Page 2 - First time-resolved electron density measurements in the C-2W advanced field-reversed configuration plasmas from long-path compact second-harmonic interferometer

10B113-2 Beall, Sheftman, and TAE Team
Rev. Sci. Instrum. 89, 10B113 (2018)
FIG. 2. Schematic drawing of one dispersion interferometer, depicting the collimator (Col), second harmonic generators (SHG1 and SHG2), half-wave plate (λ/2), compensator (Comp), filter element (F), and polarizing beam splitter (PBS).
where λ is the original laser wavelength, λ/2 is the second harmonic wavelength, and z is the coordinate along the path L of the beam. The signals received by the detectors are
􏰂2􏰃􏰅􏰄􏰆 V±=α±P0/2 β1+β2±2β1β2cos(∆φ+φ0),
where α± is the detector responsivity, P0 is the fundamental beam power, βi is the SHG efficiency, and φ0 is the phase offset. A term R can be calculated such that
FIG. 3. Positioning of the dispersion interferometer optical units on the (a) inner divertor (DIV) and (b) primary confinement vessel (CV1 and CV2), with (c) sightlines shown in confinement relative to the expected core plasma position. In the initial configuration, optical units were placed at DI CV1 and DI DIV. In the secondary configuration, the DI DIV unit was moved to DI CV2.
stainless steel boxes with a turning mirror for alignment, was clamped onto the ports, as shown in Fig. 4. The optical unit was then clamped onto an insulated mounting point. Without the adapter, the system can be moved by one individual to any matched pair of standard 23⁄4 in. ports in about one day. With the adapter, any standard 6 in. flange can be used.
In the previous experimental campaign of C-2U, the optical units were only placed on opposite sides of the cen- tral confinement vessel (CV), a distance of approximately 1.4 m. While the vessel diameter of C-2W is only increased to 1.6 m, the beam paths across the larger divertor sec- tion and axially through the main vessel are 3.5 and 5.4 m, respectively. This necessitated the addition of two achro- matic lenses to re-collimate the fundamental beam onto the SHG located after the plasma, in order to maintain fre- quency doubling efficiency. In turn, this addition required an extension to the optical mounting system and an expanded enclosure.
FIG.4. CADmodeloftheprimaryopticalenclosureandrightangleadaptor. Both are easily hand-portable and can be attached to any 23⁄4 in. or 6 in. standard vacuum flange, respectively.
R = V+ − V− = α ± V sin(∆φ + φ0) , (2)
where
and
V+ +V−
1±αVsin(∆φ+φ0)
α = α+ − α− (3)
α+ + α−
√
2 β1β2
V=β+β. (4)
12
Correct tuning can reduce α to negligible values and V to
approximately 1, allowing the plasma phase to be recovered
as the inverse sine of R minus a DC offset. When the system
.
is well aligned, it can measure electron density to a resolution 18 −2
of approximately 3 × 10 m
The dispersion system was first brought online for the C-
2W experiment during the early commissioning phase when FRC plasma formation and translation were being optimized. This process consists of two smaller FRCs being generated by magnetic pinch in quartz tubes referred to as “formation” on opposite ends of the device and then accelerated by main- reversal fields toward the central confinement region. Unlike in the prior experiment C-2U,4 the plasmoids also have to pass through the large inner divertor vessels used for magnetic- field flaring and neutral gas control. In order to verify and characterize the formation and translation processes, the two interferometer systems were installed radially in the divertor and primary confinement regions, as depicted in Fig. 3, to measure the FRC as it passed by.
Once formation and translation were able to be consis-
tently produced, the interferometer on the inner divertor was
moved to view near-axially along the central confinement
region, and the other system was left in place to view the
expected X-point of FRC plasmas. In this position, the sys-
tem was able to measure the merger of the two plasmoids and
provide information about plasma equilibria to complement
5,6
ally, a pair of right-angle port adapters, consisting of robust
the primary density diagnostics.
To mount the system axi-