Thomson Scattering on the ZaP Experiment€¦ · Aerospace & Energetics Research Program A Thomson Scattering System Will Measure the Electron Temperature of the Z-Pinch Plasma Properties

Plasma Dynamics GroupAerospace & Energetics Research Program

Temperature Measurements on the ZaP Experiment

R.P. Golingo, U. Shumlak, B.A. Nelson, D.J. Den Hartog, and the ZAP Team

12-14 February 2006


Abstract

The ZaP Flow Z-Pinch Experiment is presently studying the effect of sheared flow on gross plasma stability. During a quiescent period in the magnetic mode activity, a dense Z-pinch with a sheared flow is observed on the axis of the machine. The velocity shear agrees with the threshold predicted by linear theory. A better comparison between the experimental and analytic results can be made once the pressure profile is known. The present results are from deconvolutions of chord integrated measurements. The plasma density is measured with a HeNe interferometer and holographic interferometery. The ion temperature is measured with Doppler broadening. The electron temperature is found with coronal equilibrium. The total temperature can be found with pressure balance. Zeeman splitting measurements have confirmed the assumed magnetic field magnitude and the edge of the Z-pinch. Thomson scattering is capable of measuring the electron temperature and density at a point in the plasma. A single point Thomson scattering system is being installed to directly measure the local electron temperature in the Z-pinch. The system has a 3 mm radial resolution and can collect scattered light up to 4 cm off of the axis of the machine. (The Z-pinch has a 1 cm characteristic radius.) The expected Thomson signal has been calculated to be 10 times the measured background radiation level. Initially the system will measure the electron temperature at a single point in the plasma. The design and hardware allow for the system to be upgraded to a multipoint Thomson scattering system which would measure the pressure profile of the Z-pinch. The design of the system and initial results will be presented.


Stable, Sheared Flow Z-Pinches are Formed in the ZaP Experiment

Motivation- The ZaP Experiment studies the effect of sheared flows on the stability of an otherwise unstable configuration. The temperature of the Z-pinch is found by measuring chord-integrated quantities and using Abel-type inversions to find the local value. A Thomson scattering diagnostic can measure the local temperature and density in the Z-pinch. A Z-pinch is a simple magnetic confinement configuration, which may be stabilized with a sheared flow.A unique formation process is used, which generates a Z-pinch with a sheared flow.The plasma temperature has been measured with different methods.A Thomson scattering system has been designed using available equipment. Initial measurements of the scattered light have been made.


Sheared Flows Stabilize the Kink Mode

0

0.02

0.04

0.06

0.08

0.1

0.12

1 2 3 4 5

Threshold Shear

v' /

k V

A

rw / a

Unstable

Stable

Linear stability calculations show the kink mode can be stabilized when the flow shear, dvz/dr, is greater than 0.1kVA

[1].The wall does not affect the stability when rw/a>4.The analysis used a pressure profile which was marginally stable to the m=0 mode.

[1]U. Shumlak and C.W. Hartman. Phys. Rev. Lett., 75 3285 (1995)


Plasma is Accelerated by the Magnetic Pressure

Neutral gas is puffed between two coaxial electrodes.A voltage is applied between the electrodes.The gas breaks down, forming an annular current sheet.The JXB force accelerates the current sheet along the electrodes.Neutral gas ahead of the current sheet is ionized and entrained in the sheet.


The Current Sheet Collapses Forming the Z-Pinch

The inner region of the current sheet collapses onto the axis at the end of the inner electrode.The outer region continues traveling axially along the outer electrode.The motion continues until the current has assembled on axis.The momentum of the plasma maintains the flow.Neutral gas in the accelerator may supply plasma to the Z-pinch.Similar devices have seen pinch-like structures which last throughout the current pulse[2].

[2]J. Marshall, Phys. of Fluids, 3 1 134 (1960)


ZaP Uses a Coaxial Gun Coupled to an Assembly Region

Machine drawing showing the present configuration of the ZaP experiment.Gas is puffed into the midpoint of acceleration region (z=-75 cm).Most of the measurements shown are made at z=0 cm

(~20 cm from the end of inner electrode nose cone).A 100 cm scale is included for reference.


The Plasma Characteristics are measured with an Array of Diagnostics

Plasma density– A two chord heterodyne quadrature interferometer– A holographic interferometer

Plasma velocity– A 0.5 m spectrometer which views 20 parallel chords through the plasma (ICCD)– A 1.0 m spectrometer with a 16 channel PMT at the exit slit (IDS)

Shape and position of the emission from the plasma– An Imacon fast-framing camera– A 16 chord photodiode array and two 32 chord photodiode arrays

Magnetic fields and current location– An axial array of surface magnetic probes– Four azimuthal arrays of surface magnetic probes– Zeeman splitting measurements

Other diagnostics are used to verify these measurements– A 0.5 m spectrometer with a CCD and PMT– A bolometer and filter scopes– Gridded energy analyzer– Langmuir/Mach probe


A Quiescent Period is Seen in the Mode Activity

After the Z-pinch forms, a quiescent period is seen in the mode data.The horizontal line at 0.2, the Z-pinch radius, is an empirical level at which the mode activity changes its character. Stable Z-pinches are seen in the assembly region when the mode activity is below this line.The plasma current is shown for reference.


Impurity Line Radiation Indicates heating

The evolution impurity line emission from the C III and C V is measured throughout the lifetime of the Z-pinch.The appearance of C V later in time indicates heating of the Z-pinch throughout the quiescent period.


Lower Ionization States Burn Through

The impurity line emission from oxygen is measured by the IDS instrument for multiple chords through the plasma.Line emission from O IV is seen early in time on the edge of the Z-pinch.As the plasma heats throughout the quiescent period the line emission burns through to O V.O IV line emission is measured as the plasma cools at the end of the quiescent period.


The Ion Temperature Evolves During a Pulse

The IDS instrument measures the time evolution of the ion temperature from one chord through the plasma.The ion temperature is high during the quiescent period.The ion temperature decreases after the quiescent period ends.


Holographic Interferometer for Density Profiles

A holographic interferometer measures chord-integrated density profiles. The system uses a pulsed ruby laser in a double-pass or single-pass configuration.The holograms are reconstructed using a He-Ne laser. The chord-integrated data are deconvolved to determine the density profile.


The Density is Peaked on Axis

During the quiescent period, the density profile is peaked on the axis of the pinch.The peak density is ≈1.5×1017 cm-3.


The Temperature is Found with Pressure and Power Balance

The density profiles are analyzed using the magnetic field measured at the outer electrode and the input power to compute equilibrium temperature and magnetic field profiles.Using radial force balance,

( ) ( )e e i io

d rBB d n T n Tr dr dr

θθµ

= − +

Computing the radial thermal conduction for a specified input power,

( )[ ]1e e i i

dq r k T k Tr dr ⊥ ⊥= − +&

where the thermal conductivities are given by Braginskii[2] as

[2] Braginskii, Reviews of Plasma Physics, vol 1. (1965)

24.7 e ee

e ce e

n Tkm ω τ⊥ = 22 i i

ii ci i

n Tkm ω τ⊥ =


Holography Measures a Discrete Plasma Pinch

Temperature and magnetic field profiles are computed from the model.The model sets an upper bound to the temperature in the Z-pinch.


The Magnetic Fields can be Measured with Zeeman Splitting

Line radiation emitted from the plasma is split, due to the Zeeman effect, into π and σcomponents.Doppler broadening is wider than the Zeeman splitting.The π components are polarized parallel to B and the σ components are elliptically polarized.The Zeeman effect can be seen by viewing the plasma parallel to the B field and measuring either the left or right handed circularly polarized light.


The Spectra Shows the Zeeman Effect

580 nm RP580 nm LP581 nm RP581 nm LP

Im pact Param eter = -11.3 m m

Im pact Param eter = 0.0 m m

∆λ (nm )

-0.2 -0.1 0.0 0.1 0.2

Im pact Param eter = 11.3 m m

Pulse 51012026

The brightness of the C IV doublet at 580 nm was increased by using 100% methane for the working gas in ZaP. (These plasmas behave similarly to ones formed with hydrogen.)The spectral intensity of the left and right hand polarization is measured with the ICCD.The centroid of the spectral intensity is shifted for impact parameters whose view is parallel to the magnetic field.The shift of the centroid reverses when the quarter wave plate is rotated 900.


Zeeman Splitting Measurements show All Current Flows Within the Characteristic Radius

Radius (m m )

-20 -10 0 10 20

B (T)

0.0

0.2

0.4

0.6

0.8

1.0

The local magnetic fields are found by deconvolving the spectral intensities with a shell model[3]. Shown are the measured magnetic fields for a number of pulses.The dashed line shows the 1/r dependence of the edge magnetic field.All of the current is flowing within the characteristic radius of the Z-pinch.

[3]R.P. Golingo and U. Shumlak. Rev. Sci. Inst., 74 2332 (2003)


A Thomson Scattering System Will Measure the Electron Temperature of the Z-Pinch

Plasma Properties

Minimum Maximum Design

1016

100

0.106α

Ne (cm-3) 1015 1017

Te (eV) 25 150

0.027 0.668

The local plasma properties in the ZaP experiment are presently obtained by deconvolving chord integrated measurements.The theoretical stability of the Z-pinch is sensitive to the pressure profile.The initial system will measure the electron temperature at one location in the plasma.This system will provide the necessary information to finish designing a multipoint Thomson scattering system. A multipoint Thomson scattering system could measure the pressure profile of the Z-pinch.


Major System Components of the Thomson Scattering System

Many key components for the ZaP Thomson system are from other experiments and projects.

– Korad Laser (U. Wash./LANL)– Hibschman Spectrometer

(LANL/U. Wisc.)– ITT MCP (LANL/U. Wisc.)– Lecroy 6880 Digitizers

(U. Wash./U. Wisc.)

These components have been refurbished and characterized.Components, specific to the ZaP experiment, have been built and installed. The ICCD spectrometer is capable of viewing the scattering volume though it is not optimized to measure the scattered spectra.


The Structure Provides Stability and Safety

The beam path is enclosed to ensure safe operation of the laser.The focusing and collection optics are matched.The structure allows for alignment of the laser and collection optics, while minimizing drifts and vibrations.Access to other ports is maximized with the structural design.The alignment of the system can be checked between pulses.


Beam Path

The beam is brought into the machine with a single turning mirror on a translatable mount.Vertical adjustments are made by raising the laser rail.A 400 mm focusing lens is used to create a 2 mm waist along 8 cm in the center of the machine.A small tilt on the lens prevents back reflections into the laser.The lens is mounted on three translation stages for alignment.


View ports

View ports: The laser beam must be directed through the plasma while minimizing reflections back into the laser.

– The view ports are placed at the Brewster angle to avoid reflecting energy back into the laser system and to maximize the transmission through the window.

Laser Beam Dump

Angled View Port Angled View Port

Focusing LensTop-Down View


Thomson Scattering Collection Optics Design

Measurements along the beam from the center of the plasma to r = 4 cm can be made with the present lens.Various radial locations are viewed by moving the fibers.The fibers collect light from a region wider than the laser beam to ensure that all of the scattered light is collected so that ne can be measured.The etendue of the collection system matches the spectrometer.


MCP Filter and Instrument Function

The channels from the MCP have a 6.80 nm spacing in wavelength.The instrument function from the Hibschman spectrometer is a Lorentzian with a FWHM of 1.995 channels (12.93 nm).An interference filter rejects light from 656 to 659 nm (Hα).Three interference filters reject light from 692 to 699 nm (ruby).The present data acquisition system can measure signals as small as 5 µA. Amplifiers will reduce the MCP charge depletion between the MCP channels and allow measurements of smaller signal levels.


F/O Trigger Pulse Input

Gating Pulse Circuit

MCP Biasing Circuit

Amplifier

Anode

0

In out

G

1n

1u

MCP Out

3

2

6

+

-

OUT

200k

240k 50K15k 1Meg

50

-3kV

50k

0

MCP In

50

IGBT

Photo Cathode

0

0

V1180V

Thomson Scattered Signal Detected by an MCP and Amplifier Circuit

• During the pulse, photo-electrons flow towards MCP In.

• Secondary electrons are accelerated and multiplied through the micro-channels.

• The anode signal is amplified and recorded.

• A gated light pulse at the “Pulse Input” is used to apply -180 V across the 50 ohm resistor.

• The photo-cathode becomes more negative than “MCP In”.


The MCP Signal is Amplified

Coupling between the MCP channels can be reduced by amplifying the signals.Smaller currents can be measured with the PDA amplifiers. The amplifier is capable of measuring the time evolution of the laser pulse.The frequency response is being increased.


Scattered Light Measured by the MCP

Initial Thomson scattering measurements have been made on hydrogen plasmas.Line radiation from carbon impurities has limited the use of the MCP for spectral intensity measurements, but amplifiers remove this limitation.Shown are the brightness at 672.1 nm measured across the plasma.The inner spatial channels see a scattered signal while the edge channels do not.


ICCD Measures the Scattered Light

The ICCD spectrometer is optimized for velocity measurements. The fibers can view the scattering volume.Features near the ruby line are not present when a polarizer is placed perpendicular to the laser polarization.Three pulses are required to attempt temperature measurements.Simple background subtractions cannot be made due to motion of the plasma and small pulse to pulse variation in the background radiation.


Conclusions and Future work

A unique formation process is used, which generates a Z-pinch with a sheared flow.Dense Z-pinches are formed in the assembly regionTemperatures in the plasma are about 100 eV.A Thomson scattering system has been constructed with available equipment and is beginning to make measurements. Initial scattered light has been measured.Future work

– Build and install faster amplifiers for the MCP.– Replace AR coating on ruby rods.

Please go to http://plasma.aa.washington.edu and follow the ZaP link for reprints.

Documents

Thomson Scattering on the ZaP Experiment€¦ · Aerospace & Energetics Research Program A Thomson Scattering System Will Measure the Electron Temperature of the Z-Pinch Plasma Properties