10 Figure 3.1 Electron cyclotron resonance (ECR) frequencies of the second, third and fourth harmonics. for the KSTAR plasma at nominal operating fields from 2.0 T to 3.3 T. 27 Figure 3.12 (a) Calculated (curve) and measured (symbols) transmittance, reflectance and absorption. as a function of frequency from 75 to 110 GHz of a strip-grid beam splitter for an X-mode beam. 35 Figure 4.2 Schematic of CSS consisting of five parts such as millimeter wave source, optics,.
Introduction
Nuclear fusion energy
The main purpose of KSTAR is to conduct plasma experiments to check various instabilities, including understanding the physical mechanism, and to conduct in advance the experiments to be carried out in ITER. The construction of ITER is expected to be completed in 2024, the D-D and D-T reaction experiment will start from 2025 and 2035, respectively. The main goal of ITER is to achieve a fusion power gain of Q ≥10 through the 50 MW externally heated D-T reaction for 300 – 500 s which means that the energy production from fusion is over 500 MW (Q = fusion power / external heating power) [4].
Motivation
ETG state instabilities which cause the electron heat transport are characterized by 𝑘𝑘𝜃𝜃𝜌𝜌𝑖𝑖 ≫1 or 𝑘𝑘𝜃𝜃𝜌𝜌𝑒𝑒~1, where 𝑖 is ion. Only recently has it reported direct evidence for a small-scale turbulence driven by ETG mode through comparison of observed turbulence in the National Spherical Torus Experiment (NSTX) and the numerical results from a linear gyrokinetic stability code [7, 8]. As mentioned above, ETG mode instability has been observed in several Tokamaks, but the physical mechanism of ETG mode has not been fully elucidated.
Thesis organization
Theory of Thomson scattering
Principle of scattering measurement
The spread length (L) shown in fig. 2.1 can be represented as follows. 2.5) requires careful calculation because the value is incorrect when the scattering angle is sufficiently small. This means that the scattering length (𝐿𝐿𝑧𝑧) can be much larger than the area of the fluctuations along the probe. From the above equation, we can calculate k-space resolution if we know the information about the probe beam. and the spread angle to be measured at the spread position. 2.17) means that the spread has an angular extent related to the size of the probe beam.
Scattering radiation
- Scattering radiation by a single charge
- Scattering radiation by a density fluctuation
- Scattering radiation power
Subject to the non-relativistic assumption (β ≪1) that applies to the scattering experiment, the scattered electric field is from a charged particle. The total electron density fluctuation scattered radiation (𝑛𝑛�𝑒𝑒(𝑟𝑟���⃗′,𝑡𝑡')) observed at time 𝑡𝑡 in the scattering volume (V) is given by. We are not interested in the case of incoherent density fluctuation, so the detailed procedure is omitted.
Preparation for design of diagnostics
- Probe beam frequency and polarization
- ECR frequency and absorption coefficient
- ECE emissivity and system noise
- The CSS and MIR diagnostics
- Strip-grid beamsplitter
- Transmittance of the dielectric substrate
- The refractive index of lenses and the waist of the 300 GHz probe beam
- Ray tracing for 300 GHz in KSTAR plasma
𝑃𝑃𝑛𝑛=𝑘𝑘𝐵𝐵𝑇𝑇𝑛𝑛𝐵𝐵𝑛𝑛 (3.1) where 𝑘𝑘𝐵𝐵 J/K is the Boltzmann constant and 𝐵𝑛𝑛 is the noise temperature and bandwidth of the detection system, respectively. A glass beam splitter made of 1.1 mm thick Borofoat 33 was used to split and combine the optical paths of the two diagnostics. This may be caused by the power variation of the source beam and the manufacturing tolerance of the strip grid beam splitter.
For the S-polarized beam, the electric field is perpendicular to the plane of incidence, while that of the P-polarized beam is parallel to this plane. The dielectric loss of beam power in the dielectric material as absorption is given by. In conclusion, the grid band beam splitter enables the effective separation and combination of CSS and MIR optical systems.
The index of refraction of lenses used for optics was obtained using the measured beam radii behind a lens. Like the optics of the MIR system, the lenses of the CSS optics were made of high-density polyethylene (HDPE). Note that in determining the refractive index, comparisons in both directions were taken into account.
The red line represents the beam path of the probe optics, and the four blue lines represent
KSTAR collective scattering system
- Overview of the collective scattering system
- The optical system
- probing optics
- receiving optics
- Millimeter wave source
- Synthesizer
- Heterodyne detection system
- Electronics
- Digitizer
The curvature information of the lenses and the positions of the strip grating beam splitter and the mirror are shown in detail in Fig. 4.3. Because the antenna of the detectors is the same as that of the probe beam source, the receiving optics were designed with the waist on. The slopes of the graph are linked to the scattering angles and agree well with each other.
4.5 (a) Ray tracing result of the receiving optics, (b) Gaussian beam calculation of the receiving optics (top view), showing information about the lenses and other optical components. The reference mixer and the detector of the detection system use the subharmonic mixer, and the center frequency (IF) is 1 GHz, and so the frequency of the LO source is 149.5 GHz. The output frequency is fed back to the input of the system via a frequency divider, creating a negative feedback loop.
In the most common application, two signals are mixed at frequencies 𝑓𝑓1 (measured signal) and 𝑓𝑓2 (LO signal), creating two new signals; one at the sum of the two frequencies 𝑓𝑓1+𝑓𝑓2 and the other at the difference between the two frequencies 𝑓𝑓1-𝑓𝑓2. Typically, only one of the two heterodynes is used and the other signal is filtered out. IF signal and the IF signal from scattered beam are separated into in-phase and quadrature signals in the I/Q demodulator, and the propagation direction of the measured fluctuations can be determined by analyzing the I/Q signals.
The old 16-channel electronics of the MIR system are used for the CSS electronics after expanding the video bandwidth from 1 MHz to 4 MHz.
Small scale turbulence study
Small scale turbulence for the KSTAR plasmas
Small scale turbulence driven by electron temperature gradient
As mentioned in Section 1.2.1, the small-scale turbulence driven by the ETG mode increases as the electron temperature gradient increases. To clearly verify whether the CSS indeed measured the high k turbulence, we have compared the changes of the CSS spectrogram by increasing the electron temperature gradient near the plasma core using two ECH sources. Due to 𝑘𝑘𝜃𝜃𝜌𝜌𝑒𝑒 =~0.12 and 𝑘𝑘𝜃𝜃𝜌𝜌𝑖
Although there is no change in the toroidal plasma velocity in the core region, the measured fluctuation becomes stronger in the electron diamagnetic direction as sotron ECH is injected as shown in Fig. 5.5. Looking at the ion temperature profile and the electron density profile, there is no significant change compared to the electron temperature profile. Also, by calculating the normalized scale length of 𝑇𝑇𝑒𝑒 at gauge radius (𝑅𝑅/𝐿𝐿𝑇𝑇𝑒𝑒=R/(𝑑𝑑ln𝑇𝑟𝑟𝑟𝑒) and comparing the total spectral power of the CSS spectrogram, it appears that there is a correlation between the two parameters.
Simultaneous measurement of ITG and TEM in H-mode plasma pedestal
In plasmas without the neutral beam injection (NBI), the ionization source is mainly peripheral, causing the particle flux in the core to disappear. With the collective scattering system, we observed density fluctuations where each wavenumber channel rotates differently in the ion or electron diamagnetic direction in an H-mode plasma. In CH.4 (~26 cm−1) and CH.3 (~20 cm−1), fluctuations appear to rotate in the diamagnetic electron direction (stronger amplitude in the negative frequency), but rotate in the diamagnetic ion direction (stronger amplitude in the positive frequency) in CH.2 (~18 cm−1) as shown in Fig.
5.8, the electron density gradient decreases after the impurity drop, but there is no significant change in the electron temperature. This may indicate that the fluctuation of the electron diamagnetic direction measured in CH.4 and CH.3 before the impurity is due to the TEM instability caused by the electron density gradient. Another possible interpretation is that the plasma poloidal rotation velocities in the TEM turbulence laboratory frame of different wavenumbers are not the same due to the difference in the motion speed 𝑬×𝑩𝑩.
The plasma rotation velocity of the poloidal direction in the laboratory frame is composed of 𝑬𝑬×𝑩𝑩 drift velocity and phase velocity of micro-turbulence (𝐯𝐯𝑙𝑙𝑎𝑎𝑙𝑙𝑙𝑝 𝐯𝐸𝐸×𝐵𝐵+𝐯𝐯𝑝𝑝ℎ𝑎𝑎𝑠𝑠𝑒𝑒). This means that if the 𝑬𝑬×𝑩𝑩 drift velocity is much larger than the phase velocity of micro-turbulence in electron diamagnetic direction, TEM can be observed in the ion diamagnetic direction. The CH.2, which has a larger scattering volume than CH.3 and 4, is measured in a region with probably a large 𝑬𝑬×𝑩𝑩 velocity, and fluctuation by TEM turbulence can rotate in the ion diamagnetic direction.
In the future, which interpretation is more appropriate will be verified through simulation results.
Summary and conclusions
Lee et al, "Design of a Collective Dispersion System for the Study of Small-Scale Turbulence in Advanced Superconducting Tokamak Research in Korea," Scientific Instruments Review. Smith et al, "A collective scattering system for measuring electron spin rate fluctuations at the National Spherical Torus Experiment," Review of Scientific Instruments. Chang et al, "Design of the neutral beam injection system for the KSTAR tokamak", Fusion Engineering and Design.
특히 프로그램이 끝나갈 무렵 제가 어려울 때 응원해주신 많은 분들께 감사하다는 말씀 전하고 싶습니다. 또한 박사님께도 감사의 말씀을 전하고 싶습니다. 나의 두 번째 고문이라 할 수 있는 이우창. 박사님께도 감사의 말씀을 전하고 싶습니다. 임준억 박사와 편안한 친구이자 가이드였던 김민호.
박사님께도 감사의 말씀을 전하고 싶습니다. 제가 대전에 적응할 수 있도록 세심하게 보살펴주신 김성국 기사님과 한종원 기사님. 박사님께도 감사의 말씀을 전하고 싶습니다. 이인근 박사와 울산에서 잘 챙겨준 류민우. 원상님과 주형님에게도 감사 인사 전하고 싶습니다.
그리고 오랫동안 인연을 맺어온 장경자님, 김미혜님에게도 감사의 말씀을 전하고 싶습니다.
![Fig. 3.6 a 3D design of MIR system and CSS in KSTAR E-port (top view) [18].](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10487160.0/38.893.135.764.149.369/fig-design-mir-system-css-kstar-port-view.webp)
