Scintillator Fast Lost Ion Probe
D. Darrow
NSTX Physics Meeting
February 28, 2005
Goal & Motivations
Goal:– Predict fast ion losses from ST plasmas
Motivations:– Dimensionless parameters of beam ions similar to
3.5 MeV s in NSST (good model system)– Lost beam ion characteristics can reveal internal
physics, esp. effects of MHD instabilities
Outline
• Loss mechanisms
• sFLIP diagnostic
• Example data
• Parametric dependence of loss
• Model of detector signal
Fast ion loss mechanisms
• Prompt orbit loss: fast ion born in loss cone• Radial transport to wall (P):
– MHD– TF ripple
• Pitch angle scattering into loss cone ():– Classical collisions– ICRF heating
This work: mainly prompt loss
• Prompt loss increases with:– decreasing Ip
– decreasing outer
– decreasing Rtan
• NSTX: 80–90 keV D NBI– A: Rtan = 69.4 cm
– B: Rtan = 59.2 cm
– C: Rtan = 48.7 cm
0
20
40
60
80
100
0 200 400 600 800 1000Plasma current (kA)
outer
=3.8 cm
outer
=14.4 cm
Scintillator fast lost ion (sFLIP) probe is magnetic spectrometer
• Combination of B and aperture geometry disperse different pitch angles and energies on scintillator plate
Scintillator detector: principle of operation
Bay J
Vessel & limiters
NSTX Midplane
ScintillatorDetectorBeam C footprint
Scintillator probe assembly
Aperture
Light shield
Graphitearmor
Base &Heat sink
Scintillator(inside)
Plasma
Vacuumwindow
Bay J
Incidentions
: 5–60 cm, : 10°–70° (typ.)
Typical orbit to detector
• Commonly only a few steps contribute in each orbit
• Model includes full 3D structure of vessel & beam deposition
& map can be applied to data
NSTX sFLIP diagnostic, shot 111192, frame 5, with grid for t=169 msB=0.2692 T, rho=22 cm, E_D=84 keV, chi=23°, 32°, & 60°
Fiber optic bundle limits resolution of fast ion parameters
• Limited resolution of bundle (50 x 50) causes discretization of image & uncertainty in scintillator position in camera field of view
CCD Camera
Scintillator
Fiber bundle
Single fiber
Position calibration image of scintillator
Instrumental “line widths” also set limit on resolution
• Example case: 80 keV (=24 cm) FWHM is =8 cm
• Pitch angle line width: 6° FWHM
Beam ion loss clearly seen
112132: 800 kA, 4 MW
QuickTime™ and a decompressor
are needed to see this picture.
Higher
Lower
Lower Higher
30 frames/s= pitch angle
= tan-1(v||/v)
Several general classes of loss seen
• Few cases analyzed so far, but all consistent loss at injection energy (prompt loss)
“Bar” loss: wide range
Typically early in NBI:
low ne & deeper dep’n
(113002, 330 ms)
High loss
Typ. later in NBI: high ne
Often modulated by MHD
(112232, 400 ms)
Multiple discrete s
(111130)
Methodology of prompt loss investigation
• Compare losses from 112164 (source A only) & 112166 (source C only) to determine effect of Rtan (nominally identical shots)
• Compare different time slices within each shot to determine effect of Ip on loss, since beam injection starts during Ip ramp up
Parameters for 112164, 112166
112164: A
112166: C
Measurements show loss decreases as Ip increases
99 ms, 500 kA
116 ms, 650 kA
149 ms, 750 kA
112164
Source A
90 keV
More loss seen from source C than A under same conditions
112164 (A)–top vs 112166 (C )–bottom
100 ms 115 ms 150 ms
Are these prompt losses?
• If so, then:– Detected energy must equal injection energy– Detected pitch angle must correspond to an orbit
populated directly by the beam deposition
Gyroradius range appears consistent with loss at Einj
• 90 keV D, 0.25 T => =25 cm• Scintillator image position calib. injects uncertainty
10°
20°30°
40°50° 60°
5
10
15
20
Pitch Angle ()
Gyr
orad
ius
cent
roid
(cm
)
112164, 100 ms 112166, 150 ms
10°
20°30°
50°60°
5
10
15
20
Pitch Angle ()
Gyr
orad
ius
cent
roid
(cm
)
40° 70°
Detector signal modeling for range of detected
• Need efficient method to compare volume of phase space sampled by detector with volumes populated through beam injection
• “Constants of Motion” (COM) approach: orbit fully characterized by E, (=mvperp
2/2B), & P (=mvR+qpol)
• For prompt loss, where E does not change, problem is 2D: plot beam deposition & detected orbits in (P, ) and look for overlap
• But, conservation marginal in STs!
COM model (cont’d)
• Treat beam as ensemble of test particles deposited in 3D volume where beam passes through plasma – all velocities parallel to beam axis– ~100,000 particles typically
• Model detected ions as 2D fan of velocities at detector entrance aperture– ~100 velocities, ~1° steps in
• Plot both sets in same (P, ) space for Einj, look for overlap
Example case
• Clear overlap seen between deposited beam orbits and orbits sampled by sFLIP
• Predicts loss at detector, =20° to 54°
P (10-20 kg m2/s)
(1
0-14
J/T
)
112166, 100 ms, 500 kA, source A, 90 keV
Beam ions
sFLIP
Ra
nge
of
p
red
icte
d a
t d
ete
cto
r
Model in reasonable agreement with measured range
• Model predicts =22.7 cm, 10°≤≤35°
• Measured spot is extended due to finite aperture size, but is consistent with model &
10°
20°30°
40°50°60°
70°
5
10
15
20
Pitch Angle ()
Gyr
orad
ius
cent
roid
(cm
)
Model
112166, 150 ms, source C
Model reproduces observed differences between A & C
• C fills low orbits at detector (t>100 ms); A does not
112164: source A
97 ms 139 ms 169 ms
100 ms 140 ms 170 ms
112166: source C P
Bright, high loss often observed during MHD
• Lost at injection energy, =64°
• Prompt loss model: 48°–63°
• Loss appears too localized in to be consistent with prompt loss
10°
20°30°40°50° 60°
5
10
15
20
Pitch Angle ()
Gyr
orad
ius
cent
roid
(cm
)
Model
70° 80°
112074, 400 ms, sources A, B, & C
(with Fredrickson, Medley)
MHD-lost ions are banana orbits, near P/T boundary
• P/T boundary at 60°
• Bounce frequency changes rapidly with here–87 kHz for this orbit
PPPL Lorentz ORBIT v205 for NSTX 02/08/16 11:10:00ORBIT(S) CALCULATED ON 02/03/05Comment Orbits to sFLIP in 112074, t=403 msORBIT STRUCK LIMITER AT R= 1.502 Ph= -83.4 Z= .465
3.1416 Init gyro ang.0000 Init pitch ang1.60 Detector R-.14 Detector Z
1.33046 Detector phi1.3744 Det RZ ang
-1.5708 Det tor ang1.00 Ion charge2.00 Ion mass
9.000E-02 Init energy0.000E+00 Init mu
0 Time dir'n60/ 81 IX/NX1/ 1 IY/NY.01 Step len (m)
100.00 Orbit len.0616 Aper sep (m).0060 Col hwidth.0010 Col hheight.0463 Det hwidth.0010 Det hheight
GYRO = .3749CDPTCH= .0000RIPTCH= 115.80RMN = .00EFFIC =9.757E-09SUMEFF=3.590E-08
0.5 1.0 1.5
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
-1 01
-1
0
1
Summary
• sFLIP diagnostic now measuring beam ion loss routinely
• Beam ion loss parametric dependence, gyroradius, & pitch angles match prompt orbit loss
• (P, ) mapping provides fast calculation of prompt loss pitch angles at detector
• MHD-induced loss seen near P/T boundary
Future plans
• Make absolute calibration of loss rate with internal Faraday cups
• Higher resolution fiber bundle (?)
• Augment model to include orbit class boundaries, loss boundary
• Investigate loss at high rotation speed