디버터영역 플라즈마 진단을 위한 레일리 및 톰슨 산란 연구

Abstract

ITER를 포함한 미래의 토카막 핵융합로에는 안정적인 핵융합로 구동을 위하여 타겟의 열부하 감소와 중성입자들을 배출시키거나 내벽으로 흡수시키기 위한 폴 로이달 자장 디버터가 사용될 것으로 예상한다. 이러한 디버터를 사용함에 따라 발생하는 'H-모드'와 '분리된 디버터'에 대한 고려가 안정적인 핵융합로 구동에 필요하다. 이에 따라 안정한 핵융합로 구동을 위해서는 분리된 디버터 영역 확립 이 중요하다. 그러나 H-모드에서 SOL 플라즈마로 들어오는 강한 열 플럭스는 분리된 플라즈마를 뚫고 자기 유체 역학적 불안정성에 의해 유발되는 edge localized modes (ELMs)가 발생하는 동안 디버터 타겟에 심각한 피해를 줄 수 있다. ELMs을 분석하기 위해서는 디버터에 존재하는 중성입자밀도를 포함한 전 자온도와 전자밀도에 대한 정보가 필요하다. 디버터에 존재하는 중성입자에는 PFCs 재료 자체뿐만 아니라 플라즈마 종(수소 중성입자 및 불순물 포함)이 있다. 폴로이달 자장 디버터를 사용하는 핵융합로를 안정적으로 구동시키기 위해서는 디버터 영역의 전자온도, 전자밀도, 수소 원자 와 분자를 포함한 중성입자밀도 측정이 필요하다. 현재, 전 세계에서 연구로에서 디버터 영역의 전자온도는 ∼ 1.0 eV이고 중성입 자밀도는 1016 - 1020 m -3 정도이다. 따라서 ITER 디버터 영역에서 중성입자밀도 는 1/3-1/5 적고 전자온도는 이와 유사할 것으로 예상하고 있다. 이를 진단하고 자 ITER 위원회에서는 톰슨산란 진단계와 레이저유도형광(laser induced fluorescence)를 도입하기로 결정하였다. 디버터 영역에서 예상되는 중성입자밀도, 이온온도를 측정하기 위한 LIF에는 355 nm YAG 펌핑 dye laser를 이용하는 방향이 결정되었다. 그러나 line scanning에 필요한 흡수 스펙트럼의 수를 최적화하는데 어려움이 있다. 디버터 타겟 부근의 낮은 전자온도조건을 디버터 톰슨산란진단계로 진단하기 위하여 높은 펄스에너지를 갖는 레이저와 신호처리가 빠르고 신호감도가 좋은 분광기 등 S/N를 향상시키기 위한 다양한 연구가 진행되고 있다. 톰슨산란진단법과 같은 광 진단법은 플라즈마의 전자온도가 증가할수록 청색편 이 증가하고 스펙트럼이 넓어진다. 따라서 톰슨산란진단계를 구성하는 분광기에 는 band-pass filter를 사용하여 APD로 입사하는 각 채널영역의 파장신호를 측 정한다. 이러한 필터는 스펙트럼의 일부를 선별적으로 투과시키기 때문에 투과 파장범위가 고정되어있다. 또한 투과파장 외에 주변파장을 통과시켜서 채널과 채 널이 겹치는 경우가 발생한다. 이에 다양한 전자온도와 세밀한 전자온도 측정에 어려움이 있다. 본 연구에서는 낮은 전자온도를 갖는 ITER 디버터 플라즈마의 전자온도, 전자 밀도 그리고 중성입자밀도를 측정하기위하여 레일리산란을 이용한 수소 중성입 자밀도진단과 톰슨산란진단계에서 사용되는 분광기의 band-pass filter의 단점을 보완할 회절격자 분광기에 대하여 연구하였다. 레일리 산란을 이용한 수소 중성입자밀도 진단법에 대하여 연구하고자 OPO 레 이저와 13.56 MHz RF CCP 플라즈마소스, PMT를 이용하였다. 이러한 장치들을 사용하여 파장에 따른 레일리산란특성을 파악하고 챔버 내부압력에 따라 레일리 산란 신호를 측정하여 전자온도가 낮은 수소플라즈마 내 중성입자밀도를 진단하 였다. 실험을 통하여 도출된 레일리산란을 이용한 중성입자밀도진단법은 ITER 디버 터 내 중성입자 밀도 진단에 사용될 LIF에서 사용되는 dye laser의 흡수 스펙트 럼에 대한 고려 없이, 단일 입사파장으로 레일리 산란을 이용하여 중성입자밀도 측정이 가능할 것으로 판단된다. 회절격자를 이용한 분광기에 대하여 연구하고자 300 grooves/mm 회절격자와 광섬유 빔스플리터, APD를 이용하여 분광기를 구성하였다. 이를 바탕으로 텅스 텐 램프를 이용하여 채널 간 겹침 해결방법과 미세 전자온도측정가능성에 대하 여 연구하였다. 실험을 통하여 도출된 결과를 보아, 회절격자 분광기가 기존 분광기에서 사용 되는 band-pass filter의 한계인 투과파장 고정과 주변파장 투과로 인한 한정된 전자온도 측정범위 및 채널 간 겹침 문제를 해결하는데 활용할 수 있음을 확인 했다. 그리고 세밀한 회절격자의 각도 조절을 통하여 각 채널의 파장영역을 미세 하게 변화시켜 세밀한 전자온도 측정이 가능함을 확인했다.

ITER is a scale-up equipment based on the results of several fusion research reactors around the world, such as KSTAR, DIII-D, NSTX, EAST, ASDEX. Including ITER and various researches, the tokamak is thought as main type of reactor for achieving the fusion reaction.  In order to achieve stable fusion, detailed diagnostic studies of the plasma state are necessary. To understand the plasma state more precisely, researchers are dividing the inside of the fusion reactor into CORE, EDGE, and DIVERTOR and diagnostic tools on each sectors are developed for many plasma composing components, such as ions, electrons, and neutral particles. The poloidal magnetic diverters used are expected to be used in the future Tokamak fusion reactors, including ITER. In Tokamak with poloidal diverter, both a high-recycling regime of Divertor and the H-mode which improve core plasma confinement were found. One of the main issues on divertor is the reduction of thermal flux (up to 10 MW). Plasma near the divertor is composed PFCs material, hydrogen atom and molecule etc (netural particles). The main processes in divertor region of the radiative divertor are impurity radiation loss, the plasma-neutral interactions and the recombination of the plasma. When some conditions of the heat flux coming into SOL from core, the impurity radiation loss in the SOL and divertor regions and the upstream plasma parameters are satisfied, Ultimate detachment is possible. However, in H-mode the magnitude of the heat flux coming into SOL can change vary largely in time due to bursts of energy and particle loss from the core associated with excitation of ELMs driven by magnetohydrodynamics instabilities. ELMs can break the detached divertor by heat flux of come into SOL. So, the control of ELMs is related to detached divertor stability. Article by S.I. Krasheninikov [1], in low temperature plasma, the inequality of the effective ion-neutral drag force causes the reduction of the plasma flux to the divertor target and the plasma detachment occurs. However, both the impurity radiation loss and plasma recombination are responsible for the reduction of the plasma flux to the divertor targets, by ion-neutral interaction is crucial to balance the high pressure further upstream from the recycling region. Therefore, the physical effect of hydrogen neutral particle takes crucial role in recycling regime. The electron density, the electron temperature and hydrogen neutral density are important parameters in divertor diagnostics. Thomson scattering is the only method to measure the absolute electron temperature and electron density both. LIF and DTS (Divertor Thomson scattering) are the method to measure the neutral density based on CRM(collisional radiative-model). However, Thomson scattering diagnostics is difficult to distinguish the signal from noise due to the low electron temperature in the Divertor, LIF introduced in ITER recently, is not easy to optimize the number of laser absorption wavelengths. In order to identify the boundary conditions in the Divertor at low temperature, this thesis studied both the polychromator with the grating to measure the signal of Thomson scattering and the measurement of hydrogen neutral density using Rayleigh scattering with several wavelength of OPO laser (optical parametric oscillator laser). Rayleigh scattering is the result of interaction between the incident light with specific wavelength and the particles of smaller size than the wavelength. Therefore this scattering is possible to measure the density of hydrogen atom which comes from the recombination of hydrogen ion and electron at low electron temperature condition in the Divertor. The cross-section of the scattering is inversely proportional to the square of the wavelength. xi In this study, first we calculated the theoretical intensity and cross-section of Rayleigh scattering using both the emission spectrum of hydrogen CCP(capacitively coupled plasma) and the operating wavelength range of OPO laser to avoid the interference with Rayleigh scattering. The output of Rayleigh scattering in UV range showed similar or stronger intensity despite the intensity of OPO (visible to NIR) output is up to 40 times weaker in theoretical calculation. Usually Rayleigh scattering is advantageous for short wavelengths. Rayleigh scattering can be stronger although the weak power of OPO laser at UV wavelength. However, UV was not used in the experiment because it has higher energy (5.07 eV at 245 nm) than the binding energy (4.52 eV) of hydrogen molecules and the excitation energy (13.6, 3.4, 1.5 eV) of hydrogen atom. So, the wavelengths of 440, 532 nm were chosen to measure the neutral density of CCP type hydrogen plasma at 22 - 95 mTorr of pressure. In the RF hydrogen neutral density CCP, the minimum hydrogen atom density, was calculated as 1.31 × 1019 m-3 when Hydrogen neutal density was 7.29 × 1020 m-3 at the pressure of 22 mTorr. This value is similar than neutral density of ITER divertor. To use Thomson scattering diagnostics, it is required both a high-energy pulse laser and a polychromator to measure the wide wavelength range. Polychromator is comprised of band-pass filter to separate the wavelength ranges, and APD to measure very weak signal of Thomson scattering. These filters have the fixed wavelength ranges. It is not suitable to detect wide range of electron temperature. In this study, the polychromator with diffraction grating was develop in order to overcome the disadvantage of the fixed transmitting wavelength range. The polychromator using a diffraction grating, it is composed 3 major process: dispersion, separation and detection. The main device of dispersion part is the 300 grooves/mm grating. it was chosen by considering the size of optical component and polychromator. Three methods have been studied to separate the dispersed wavelength image into several channels(the separation part). These are the beam splitter using reflection/transmission, the reflective beam splitter and the plastic fiber beam splitter. Among them, a plastic fiber beam splitter was used due to easy in focusing the image and efficiency of wavelength selection. When the wavelength range was divided into 150 - 160 nm, 40 nm was overlapped. This value was obtained by installing a block. It is lower than the 100 nm overlap of wavelength before the block is installed. The low intensity peak was shown at 900 nm, which was caused by the material properties of the plastic optical fiber used for channel separation. The detection part which measures the wavelength signal of each channel, is composed of APD, power supply and DAQ. This part can detect the integrated plasma emission spectrum of each channel using APD with signal sensitivity higher than CCD. In conclusion, the plasma diagnostics using Rayleigh and Thomson scattering in ITER divertor was studied. the results are mention below: First part, the effect of wavelength on Rayleigh scattering were studied in this thesis. The hydrogen neutral density was measured by Rayleigh scattering using 440 and 532 nm. When comparing the two results, almost identical values were obtained. The usefulness of 440 nm, incident wavelength in Rayleigh scattering was confirmed environment for plasma diagnostics. The density of hydrogen atom was obtained as 1.31 × 1019 m-3 when 7.29 × 1020 m-3 of neutral density at 22 mTorr by Rayleigh scattering. Second part, Polychromator for Thomson scattering system has fabricated using the diffraction grating to overcome the disadvantage of fixed band-pass filter. By changing the wavelength range of channel, the details of electron temperature can be measured with the polychromator.