# Presentation: 926455

## 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, 2004
- Savannah, 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 arrays

**Basic 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**

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

## “Classical” soft x-ray (SXR) photodiode arrays

**Limitations:**
-      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**

**2/1  + 1/1 **

**E > 0.4 keV (5 μm Be filter)**

**Mid-plane**

**30  -**

**120 -**

**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 arrays

**Current USXR **
**system in NSTX**

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

**Current 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

- 0.3 m Ti and 0.5, 5, 50 and 100 m Be **foils**
**Columnar (crystal growth) CsI:Tl (30 ********m) deposited on a 2mm fiber optic plate (FOP, NA ******** 1).**
- 40 mm in diameter, 1/3” thickness UHV **fiber optic window**  (6” CF).
- Hi throughput, **multi-clad**, non-scintillating, 1.5 m long **fiber optics** (NA  0.7 T  40%).
**Current pre-amplifiers** (104 - 1011 V/A).

**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!**
**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 fiber
- optics (NA  0.7 T  40%).

- PMT aluminum box with:
- 0.05” thick -metal, and 1/4”-2.0 kg iron cylinder and lids for magnetic shielding.
- Delrin-based structure for fiber optics support and alignment.
- 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 - 101019 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) 

**Compactness**  & portability.
- Almost** unrestricted plasma accessibility** (multiple toroidal & poloidal locations).
- The **initial photon statistics is conserved** due to optimized scintillation (**CsI:Tl**) properties.
**Electronics** far away of the machine.
- Less expensive than the diode system **($)**.
- 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) 

- Optically **thin to neutron bombardment**.

- Shot #114022,  0.8 MA, 2-4 MW NBI, 1.01014 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)

**Diode**
**SXR**

**Optical**
**SXR**

**Noise comparison between diode**
**and “optical” SXR arrays**

- NBI

## Benefits of the “OSXR” system (III) 

- The **time response of the OSXR system** is comparable to that of the AXUV photodiodes (**2-4 ********s**).

**Fast ELM measured with H****up**** diode array**

- Chord#

- t (ms)

- t (ms)

**Diode**
**SXR**

**Optical**
**SXR**

- 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 H8711
- Multi-anode
-  TTS (FWHM)  0.4 ns

- +

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

**Optimization**

**OSXR array**
** ****head  tested on NSTX**

**NSTX (2005)**

- Scintillator
- Reflective coating
- Fiber optic plates (FOP)
- Filter transmission.
- In vacuum fiber optics
- Fiber optic window (FOW)
- Atmosphere fiber optics
- PMT and/or APD
- Trans-impedance amplifier
- 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?

**M****u****lt****i****-****c****ol****or**** (****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 ****T****e**** 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

**Compare**
**with**
**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

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

***Mid-plane tangential reconstruction***

***Emissivity***

***Brightness***

**multi-energy**
**cut**

- m

- m

## Poloidal multi-color OSXR for NSTX

- 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:
- 0.05” thick -metal, and 1/4”-2.0 kg iron cylinder and lids for magnetic shielding.
- Delrin-based structure for fiber optics support and alignment.
- Attached electronics with PMTs, trans-impedance amplifiers and RG-58 coaxial cables.

- Hi throughput, multi-clad,
- non-scintillating, 1.5 m long fiber
- optics (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)

- 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 test

- k x-rays from Manson SXR source

**CsI:Tl visible **
**550 nm **
**green light**

- CsI:Tl visible 550 nm
- green light

- Epoxied

- Fused

**Assembled**
**fiber optics**

## In-vacuum fiber optic test (II)

**Manson source**

- 90o

***Anodes***

**k******** x-ray**
**beam 1**

**Beam 2**

- Point-like **Al-k******
- (1486 eV) emissivity: 51010 photons/sts

**2-3/4” **
**flange**

**BNC **
**ceramic break**

**k******** SXR lines**

**5 ********m Be**
**foil**

**Filter**
**CsI:Tl**
**FOP**

**AXUV**
**100**

- We need to determine first how much visible
- light we can collect from the conversion of
**Al k******** (8.35 Å)** into green (550 nm) light from **CsI:Tl**

- Experiment with Be filter,
-     and AXUV-100
-      ****** ****I****x-rays**** = 79 nA**
- Experiment with Be filter, **CsI:Tl + FOP** and AXUV-100 ****** I****visible**** = 4.1 nA ****(next slide)**

- 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**

**Filter**
**CsI:Tl**
**FOP**

- (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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