A combined millimeter wave and CO2 interferometer on the C-2W Jet plasma

REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10B110 (2018)
A combined millimeter wave and CO2 interferometer on the C-2W
Jet plasma
R. J. Smitha) and TAE Teamb)
TAE Technologies, Inc., Foothill Ranch, California 92610, USA
(Presented 17 April 2018; received 23 April 2018; accepted 9 June 2018; published online 7 September 2018)
A two wavelength tangentially viewing multi-chord interferometer has been built for the Jet plasma of the C-2W experiment at TAE Technologies. A novel 1 mm wavelength interferometer has been developed to be used simultaneously with a CO2 laser interferometer to provide full coverage of the Jet plasma and the translating field-reversed configuration (FRC) plasma before merging. With CO2 and millimeter wave sources, the interferometer proposes to cover a combined dynamic range of line integrated density of more than 1000 although the CO2 interferometer sub-system is not yet operational. Sited at the axial location of the mirror field of C-2W, the interferometer will play a pivotal role in assessing the FRC before merging and the operation of the inner and outer divertors and particle outflow. The performance of the millimeter wave interferometer and recent measurements is discussed. Published by AIP Publishing. https://doi.org/10.1063/1.5037332
I. INTRODUCTION—C-2W DEVICE
The C-2W device generates a field reversed configuration
(FRC) plasma in a 1.6 m diameter Inconel confinement vessel
(CV) by forming and accelerating two FRCs in opposite direc-
tions to collide and merge in the CV. This program has been
highly successful in forming and sustaining the FRC using
outside the plasma. The technique is sensitive to movements
(vibrations) which produce a relative displacement in the paths,
∆l, as well as a relative change in index of refraction along the
path through the plasma of length, Lp. The refractive index of
the plasma relative to air (unity) is proportional to the local
4
􏰀 Lp 0
where the laser wavelength λ represents a phase of 2π radians. A two wavelength, λ1 and λ2, interferometer can discriminate between phase due to the plasma and phase from relative dis- placements. The plasma’s line-integrated density (LID) is a profile independent quantity given by
(φ1/λ2 − φ2/λ1) 1
−15 (λ /λ −λ /λ ), (2)
12
neutral-beam injection (NBI) on the previous C-2U device.
The FRC is stabilized by biasing from distant electrodes, and
a large divertor structure has been implemented. On C-2W, an
inner divertor has been added between the formation section
and the CV. This allows a volume for flux expansion to stag-
nate the flow of particle outflow of the scrape-off-layer (SOL)
and allow the electron temperature to increase. The goal of
C-2W is to explore FRCs with a high electron temperature, as
high as 1 keV; additionally, the NBI power has been increased
and the capability of raising the external magnetic field in the
CV from 0.1 to 0.3 T is in place. Plasma pulse duration can
be eventually extended up to 30 ms on C-2W. In sequence, the
C-2W device has a mirror field at the ends of the CV, an inner
divertor, a formation stage, an outer divertor, and end electrode
plates with a plasma gun on axis and mirror symmetric with
φ(t)=2.82 × 10−15λ
ne(t)ds + 2π∆l(t)/λ (rad), (1)
electron density, ne. given as a phase,
The difference in optical path length is
􏰀 Lp 0
neds=
and the relative displacement, ∆l, is given by
respect to the CV mid-plane. The Jet plasma is the outflow of 31
particles through the X points of the FRC equilibrium. The Jet outflow is important on experiments with longer FRC life- times as the Jet plasma may dominate the loss of inventory, and its significance is not yet known for C-2W plasmas. The Jet interferometer is located at the north mirror region where the CV necks down to a 0.7 m inner diameter.
Plasma interferometers measure the difference in optical path length of a path containing the plasma relative to a path
Note: Paper published as part of the Proceedings of the 22nd Topical Confer- ence on High-Temperature Plasma Diagnostics, San Diego, California, April 2018.
a)Electronic mail: rsmith@tae.com
This system combines two millimeter wave (λ = 1 mm)
2.82×10 1 2 2 1
(λ2φ1 − λ1φ2) 1
∆l=− · .(3)
2π (λ1/λ2 − λ2/λ1)
and three CO2 (λ2 = 10.6 μm) interferometers over five chords.
Since the millimeter wave and CO2 interferometers are not
implemented on the same chord, Eqs. (2) and (3) do not
strictly apply. However, for the two adjacent diameter sight-
lines with mixed interferometers, these relations should hold.
The movements are expected to be largely the same for all
interferometers. This can be checked with vacuum discharges.
With λ1/λ2 = 95, the two effects decouple with LID approx-
imately the first term of Eq. (1) using φ1 and λ1 and, ∆l, the
second term using φ2 and λ2. For large LID, the CO2 sys-
tem can contribute to the first term and will be relied on if the
89, 10B110-1 Published by AIP Publishing.
b)TAE Team members are listed in Nucl. Fusion 57, 116021 (2017). 0034-6748/2018/89(10)/10B110/5/$30.00
) as the FRC moves past, but this is an exceedingly brief time in which
21 −3 millimeter wave system is cutoff (peak ne > 10 m