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