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

10B110-2
R. J. Smith and TAE Team
Rev. Sci. Instrum. 89, 10B110 (2018)
FIG. 1. (a) The Jet interferometer layout showing the port array and gate valve separating the CV from the inner divertor and (b) the arrangement of CO2 and mmwave probe beams. The CO2 beams are pencil beams of about 6 mm diameter; the millimeter beams are extended Gaussian beams with a 1.7 cm waist on the machine axis.
∆l does not change and two wavelength interferometry is not needed.
The Jet interferometer represents a challenge in dealing with plasmas that span a wide range of LID but also an opportu- nity to provide key measurements on the FRC before merging, the establishment of the equilibrium and Jet outflow, and the interaction of the inner and outer divertors as well as biasing and the use of the end plasma gun. The Jet plasma density is not well known but must substantially be less than the bulk plasma density. The translating FRC, at velocities exceeding 300 km/s, is compressed in the magnetic mirror region enhancing its peak density and reducing its length (FRCs contract axially when compressed radially), although the translating FRC represents only half of the particle inventory of the merged FRC equi- librium in the CV. A compact source is needed for this region with enough sensitivity to quantify a Jet plasma of relevance and yet have a high cutoff density. A longer wavelength far infrared (FIR) source is rejected by the limited space and by constraints placed on a Gaussian beam, given the 1.6” port diameters separated by 1 m. A CO2 interferometer system deployed on C-2U5,6 was shown to be appropriate for the FRC. A novel 300 GHz interferometer was implemented on C-2W, as shown in Fig. 1; the arrangement of chords is also shown.
II. DESCRIPTION OF THE INTERFEROMETER A. Mechanical support structure
Several considerations have dictated the design of the interferometer frame, as shown in Fig. 2. The 2.5 m long stain- less steel optical table is supported by the C-2W frame, but its width is limited to 0.2 m. The frame was designed as a stiff U shape with a garolite rod spanning the top to complete a rigid box structure. Stainless steel shear walls support the ver- tical panels off the optical table. The two panels are optical platforms for mirrors and beam splitters. The panel material is 0.5” thick phenolic, chosen to minimize phase distortion and beam misalignments due to the high magnetic fields in this region. These fields are static or changing on a slow time scale, and their effects should not be difficult to remove from the phase signals. The top of the panels is stabilized by three supports to the C-2W frame. Figure 2 shows the layout of the interferometer.
On the CV, the CO2 system uses ZnSe viewports mounted on 23⁄4” conflat flanges. The windows are anti-reflection (AR) coated. For the millimeter wave system, crystalline quartz win- dows mounted on a 3-3/8” conflat are used to provide the Gaussian beam with a generous aperture that matches the fun- damental restriction of the 1.6” inner diameter of the ports. The quartz window aperture is a full 1.6” (Laser Optex Co.). The transmission through the quartz window can be signif- icantly reduced by Fresnel reflection losses. Two windows were coated with an AR coating of parylene film (Tydex Co.). Parylene is a high vacuum compatible plastic, stable at high temperatures. Both surfaces of the window were coated yield- ing a transmission of 89%. The windows on the CV are at present uncoated but will be replaced.
The Gaussian beam of the millimeter wave sources has a diameter of 1.7” at the focusing lenses, and the insertion loss of a window and its vacuum hardware was tested and found to be low.
There are 3 CO2 probe beams, two independent millimeter wave interferometers, and a 4th reference CO2 interferometer independent of the plasma. Only for the two diameter chords,
FIG.2. OpticallayoutoftheJetinterferometertogetherwithasectionoftheCVwithanarrayofportstoscale.