Measurement of Ion Temperature in a Laboratory Plasma Jiachen Liu, Seth Dorfman, Walter Gekelman, Patrick Pribyl, Anton Bondarenko, Troy A. Carter

University of California, Los Angeles

What is Plasma?

Plasma is one of the four fundamental state of matter,

and more than 99% of the visible matter in the universe

consists of plasma.

A plasma is a “quasineutral” gas of charged and neutral

particles which exhibits collective behavior[1]. And plasma

physics is the study of how plasma interacts with electric

and magnetic fields.

Some examples of plasma would include

the Sun, fluorescent lamps and this

plasma globe.

Objectives

This research primarily focuses on measuring the

temperature of the plasma by measuring the width of a

prominent ion spectral line. By the theory of Doppler

broadening, the hotter the plasma is, the larger the

width of this line. We measure the width of the

convolution of the plasma simulation output and the

calibration line. The result of this convolution is used to

account for the instrumental broadening. We then

change the plasma temperature in the simulation.

Afterwards, we are able to compare the result with the

width of actual spectral line of the plasma measured in

LAPD (Large Plasma Device). Therefore, we are able

to obtain the temperature of the plasma.

Hypothesis: The new LaB6 cathode implemented in

LAPD will produce a plasma ion temperature of

10eV.

The result of this research will be crucial in future

research focusing on the behavior of Alfvén wave in hot

plasma, which may contribute to the applications

mentioned above.

LAPD and the Cathode Plasma Simulation Conclusion

Temperature versus Time

References

After the calibration of the monochromator, we moved

the apparatus to LAPD to measure the spectral line of

the actual plasma. In order to determine the ion

temperature, we compare the width of actual plasma line

with a simulated spectral line. To achieve this, we use a

software called PrismSPECT that simulates plasma

conditions and produces a simulated spectral line.

The Large Plasma Device

(LAPD), is a low maintenance

device for studying a variety of

waves and nonlinear effects in

magnetized plasma.

By comparing the convolution results with the actual

plasma spectral line, we are able to determine that the

plasma ion temperature is about 5eV.

Even though the plasma temperature does not reach

10eV as we expect it to be, LAPD can still produce an

average of 5eV which is sufficient for some new

experiments investigating Alfvén waves in LAPD.

Future Plans

What is an Alfvén wave?

Method of measurement

Calibration

Temperature ResultAn Alfvén wave is a low frequency hydromagnetic wave

that may occurs in a plasma with a background magnetic

field. Here are some applications for the Alfvén wave[2]:

Heating of solar and stellar

corona:

The instrument we use to

obtain the spectral line is a

two-meter monochromator

that connects to a photo-

multiplier. A diffraction

grating moves after a

number of shots are taken.

For each shot, the

photomultiplier converts

light to voltage and it is

recorded by the digitizer.

For the calibration process, we use the monochromator to

measure the width of the spectral line of a mercury lamp.

Since the mercury lamp is relatively cold (less than

0.1eV), the width of the spectral line is supposed to be

really small. There are multiple contributions to the

instrumental broadening: the alignment of the mirrors in

the monochromator, the focus point at the exit slit, the

height and width of the entry and exit slit. For some

contributions, we are able to calibrate the monchromator

to reduce the instrumental broadening.

This new LaB6 cathode

has a higher electron

emission rate compared

to the old BaO cathode.

More electron emission

means the cathode is

able to create hotter

plasma.

Czerny-Turner Scanning Spectrometer

--the monochromator we were using.

A graph shows how

our calibration helps

lower the width of

the spectral line of

the monochromator.

As one can

see, as the

temperature

increases, the

width of the

simulated

spectral line

broadens.

I perform a convolution of the simulation spectral line and

calibration line in order to account for instrumental broadening.

Then I compare the convolution result with the actual plasma

line to determine the temperature of the plasma.

These two

graphs

suggest that

the plasma

temperature

is about 5eV,

less than

what we

expect it to be

(10eV).

One of the benefits of using a monochromator to examine

the spectral line of a plasma is that we are able to obtain

time resolved relations. After fitting the spectral lines to

Gaussian distributions[4], we are able to determine the

relationship of the temperature versus time.

How the monochromator

works:

The light from the plasma

shines onto the grating and

only the wavelength of the light

we want eventually reaches the

exit slit.

UC Davis, ChemWiki

This

demonstrates that

as the time

increase, the

width of the

Gaussian

distribution

increases as well.

TRACE image

Despite the temperature being lower than what we

anticipated, a temperature of 5eV still allows us to

perform some experiments examining the behavior of

Alfvén wave in warm plasma.

One of the possible experiments is to investigate the

dispersion relationship of an Alfvén wave in the presence

of warm ions.

The next step of this research is to measure the ion

temperature of the plasma in the Enormous Toroidal

Plasma Device (ETPD).

A picture of

ETPD, the red

column enables

the device to

generate toroidal

magentic field and

the blue magnetic

ring generates

vertical magnetic

field.

It is expected that the ion temperature of the plasma is

more than 10eV. The reason is that longer column length

makes it harder for ions and electrons to escape.

In the future, we will be designing an optical system to

let the light of the plasma shine on the entry slit of the

monochromator.

[1] Francis, F. Chen. 1974. Introduction to

Plasma Physics and Controlled Fusion. 2nd ed.

New York: Plenum Press. 3 p.

[2] Zhukov, Andrei, and Yuriy Voitenko, 2013.

Highlight of the STCE Workshop on Alfvén

Waves in Solar and Space Plasma. STCE.

[3] W.Gekelman, S.Vincera, B. VanCompernolle,

. G. J. Morales, J.E. Maggs, P. Pribly, T.A. Carter.

2011. The Many Faces of Shear Alfvén Wave.

Phys. Plasmas 18, 055501.

[4] Strong Lines of Helium ( He ). Strong Lines of

Helium ( He ). NIST, n.d. 2014.

<http://physics.nist.gov/PhysRefData/Handbook/

Tables/heliumtable2.htm>.

The Many Faces of Shear Alfvén

Wave[3]

The figure on the left

shows that the

temperature rises as

the discharge current

increases. The

maxima of

temperature is about

5.5 eV.

This work is performed at Basic

Plasma Science Facility, funded by

DOE and NSF.

J.De Keyer

The possible cause of

particle accelerations in

auroras.

How the Alfvén wave is

generated and detected: