We report recent experimental results from HL-2A and KSTAR on ELM mitigation by supersonic molecular beam injection (SMBI). Cold particle deposition within the pedestal by SMBI is verified in both machines.The signatures of ELM mitigation by SMBI are an ELM frequency increase and ELM amplitude decrease.These persist for an SMBI influence time τI. Here, τI is the time for the SMBI influenced pedestal profile to refill. An increase in f SMBI ELM /f 0 ELM and a decrease in the energy loss per ELM WELM were achieved in both machines. Physical insight was gleaned from studies of density and vφ(toroidal rotation velocity) evolution, particle flux and turbulence spectra, divertor heat load. The characteristic gradients of the pedestal density soften and a change in vφwas observed during a τI time. The spectra of the edge particle flux Г~ < ˜vr˜ne> and density fluctuation with and without SMBI were measured in HL-2A and in KSTAR, respectively. A clear phenomenon observed is the decrease in divertor heat load during the τI time in HL-2A. Similar results are the profiles of saturation current density Jsat with and without SMBI in KSTAR. We note that τI/τp (particle confinement time) is close to ~1, although there is a large difference in individual τI between the two machines. This suggests that τI is strongly related to particle-transport events. Experiments and analysis of a simple phenomenological model support the important conclusion that ELM mitigation by SMBI results from an increase in higher frequency fluctuations and transport events in the pedestal.
Since the successful first plasma generation in the middle of 2008, three experimental campaigns were successfully made for the KSTAR device, accompanied with a necessary upgrade in the power supply, heating, wall-conditioning and diagnostic systems. KSTAR was operated with the toroidal magnetic field up to 3.6 T and the circular and shaped plasmas with current up to 700 kA and pulse length of 7 s, have been achieved with limited capacity of PF magnet power supplies. The mission of the KSTAR experimental program is to achieve steady-state operations with high performance plasmas relevant to ITER and future reactors. The first phase (2008–2012) of operation of KSTAR is dedicated to the development of operational capabilities for a super-conducting device with relatively short pulse. Development of start-up scenario for a super-conducting tokamak and the understanding of magnetic field errors on start-up are one of the important issues to be resolved. Some specific operation techniques for a super-conducting device are also developed and tested. The second harmonic pre-ionization with 84 and 110 GHz gyrotrons is an example. Various parameters have been scanned to optimize the pre-ionization. Another example is the ICRF wall conditioning (ICWC), which was routinely applied during the shot to shot interval. The plasma operation window has been extended in terms of plasma beta and stability boundary. The achievement of high confinement mode was made in the last campaign with the first neutral beam injector and good wall conditioning. Plasma control has been applied in shape and position control and now a preliminary kinetic control scheme is being applied including plasma current and density. Advanced control schemes will be developed and tested in future operations including active profiles, heating and current drives and control coil-driven magnetic perturbation.
A beam emission spectroscopy (BES) system based on direct imaging avalanche photodiode (APD) camera has been designed for Korea Superconducting Tokamak Advanced Research (KSTAR) and a trial system has been constructed and installed for evaluating feasibility of the design. The system contains two cameras, one is an APD camera for BES measurement and another is a fast visible camera for position calibration. Two pneumatically actuated mirrors were positioned at front and rear of lens optics. The front mirror can switch the measurement between edge and core region of plasma and the rear mirror can switch between the APD and the visible camera. All systems worked properly and the measured photon flux was reasonable as expected from the simulation. While the measurement data from the trial system were limited, it revealed some interesting characteristics of KSTAR plasma suggesting future research works with fully installed BES system. The analysis result and the development plan will be presented in this paper.
A 280 GHz single-channel horizontal millimeter-wave interferometer system has been fabricated and installed for plasma electron density measurements on the Korea Superconducting Tokamak Advanced Research (KSTAR). A retractable cassette system has been adopted for deep positioning of the interferometer system on large cryostat. The cassette system contains a pneumatic vacuum window shutter and a beam focusing module. The focusing module consists of antennas and aluminum concave mirrors, where an incident beam is reflected on a specially designed carbon inner-wall tile. The module enhances receiving beam power and reduces phase errors due to unexpected beam reflections on a vacuum vessel. Microwave components such as oscillators and mixers are located 2 m away from cryostat with a shielding box. Intermediate frequency signals generated by mixers are transmitted to a diagnostics room, and the phase difference between these signals is measured using a multifringe counting phase comparator. A beam path analysis has been performed for a triangular beam path geometry. An effective line-integrated density can be deduced from measured line-integrated density with these results. A beam path length error due to plasma refraction effect has been determined with various plasma conditions.
One of the main diagnostic tools for measuring electron density profiles and the characteristics of long wavelength turbulent wave structures in fusion plasmas is beam emission spectroscopy (BES). The increasing number of BES systems necessitated an accurate and comprehensive simulation of BES diagnostics, which in turn motivated the development of the Rate Equations for Neutral Alkali-beam TEchnique (RENATE) simulation code that is the topic of this paper. RENATE is a modular, fully three-dimensional code incorporating all key features of BES systems from the atomic physics to the observation, including an advanced modeling of the optics. Thus RENATE can be used both in the interpretation of measured signals and the development of new BES systems. The most important components of the code have been successfully benchmarked against other simulation codes. The primary results have been validated against experimental data from the KSTAR tokamak.
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