Multipoint Thomson scattering system for the extrap Z-pinch experiment

391

Nuclear Instruments and Methods in Physics Research A257 (1987) 391-397 North-Holland, Amsterdam

MULTIPOINT THOMSON SCATTERING SYSTEM FOR THE EXTRAP Z-PINCH EXPERIMENT Per KARLSSON

Royal Institute of Technology, S-100 44 Stockholm, Sweden

Received 22 January 1986 A Thomson scattering system for simultanous measurements of the electron temperature and density at three different positions and at two different times during a single plasma shot has been developed for a Z-pinch experiment. The plasma in the pinch is characterized by densities in the range from 5 x 10 Z° to 10 22 m-3, temperatures up to 30 eV and a pinch radius of the order of 1 cm . The spatial resolution of three points with 3 mm between points is obtained by imaging the plasma onto a matrix of quartz optical fibres at the output slit of the spectrometer . Thus only one spectrometer is used together with 15 PM-tubes to detect the scattered radiation as well as the background radiation. Due to the relatively dense plasma prevailing in this Z-pinch discharge the number of scattered photons is large and the photon to electron conversion noise is small. Therefore the background radiation is the most important factor limiting the accuracy of the measurements.

1. Introduction A Thomson scattering system has been developed to perform measurements of the electron temperature and density on the EXTRAP-Ll device [1-3]. EXTRAP-Ll is a linear Z-pinch stabilised by an external octupole field generated by four conductor rods where the currents run antiparallel to the plasma current. The EXTRAP Ll device has been used as a test device where the effects of changes in the configuration have been tested. These configuration changes have affected the operating conditions for the Thomson scattering diagnostics. The plasma parameter range is characterized by densities in the range 5 X 10 2° -1022 m -3 and temperatures from a few eV up to 30 eV . In this parameter regime it is justified to assume ion and electron temperature to be equal. The pinch diameter is of the order of 1 cm and the duration of the discharge is 80 lis. The configuration changes have also affected the other aspects of the diagnostics including stray laser light levels and plasma light levels from impurities . A particular configuration change affecting these light levels that will be discussed when describing the performance of the Thomson scattering system, is the presence of a glass discharge tube placed inside the rod conductors. A schematic cross section of the device is shown in fig. 1. The present Thomson scattering system is capable of simultaneous measurements at three points across the pinch diameter at one axial position. The laser can be operated in a double pulse mode so measurements can be made at two different times during the discharge. Spatially resolved Thomson scattering is usually achieved by 2 different methods: The first involves one 0168-9002/87/$03 .50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

spectrometer and 2-3 PM-tubes for each spatial position [4] while the second method means using one spectrometer and a microchannel plate [5] or an intensified charge coupled device [6] as a detector. Our scattering system obtains a 3 point resolution with one spectrometer and 15 PMTS with an array of quartz optical fibres at the output of the spectrometer. For the low temperature plasmas that are obtained in the pinch experiment the nonrelativistic expression

Supporting Frame (Fibre Glass Epoxy) 6cm spocing

10cm

Fig. 1. A cross section of the vacuum chamber.

P. Karlsson / Multipoint Thomson scattering system

39 2

for the Thomson scattering of the laser light is adequate N»J ss) da s dSl =

ró Lcn sinz~ 2Fv,h

xexp

sin(

-

2) Al c(Xs- ~'J

2v a , X 1 sin(

z da s dQ,

where N1 is the number of laser photons in a pulse, NS is the number of scattered photons/(solid angle and wavelength), L is the distance observed along the laser beam, ro is the classical electron radius (= 2 .82 x 10 -1s m), tP is the angle between the electric field of the laser radiation and the direction of the scattered radiation, c is the velocity of light, B is the angle between the laser radiation and the detected radiation, u n , _ (2kT/me)l/z is the electron thermal velocity, k is the Boltzmann constant, m e is the electron mass, al is the laser wavelength, Sl is the solid angle of detection and A s is the wavelength of the scattered radiation . SI-units are used throughout this paper unless otherwise stated . The geometry of our Thomson scattering is outlined in fig . 2. The laser beam enters the vacuum horizontally and is focused on to the plasma volume perpendicular to the pinch axis. The electric field vector of the laser radiation is parallel to the pinch axis and thus we have xp = 90 ° . The radius of the pinch is denoted by a and the laser beam can be approximated by a cylinder of diameter d during the passage of the pinch . The scattered photons are collected at the angle B to the laser beam and over the solid angle A92 of the collection optics . In our case we have dSl = 0.01 sr and, due to the mechanical construction of the vacuum vessel, B = 112.5 ° . The image of the entrance slit of the spectrograph, projected on to the horizontal plane, is a rectangle of length L along the laser beam and a width d z along the pinch axis . It is assumed that 4 z < d . The standard values of L and Az are 10 mm and 2 mm respectively, but they can be reduced by a factor 2 in the the present setup. The plasma scattering volume is of the order of L dzd = 2 x 10 -7 m3 . The total number of scattered photons collected by the detection optics and transmitted to the PM-tubes, provided that the detectors cover the spectral range of the scattered radiation, is Nswc = N1nLrp 411T,

(2)

where z is the transmission of the detection optics including reflection losses from the lenses, the transmitance of the spectrograph and the packing fraction of the fibre bundle and the fibre matrix . If the number of PM-tubes is denoted by m each with a quantum ef-

Fig. 2 . The geometry of the Thomson scattering .

ficiency rl, then the average number of photoelectrons for one PM-tube becomes Ne = NSI°tr1/m . With typical parameter values N1 =1019, n = 1021 m-3, L = 0 .01 m, d Sl = 0 .01 sr, z = 0.03, 11 = 0 .06 and m =15 we obtain Ne =1 .7 x 10 3 . Since Ne is Poisson distributed the signal to noise ratio due to photon-electron conversion statistics is Ne . In our case Ne = 41 which means that the noise due to photon-electron conversion statistics is not expected to be a problem, due to the relatively cold and dense plasma in EXTRAP .

2. Experimental setup The experimental setup is shown in fig . 3 . First we have the laser input system consisting of the laser, input optics and stray light climination system . Then comes the detection system consisting of detection optics, spectrograph, detectors and data recording system . 2 .1 . Laser input system A QUANTEL double pulse ruby laser is used giving 10 J in a single pulse or 2 x 7 J when operated in the double pulse mode . The pulse duration is 20 ns (fwhm) and the divergence of the laser beam is less than 1 mrad . The interval between the pulses in the double pulse mode can be set in the range 2-500 Its . In fig . 3 we can follow the laser beam from the laser to the pinch. The laser beam has a diameter of 16 mm when leaving the laser and is horizontally polarized. It undergoes a horizontal deflection by mirror Ml and is then

393

P. Karlsson / Multipoint Thomson scattering system

Fig. 3. Overview of the experimental setup of the Thomson scattering system. Ml-M3 are high energy laser mirrors, Ll is a plano-concave lens f = - 200 mm, L2 is a plano convex lens f = 300 mm, L3 is a plano convex lens f =1000 mm, L4 is a camera lens f = 85 mm/1 .8, L5 is a camera lens f = 50 mm/1 .4, L6 is a camera lens f = 58 mm/1 .8, LD is the laser dump, VD is the viewing dump, P is a sheet polarizer, FM is the fibre matrix, PM(1)-PM(15) are photomultiplier tubes, OF is a 400 Am quartz optical fibre and PD is a photodiode . magnified to a diameter of 24 mm by the lenses Ll and L2 before being deflected by mirrors M2 and M3 . Although the average power density of the laser beam is almost one order of magnitude smaller than the damage threshold of the mirrors, 3 GW/cm2 for s-plane reflection, it is unevenly distributed over the beam cross section and could locally be over 1 GW/cm2. The magnification of the beam reduces the power density by a factor of 2 thus increasing the safety margin of mirror destruction. Since the damage threshold for p-plane reflection is twice that of the s-plane reflection no magnification is needed before Ml . After M2 and M3 the beam is focused with L4 down to a diameter of 2 mm inside the pinch. The input quartz window is orientated at the Brewster angle to minimize reflection losses . To reduce the stray light coming from the scattering of the laser light on the input window a set of blue glass apertures is inserted between the input window and the pinch. This will prevent any incident radiation from hitting the wall of the vacuum chamber or the current rods. The beam dump consists of a vacuum quartz window and a blue glass plate orientated at the Brewster angle. A further reduction of stray light entering the detection optics is obtained by installing a viewing dump opposite the collection lens as shown in fig. 4. The design of the viewing dump has been taken from ref. [8].

used to transport the scattered light from the experimental hall out to an adjacent room where the detectors are protected from electromagnetic disturbances . The fibre bundle has an overall transmission of 50% and its ends have a rectangular shape, 1 x 5 mm2 to fit the entrance slit of the spectrometer. The end of the fibre bundle is imaged onto the pinch with a camera lens, f = 85 mm/1 .8, to ensure low aberration . The lens and the fibre bundle are mounted inside a light tight, blackened brass tube . This arrangement permits a magnification of the fibre bundle end on to the plasma with a factor ranging from 1.35 to 3.0 . The geometry of the vacuum vessel and the 4 current rods that create the

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2.2. Detection system A schematic view of the detection system is shown in fig. 3. A coherent quartz fiber bundle of 25 m length is

Fig. 4. The viewing dump with an enlarged drawing showing the details of the profile.

P. Karlsson / Multipoint Thomson scattering system

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octupole field determine the angle of collection and the solid angle of detection, 0 =112 . 5 ° and A 92 = 0.07 sr . The other end of the fiber bundle is imaged on to the entrance slit of the spectrometer by L5 . The spectral dispersion is provided by a Monospek 1000, 1 m, f110, plane grating spectrometer (Hilger and Watts). A 1200 1/mm grating is used with a blaze wavelength of 5000 A which give a dispersion of 8.2 A/ mm. The input slit the corresponding etendue or and is 2 x 10 mm2 throughput is TP = 0.19 mm2 sr. This is the lowest throughput of any part of the detection system thus setting an upper limit of the radiant flux in the system . Particularly it sets a limit of the solid angle AQmax over which the plasma light is collected;

where App = the area of the image of the fibre bundle onto the plasma . With the standard magnification of a factor two of the fibre bundle end onto the plasma we get dgmax = 0.01. The output slit of the spectrograph is imaged onto an array of 3 rows and 5 columns of optical quartz fibres each connected to a PM-tube. The fibres are 2 m long and have a core of 1 mm diameter coated with a 0.1 mm thick cladding . The imaging element is a f= 56 mm/ 1.6 camera lens and images the output slit on to the fibre array with an inverse magnification of 2 :1 . This arrangement permits a simultaneous measurement at 5 different wavelengths with a wavelength width of 25 t1 and at three different points across the pinch with a separation of 3.3 mm as seen in fig. 5 . The stray light rejection ratio between two adjacent channels is measured to be 3 x 10 ° . The PM-tubes (Hammamatsu) employed are of the side on, multialkali cathode type with a cathode area large enough to capture the light emitted from the fibre without collection optics . We have employed two versions of the same PM-tube which differ regarding the cathode sensitivity and current amplification. R 928 has a quantum efficiency of 5% at 7000 Á and R 1477 has

E

0 000 (D 00000 O 6Ä1

F~

0,00

00 25Ä

Fig . 5 . A schematic drawing of the fibre matrix showing the spectral dispersion and the spatial resolution.

8%, while the current amplification is typically 10 7 for R 928 and 5 x 10 6 for R 1477 . The former is used where large proton flows are expected, i.e . the wavelength channels nearer to the laser wavelength and the latter is used where small photon fluxes are expected in order to improve the SNR due to the quantum statistical noise. 3. Data recording and evaluation 3.1 . Calibration and stray light A relative calibration of the channels was obtained by pulsing a light diode (Stanley 2K) having a maximum emission at 6600 Á with 50 ns pulses . The LED was placed in front of the plasma end of the fibre bundle and the spectrograph was adjusted so that every channel detected radiation at the same wavelength, 6800 A. A relative calibration is sufficient for determination of the electron temperature . In order to measure the plasma density an absolute calibration is necessary which in our case is performed by Rayleigh scattering on N2 . In the version of EXTRAP-Ll with the glass tube liner the stray light level was found to be 2 orders of magnitude greater than the Rayleigh scattered light making absolute calibration impossible . The glass tube did not have holes between the collection optics and the viewing dump which contributed to the high stray light level . In the EXTRAP-L1 version with a metal tube liner including holes the stray light was measured to correspond to the Rayleigh scattering from 60 Ton N2 and absolute calibration was performed. 3.2. Background plasma radiation The plasma background radiations has been measured at different wavelengths and at several values of the external plasma parameters. The measurements show that for a range of external plasma parameters the background radiation makes a significant contribution to the signal . The background plasma radiation affects the measurement of the scattered radiation in 2 ways : First background photons add to the measurement of the scattered photons. Second the background radiation may cause the saturation of the PM tubes. The condition for our PM-tubes not to saturate is ts i dt < 10 -8 C, Jt o

(4)

where i s is the anode current of the PM-tube, to is the time at which the plasma starts to form and i s is the time at which the laser scattering occur. In our case, saturation of the PM-tubes is not important, basically due to the short lifetime of the plasma ; i s - to < 80 lAs.

P. Karlsson / Multipoint Thomson scattering system To get a proper estimate of the background plasma radiation that must be deducted one must either measure the background at a different time, at a different wavelength, or at a different position from the scattering volume. In this pinch experiment, we wish to measure the density and temperature even under conditions where MHD fluctuations are present . These fluctuations can have a time scale of the order of microseconds and scale lengths of the order of the pinch diameter. In fig . 6, we show recordings of the background radiation for several plasma shots. The time scale of the recordings are the same but the calibration factors are not accounted for which means that the amplitudes are not comparable. The shape of the curves, however, contains information. Fig. 6a and 6b show the background radiation recorded by the same channel at two different shots with the same external parameters. The shot to shot

39 5

variation is seen to be significant . A comparison between figs . 6a and 6c shows the spatial variation of the background radiation. Fig . 6c is a recording for the same shot as fig. 6a and at the same wavelength interval but collects light from the central part of the plasma pinch while 6a looks at a part 3 .3 mm away from the centre. Figs . 6b and 6d illustrate the wavelength dependence of the background radiation . They are recorded from the same shot and at the same spatial position but 50 A apart . Because of the MHD fluctuation level that can exist, it is preferable to measure the background radiation at a different wavelength rather than at a different time . The data acquisition system requires 10 As to digitize the signals, and this is too slow compared to the frequency of the fluctuations . For the plasma parameters discussed here, we measure the background at a wavelength 125 A from the laser wavelength where no scattered light appears . At each spatial point we use one channel to detect background radiation . A series of measurements of the background radiation only is taken in order to give a relative calibration between the 3 channels detecting background light and the 12 channels detecting scattered light with respect to the amount of background light. 3 .3 . Data acquisition

E

10

i

to +50

~

t

t0 +100

Fig . 6 . Plasma background radiation recordings .

The present experimental setup allows measurements at 3 different positions and at 5 different wavelengths at each point . It has been found that, in order to obtain sufficient accuracy in the determination of the electron temperature, measurement of scattered light at 4 different wavelengths is required . The fifth wavelength channel at each position is used for measuring the plasma background radiation since no scattered radiation has been detected there . The laser pulse energy varies from shot to shot and also needs to be recorded for the absolute electron density measurements. In order to measure the laser pulse energy a 400 Am quartz fibre is mounted behind the mirror M2, shown in fig. 3, which samples and transmits the light to a silicon photo diode. A lens is used to ensure that the sample is a good average value. There are thus a total of 16 signals that need to be registered at each measurement. The Thomson scattering system is connected to a CAMAC data acquisition system which is interfacing a L5I 11/23 computer. The signals from the PM-tubes are integrated and digitized by two LeCroy 2250L CAMAC modules. The integration interval is specified by a gating pulse which is delivered by a pulse generator . The pulse generator is trigged by the signal from a photo diode that detects the laser light that is transmitted through mirror M3 . A second measurement is possible 10 As or more after the first when the laser is operated in the double

P. Karlsson / Multipoint Thomson scattering system

396

pulse mode . It is the digitizing time of the ADC that sets the lower limit of the time interval between the pulses. The data is evaluated by standard techniques . For each spatial position a straight line is fitted to the log of the signal vs the square of the scattered wavelength .

Shot#1572 Te= 3.3eV

Point#1 20 n . =4 2 -10

4. Results Because of the wide range of density in the experiments, the high voltage driving the PM-tubes must be adjusted to provide a good signal level given the maximum rating, 256 pC, of the ADCs . Results from a typical shot are shown in fig. 7. The data points are shown with error bars. The measurements of the background radiation showed that the shot to shot variation in the background level between the 3 channels employed to record background radiation and the other 12 channels was quite large. It has been found that this uncertainty gives the major contribution to the error bars. There are indications that the main part of the background radiation originates from impurities. Recordings of the H. line, the line from the 3p2D°-3d2 F transition in O II at 4705 Á and the background radiation at wavelengths of the scattered radiation during a shot show a good correlation between the O 11 line and the background radiation. This correlation is not found between H« and the background . 5. Sununary

Shot#1572 Te = 6 .4 eV

[arb . units]

Point-*2 20 n e =5 5'10

Shot#1572 Point*3 20 n e =4 .910 Te =3 .8eV

2 LnS

A multichannel Thomson scattering system for small, 1-cm diameter, Z pinch discharges has been developed. It is capable of simultaneous measurements of the electron temperature at three different positions and two different times during the discharge. The spatial resolution of 3 points with 3 mm between points is obtained with only one spectrometer and 15 PM-tubes . The 1-m, f/ 10, plane grating spectrometer disperses the incoming radiation. The output slit of the spectrograph is imaged on to a 3 x 5 matrix of quartz optical fibres which provides the spectral and spatial resolution . A silicon photo diode is used for monitoring the laser pulse energy . The detectors are connected to a CAMAC data acquisition system which evaluates the data after each plasma shot . The background radiation rather than noise form photon to electron conversion statistics has been found to be the greatest source of uncertainty in determining the electron temperature and density. Both theoretical and experimental investigations indicate that the background radiation originates from impurities . 6. Acknowledgements

Fig. 7. A slightly modified computer output of the results from a typical shot. The logarithms of the signals are plotted vs (AA s ) 2 for the three positions. Straight lines are fitted to the data points in order to evaluate the electron temperature Te and the electron density n e .

The author is greatly indebted to Mr . J. Tonks for designing the interface between the plasma and the fibre bundle, as well as constructing the fibre matrix holder and numerous other mechano-optical components of the system . For introducing the author to the ruby laser and designing part of the laser beam guide,

P. Karlsson / Multipoint Thomson scattering system Dr . B. Wilner is given thanks . Special thanks go to Dr . J.R. Drake for valuable comments regarding the manuscript and to Mr. S. Holmberg for writing part of the software for the data evaluation . This work has been supported by the European Communities under an association contract between Euratom and Sweden.

[4j [5) [6]

References

[7]

[1] B. Lehnert, Phys. Scripta 10 (1974) 139; B. Lehnert, Phys. Scripta 16 (1976) 147. [2] J.R. Drake, Plasma Phys . 26 (1984) 387. [3] J.R. Drake, T. Hellsten, R. Landberg, B. Lehnert, and B.

[8]

397

Wilner, Plasma Physics and Controlled Nuclear Fusion Research 1980, Nuclear Fusion Suppl. II (1981) 717. M. Bassan, A. Buffa and L. Guidicotti, Rev. Sci. Instr. 56 (1985) 1027 . F.M . Levinton and G.A . Navratil, Rev. Sci. Instr. 54 (1983) 35 . D. Johnson, D. Dimock, B. Grek, D. Long, O. McNeil, R. Palladino, J. Robinson and E. Tolnas, Rev. Sci. Instr. 56 (1985) 1015 . J. Sheffield, Plasma scattering of electromagnetic radiation (Academic Press, New York, 1975). E. Desoppere, G. Van Oost, G. Bosia, R. Koch and D. Pearson, Laboratory Report No . 75 (March 1981) Laboratoire de physique des plasmas, Ecole Royale Militaire, Brussels, Belgium.