Development of ‘multi-color’ optical soft X-ray arrays for MHD and transport diagnostics in

fusion experiments

L. F. Delgado-Aparicio, D. Stutman, K. Tritz, G. Suliman and M. Finkenthal

The Plasma Spectroscopy Group

The Johns Hopkins University

R. Kaita, L. Roquemore and D. Johnson

Princeton Plasma Physics Laboratory

American Physical Society (APS), Division in Plasma Physics (DPP),

November, 15 - 19, 2004Savannah, GA, USA

Abstract

We are developing fast ( 100kHz), “multi-color” (multi-energy) optical soft x-ray arrays as an improved alternative to conventional photodiode arrays for MHD and transport diagnostic. Tests of a “single-color” array on NSTX show that the optical device has SNR and immunity to noise, superior to those of conventional diode arrays, while having similarly fast time response [1]. In addition, the optical design enables improved plasma access and spatial coverage at reduced cost, making possible large channel-count, “multi-color” tomographic systems. Due to the “layered” structure of the plasma emissivity, such systems would enable simultaneous imaging from the plasma edge to the core. A re-entrant “multi-color” OSXR array will be prototyped on NSTX, where it will also augment the coverage of the existing diode system. The optimization of this diagnostic is discussed and applications of the “multi-color” technique illustrated with NSTX results and numerical simulations.

[1] Luis F. Delgado-Aparicio, et al., Rev. Sci. Instrum, 75, 4020, (2004).

“Classical” soft x-ray (SXR) photodiode arraysBasic diagnostic:• A SXR system (based on filters & photodiode arrays) does 2D tomographic reconstructions in a well defined (and limited) energy range, mainly for diagnostics of core and edge MHD activity.

Current USXR system in NSTX

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Energy (eV)

Transmission

Ti, th=0.3 microns

Be, th=5 microns

Be, th=50 microns

Why only at a well defined & limited energy range? FILTER’s TRANSMISSION

“Classical” soft x-ray (SXR) photodiode arraysLimitations:1. Due to “layered” structure of the plasma emissivity as a function of energy, important core perturbations are “hidden” by strong peripheral edge emission need to peel off the peripheral contribution (onion model)!

Current USXR system in NSTX

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2/1 + 1/1

E > 0.4 keV (5 µm Be filter)

Mid-plane30 -

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Z (cm)

1/1

t(ms)

-120 -

- 30 -

m/n modes

E > 1.4 keV (100 µm Be filter)

“Classical” soft x-ray (SXR) photodiode arraysCurrent USXR system in NSTX

Limitations:2. Tomographic reconstruction of complex structures requires a very large # of integration chords (300), therefore, such an array would be prohibitively expensive.

3. Lack of plasma access due to a tight fitting vacuum vessel, access to ports, port space and geometry, and # of ports become an engineering issue.

4. In-vacuum diode SXR cameras have to be designed accordingly, in order to sustain elevated bake-out temperatures (300 oC) and UHV conditions (10-7 torr).

5. The NBI-high neutron flux induced noise and fast electromagnetic activity (or current ramp up/down), impair the silicon-based diode arrays with transients “triangularly shaped peaks” and permanent defects.

Classical” soft x-ray (SXR) photodiode arraysCurrent USXR system in NSTX

Solution: In order to solve the above problems we proposed to REPLACE the filtered diode system with a multi-color (multi-energy) re-entrant (in-vacuum) “optical” SXR system.

Such system enables simultaneous time, space and energy resolved ‘multi-color’ tomographic measurements.

Concepts to have in mind:Multi-color: technique with different filters (or cutoff energies) so MHD phenomena from the core to the peripheral edge can be simultaneously recorded in time using the SAME integration chords (observed from the same toroidal locations, for instance).

Re-entrant: system witch satisfies bake-out temperatures (300 oC) and UHV (10-7 torr) requirements.

“Optical”: system that uses columnar (crystal growth procedure) thallium doped cesium iodine thallium (CsI:Tl) to efficiently convert SXR photons to visible light!

What is an “optical” soft x-ray (OSXR) array system?

OSXR array head tested on CDX-U & NSTX spherical

tokamaks [1]

It’s a system that uses a fast (1 s) and efficient scintillator (CsI:Tl) in order to convert soft x-ray photons (0.1<Eph<6 keV) to visible green light (550 nm).

[1] Luis F. Delgado-Aparicio, et al., Rev. Sci. Instrum, 75, 4020, (2004).

CsI:Tl deposited on a FOP

Vacuum side

Air side

Components of the “optical” SXR Array

1. 0.3 m Ti and 0.5, 5, 50 and 100 m Be foils

2. Columnar (crystal growth) CsI:Tl (30 m) deposited on a 2mm fiber optic plate (FOP, NA 1).

3. 40 mm in diameter, 1/3” thickness UHV fiber optic window (6” CF).

4. Hi throughput, multi-clad, non-scintillating, 1.5 m long fiber optics (NA 0.7 T 40%).

5. Current pre-amplifiers (104 - 1011 V/A).

6. Current detector: Bi-alkali (Sb-Rb-Cs, Sb-K-Cs), multi-anode, low cross talk, high gain ( 106) Hamamatsu photo-multiplier tube (PMT). Magnetic shielding is necessary!

7. Future candidate: Advanced Photonix’s large area avalanche photodiode (APD). One channel, 5 mm in diameter, modified, hi quantum efficiency ( 90%), high internal gain ( 300), internally amplified, cooled (-20 to 0 oC) module.

NOTE: NaI (hygroscopic scintillator) has been used for 1D ion heating studies in a magnetic reconnection current sheet (N. Ohyabu, PoF, 17, 2009, 1974).

“Optical” array installation in NSTX (PPPL)

Fiber optic window, 5 m Be filter and CsI:Tl scintillator, mounted on an 8” gate valve.

Hi throughput, multi-clad,non-scintillating, 1.5 m long fiberoptics (NA 0.7 T 40%).

PMT aluminum box with:a) 0.05” thick -metal, and 1/4”-

2.0 kg iron cylinder and lids for magnetic shielding.

b) Delrin-based structure for fiber optics support and alignment.

c) Attached electronics.

Results from NSTX (PPPL)

• Ro= 85 cm

• a = 68 cm

• A = Ro/a ~ 1.25

• BT(0) = 0.3 - 0.6 T

• Ip < 1.5 MA

• (elongation) • discharge 1 s• Te(0) ≤ 1.5 keV• ne(0) 1 - 101019 m-3

• PNB 1 - 6 MW• PHHFW 1 MW

The “OSXR” array have been compared with the present SXR

arrays used for tomographic

reconstruction and MHD studies.

Test of a “single-color” array in NSTX (PPPL)

MHD results with the “OSXR” shown at High Temperature Plasma Diagnostics meeting (San Diego, 2004) & Rev. Sci. Instrum, 75,

4020, (2004).

Shot #: 112069 Ohmic Shot Shot #: 112036 H-mode

Benefits of the “OSXR” system (I)

1. Compactness & portability.

2. Almost unrestricted plasma accessibility (multiple toroidal & poloidal locations).

3. The initial photon statistics is conserved due to optimized scintillation (CsI:Tl) properties.

4. Electronics far away of the machine.

5. Less expensive than the diode system ($).

6. The gain of the system can be remotely changed by the use of a PMT and/or an APD.

Benefits of the “OSXR” system (II)

7. Optically thin to neutron bombardment.

Sho

t #11

4022

, 0

.8 M

A, 2

-4 M

W N

BI,

1.0

10

14 n

/s

The NB-induced noise in the Si-based photodiode system seem to be due to elastic and inelastic scattering of 2.5 MeV DD neutrons in the photodiode’s silicon lattice.

NSTX Shot # 114022 (2 - 4 MW NBI)

DiodeSXR

OpticalSXR

Noise comparison between diodeand “optical” SXR arrays

NBI

Benefits of the “OSXR” system (III)

8. The time response of the OSXR system is comparable to that of the AXUV photodiodes (2-4 s).Fast ELM measured with Hup diode array

Chord#

t (ms)

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t (ms)

DiodeSXR

OpticalSXR

The time difference from peak intensities is approximately 50 s, and only indicates that the mode passed in front of the detectors in its trajectory from the bottom divertor to NSTX's midplane.

Gamp = 107 V/A (400 kHz)

Remote controlled Femto current pre-amplifier

Hamamatsu H8711Multi-anode

TTS (FWHM) 0.4 ns

+

What is a re-entrant (in vacuum) “optical” SXR array?

OptimizationOSXR array

head tested on NSTXNSTX (2005)

1. Scintillator

2. Reflective coating

3. Fiber optic plates (FOP)

4. Filter transmission.

5. In vacuum fiber optics

6. Fiber optic window (FOW)

7. Atmosphere fiber optics

8. PMT and/or APD9. Trans-impedance

amplifier10. DAQs

It’s a system based on the OSXR concept with an optimized plasma access. The so-called “re-entrant” system will have the “FOP+scintillator” combo located INSIDE the tokamak, and the extraction of light will be done with a 1-D “ribbon” of UHV compatible fiber optic bundles!

What is a re-entrant “multi-color” OSXR system?

OSXR array head tested on NSTX

NSTX (2005)

It’s a system based on the OSXR concept with an optimized plasma access and the capability of recording MHD phenomena from the core to the periphery SIMULTANEOUSLY!

What can a re-entrant multi-color OSXR can do?

• Multi-color (2-3 color) tomographic reconstructions.

• Time & space dependent transport measurements of intrinsic (e-, ions and C) and injected (neon) impurities; including SGI.

• MHD mode recognition (ELMs & RWMs) and their effects on core & edge plasma parameters.

• ELMs characterization & perturbative effects (see K. Tritz, et al., poster JP1.031).

• ‘Two -color’ modeling of Te perturbations during MHD events (NEXT SLIDE).

“Two -color” modeling of Te perturbations

during MHD events

• The ratio of low to high energy USXR signals is Te sensitive (MPTS)• USXR profiles modeled using C, O and B coronal equilibrium radiative coefficients.

Chord #t (ms)

Te (

keV

)

R (cm)

How sensitive is this method ?

First assessment (proof of principle, using a type-I ELM)Using the diode-based SXR system in NSTX!

MPTS

USXR

Comparewith

MPTS

The correct application of the ‘multi-color’ technique requires imaging the SAME plasma region, rather than piecing together images from different plasma regions at

different energies.

METHOD

Tangential multi-color OSXR for NSTX

γ(Te) =1.0+29.0exp−2.75−1.1Te

−0.4(keV)( )2

{ }

Enhancement of radiation over bremsstrahlung due to free-bound (recombination) radiation; caused predominately by

highly ionized carbon.

εph(cm−3s−1) =1hνEcut

∞

∫dPd(hν)

d(hν)

=3×1011γZeffne

2(1013cm−3)Te(keV)

E1

EcutkBTe

⎛

⎝ ⎜ ⎜

⎞

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dPff+fb

d(hν)(cm−3s−1) =3×1011γ(Te,Z)Zeff

ne2(1013cm−3)Te(keV)

exp−hνkBTe

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

Mid-plane tangential reconstruction

bph Rtg( )= εph∫ ds= εphRtg

R0 +a

∫ (RP −R0)2RpdRpRp −R

tg

2

Emissivity

Brightness

multi-energycut

m

m

16 to 48 channels of a multi-color (2 or 3 cutoff filters) OSXR array, mechanically attached and optically coupled to FOP & FOW.

PMT aluminum box with:a) 0.05” thick -metal, and 1/4”-2.0

kg iron cylinder and lids for magnetic shielding.

b) Delrin-based structure for fiber optics support and alignment.

c) Attached electronics with PMTs, trans-impedance amplifiers and RG-58 coaxial cables.

Poloidal multi-color OSXR for NSTX

Hi throughput, multi-clad,non-scintillating, 1.5 m long fiberoptics (NA 0.7 T 40%).

Components• 30 m CsI:Tl (RMD, Inc)

deposition on a 2 mm FOP (Collimated Holes, Inc).

• Ti and Be filters (Lebow, Inc)• Vacuum compatible fiber optic

bundle (L=0.5 - 2.1 m) with epoxied NA=0.64, 50 m individual fiber optics (Collimated Holes, Inc).

• Fiber optic window (XRSINC)

• 1.5 m, NA=0.74, fiber optic canes (Bicron - Saint Gobain).

• AXUV (IRD, Inc) and/or PMT (Hamamatsu)

In-vacuum fiber optic testk x-rays from Manson SXR source

CsI:Tl visible 550 nm

green light

CsI:Tl visible 550 nm green light

Epoxied Fused

Assembledfiber optics

In-vacuum fiber optic test (II)

Manson source

90o

Anodes

k x-raybeam 1

Beam 2

Point-like Al-k (1486 eV) emissivity: 51010 photons/sts

2-3/4” flangeBNC

ceramic break

k SXR lines5 m Befoil

FilterCsI:TlFOP

AXUV100

We need to determine first how much visiblelight we can collect from the conversion of

Al k (8.35 Å) into green (550 nm) light from CsI:Tl

a) Experiment with Be filter, and AXUV-100 Ix-rays = 79 nA

b) Experiment with Be filter, CsI:Tl + FOP and AXUV-100 Ivisible = 4.1 nA (next slide)

CECsI:Tl =RΩ

CEx

CEv⋅Iv(nA)I x(nA)

CECsI:Tl(E=1486 eV) 37 26/keV

CsI:Tl conversion efficiency check

10 mm

In-vacuum fiber optic test (III)

FO Bundle

(Collimated Holes, Inc.)

FO bundle active area

SS ferrule

Fiber optic window

FilterCsI:TlFOP

(to be completed with experiments at Johns Hopkins)

Conclusions• A single-color “OSXR” array has been successfully tested on NSTX.

• The time response of the system is in the order of 2 - 5 s, thus the system would be able to be sampled at frequencies in the order of 100 - 400 kHz.

• The NB-induced noise in the diode system seems to be due to elastic and inelastic scattering of 2.5 MeV DD neutrons in the photo-diode’s silicon lattice.

• The “optical” SXR array appears to be “thin” to neutron bombardment in comparison to the photo-diode (silicon) based arrays.

• A re-entrant ‘multi-color’ OSXR array has been proposed to replace the photo-diode based USXR system in NSTX and thus, correctly apply the ‘multi-color’ technique for MHD mode recognition and transport measurements.

• Tests of in-vacuum fiber optic bundles have been done at the JHU laboratories for NSTX and NCSX applications.

Current applications• Comparison between PMT and APD technologies. Time response and sensitivity for

UV/visible light. Test with columnar CsI:Tl (0.05 - 1 MHz).

• Development of optical array for turbulent SXR measurements (see G. Suliman, et. al. poster BP1.088).

• RWMs, ELMs and neutron impact studies. Correlation between the “optical” array (NSTX Bay J) and the diode arrays (NSTX Bay G).

Future plans• Test of new fast scintillator (LaCl3:Ce) for MFE applications. (1 - 10 MHz system)

• Investigation of x-ray tangential imaging for current profile reconstructions.

• Construction of multi-color x-ray toroidal and poloidal arrays.

• Development of compact in-vacuum SXR cameras for future devices like NCSX

Acknowledgments

• The Johns Hopkins University: Scott Spangler and Steve Patterson.

• Princeton Plasma Physics Laboratory: Robert Majeski, Jeff Spaleta, Tim Gray, Jim Taylor, John Timberlake (CDX-U). James Kukon, Brent Stratton, Joe Winston and Bill Blanchard (NSTX).

• This work was supported by The Department of Energy (DOE) grant No. DE-FG02-86ER52314ATDOE

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