Space-time resolved spectroscopic system based on a rotating hexahedral mirror for measurement of visible and

ultraviolet spectral line emission

Z.W. WU, B.N. WAN, Q. ZHOU, J. HUANG

Institute of Plasma Physics, Academia Sinica, 230031, Hefei, China

zwwu@ipp.ac.cn

HT-7

ASIPP

AbstractAbstract HT-7

• Using a rotating hexahedral mirror and two monochromators coupling with branchy quartz fiber bundle, a space-time resolved spectroscopic system has been developed on HT-7 Tokamak.

• A center monitoring system has been used to indicate the absolute center of the measurement in order to reduce the error caused by uncertainty of place;

• By using of the asymmetric ABEL inversion, the space-time resolved local emission coefficients of the spectroscopic lines emission have been obtained.

• Simultaneous measurements of two spectral lines emission such as CV~227.1 nm and OV~278.1 nm during a single plasma discharge on the HT-7 Tokamak are presented.

• Compared to traditional multichannel detection system, this system improved measurement efficiency, and reduced uncertainty, which is suitable for impurity transport study.

IntroductionIntroduction HT-7

• Space-time resolved measurement of impurity and global particle radial profile is a crucial issue in the study of impurity and particle transport.[1],[2]

• It is desirable to have the maximum amount of information on a single discharge, thus preventing spurious effects due to plasma irreproducibility.

• This requirement will, however, become a necessity for the next generation of Tokamak devices, because of their high cost of operation and low repetition rate such as superconducting Tokamak.

• Using a rotating hexahedral mirror and two monochromators coupling with branchy quartz fiber bundle, we can realize the space-time resolved measurements of two spectral lines during a single plasma discharge.

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Experimental apparatus HT-7 Experimental apparatus HT-7 superconducting Tokamaksuperconducting Tokamak

R = 1.22 m, a = 0.27 m

Ip = 100~250 kA

BT = 1~2.5 T

ne = 1~8x1013 cm-3

Te = 1~5 KeV

Ti = 0.2~1.5 KeV

Limiter:

graphite limiter with poloidal and toroidal configuration

ICRF:

f = 15~30MHz, P = 0.3MW

f = 30~110MHz, P = 1.5MW

LHCD:

f = 2.45GHz, P = 1.2MW

Main Goal:

Advanced Steady-state operation and related physics

HT-7HT-7 ExperimentalExperimental setupsetup

Space-time resolved Space-time resolved

spectroscopic detection spectroscopic detection systemsystem

PL--plasma, M--rotating hexahedral mirror

HN--He-Ne laser, PD--photodiode detector, L-- collecting lens ( Ø50 mm ), SP1,2--monochromators , AP1,2--amplifiers, PM1,2--photomultipliers, FB--quartz fiber bundle, DAS--data acquisition system, HF--one head of the FB near the lens

For a vertical (poloidal) scan, the L forms an image of HF in the plasma. The rotating mirror (M) repeatedly sweeps the image vertically across the plasma.

The quartz fiber bundle is 2 meters length totally. At the length of 0.5 m, it is divided into two bundles (from AB into A’ and B’). It is composed of 20 branches of quartz fibers with each diameter of 250µm. One head near the collecting lens is arrayed with a 5 × 4 rectangular-like array (AB). Other two heads connecting with the SP1 and the SP2 are arrayed with two 1 × 10 line arrays respectively (A’ and B’).

Special configuration of Special configuration of branchybranchy quartz quartz fiber bundle (enlarged fiber heads array)fiber bundle (enlarged fiber heads array)

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AB -- one head near the lens. A’ and B’ -- other two heads connecting with the monochromators respectively.

Center monitorCenter monitoring systeming system

Firstly setting the wavelength of spectrometer as 632.8 nm, we use a He-Ne laser for incidence from the exit slit center of some one spectrometer and let the laser come out from the incidence slit center. Then we change the angle of rotating mirror by hand until the laser goes through the center of vacuum chamber. After fixing this angle of mirror we adjust the displacement and the direction of the PD relative to the HN until the PD can receive the laser from the HN. Then we fix both of the HN and the PD. The PD is triggered by rising edge of laser pulse. It is quick enough for the time response (ns level). Because of very small optic spot of the HN (about 2 mm of diameter), very small receiving area of the PD (about 3 mm of diameter) and a very short passing time for a very high speed of the rotating mirror, it can be neglected for the difference between the center of receiving area of the PD which correspond to vacuum chamber center and rising edge of the HN pulse. So we can determine the vacuum chamber center by using this way during every cycle time of scan. With help of counting out the rate of passing time of vacuum chamber section and whole cycle time, it is easy to determine the relationship between the signal observed by detector and the emissive region of the plasma.

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He-Ne laser + photodiode detector

Capability and configurationCapability and configuration

1. Rotating hexahedral mirror: six facets with each 80×60 mm2, variable speed with a highest up to 3000 rev. min-1

2. Fiber: branchy quartz fiber bundle (is divided into two bundles at the 0.5 m, total length: 2 meters, fibers: 20 branches, each diameter :250 µm, head array: one of 54, two of 110 )

3. Monochromators (two 0.3-meter plane grating monochromators WDG30 ):

blazing wavelengths: 250 nm and 300 nm respectively.

wavelength resolution: 0.2nm ( 1200 grooves mm-1 and entrance slit 200 µm, exit slit 200 µm )

response of spectrum: 200 - 700 nm.

relative aperture: F/6.

4. Photomultiplier (Model R1463 made in Japan ):

response of spectrum : 200 - 800 nm

5. Entire system response of spectrum : 200 - 700 nm

Space resolution: 1 cm in scan ( poloidal ) direction

Time resolution: 2.7 ms (the highest : 0.5 ms with a 3000 rev. min-1 )

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HT-7Features of the systemFeatures of the system

1). Use of a rotating hexahedral mirror to scan entire plasma poloidal profiles during a single plasma discharge;

2). Use of a simple optical coupling system with a branchy quartz fiber bundle to allow use of two plane grating monochromators capable of simultaneous measurement of two spectral lines emission during a single plasma discharge;

3). Use of an He-Ne laser and a photodiode detector to indicate the absolute center of the measurement in order to reduce the error caused by uncertainty of place;

4). Use of ultraviolet spectrometer and matched detector to extend the working region from visible into ultraviolet

Experimental resultsExperimental results HT-7

Scan in the ohmic heating discharge Ip--plasma current, Ne--chord averaged electron density, CV,OV--chord integrated signals of CV line and OV line respectively, HN--the He-Ne laser signal act as a center monitor.

An expanded time scale from 0.36 s to 0.395 s. It is clearly to show that the period time of scan (τ) is about 18ms. A very prominent feature of these scans is the strong top-bottom asymmetry. It was caused by the B-curvature drift of impurity ions. [3]

By using of the asymmetric ABEL inversion[3], the local emission coefficients of the CV and OV lines emission have been obtained at o.3852 s. It is clearly to show the position of radiation layer.( CV~ 0.7, OV~ 0.85 )

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One such scan in an expanded time scale from 0.383 s to 0.388 s. It indicates the signals with relationship to the center of vacuum chamber and carbon limiter. From that we can get the period time of passing entire plasma (T=2.7ms). And it is also the time resolution.

local emission coefficientslocal emission coefficients

HT-7Space-time resolved local emission Space-time resolved local emission coefficient of the CV line emissioncoefficient of the CV line emission

Space-time resolved chord integrated the CV line emission

Space-time resolved local emission coefficient of the CV line emission

Space-time resolved local emission Space-time resolved local emission coefficient of the OV line emissioncoefficient of the OV line emission

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Space-time resolved chord integrated the OV line emission

Space-time resolved local emission coefficient of the OV line emission

DiscussionDiscussion HT-7

The system described in this report brings a large improvement to visible and ultraviolet spectroscopy of Tokamak plasmas, because it avoids problems due to plasma irreproducibility and because it considerably shortens the time needed to accumulate data. This will be necessity on future large Tokamaks. It has, however, some drawbacks. It is not possible to trigger the system at a desired time in the discharge, since the mirror rotates continuously. The reflectivity of each surface of the rotating hexahedral mirror is slightly different. Before the installation of the system, we calibrated these effects using a tungsten strip lamp , and applied the calibration to the data company with the absolute calibration data before Abel inversion.

ConclusionConclusion HT-7

An optical system for space and time resolved simultaneous measurement of two spectral lines emission during a single plasma discharge has been developed together with the center monitor system on the HT-7 superconducting Tokamak. High-speed rotating hexahedral mirror, branchy quartz fiber bundle, ultraviolet spectrometer and center monitor system endows it with a powerful function. Compared to traditional multichannel detection system, this system improved measurement efficiency, and reduced uncertainty, which is suitable for impurity transport study.

[1]. TFR Group, Phys. Rev. Lett. 36, 1306 ( 1976 ).

[2]. Grove, D. , et al. , in Plasma Physics and Controlled Fusion Research ( Proc. 6th Int. Conf. , Berchtesgarden , W. Germany. 1976 ) paper IAEA-CN-35/A2.

[3].TERRY,J.L.,MARMAR,E.S.,CHEN,K.I.,MOOS,H.W., Phys. Rev. lett. 39 (1977)1615

[4]. Ruan Huailin  and  Wan Baonian, International Journal of Infrared and Millimeter Waves  21 (12)   p.1973-1987  December 2000

HT-7ReferenceReference