Research Center Plasma Research Articles

Instantaneous measurement of high-power millimeter-wave beam for 28 GHz gyrotron.

An instantaneous measurement system of high-power millimeter-wave was proposed and demonstrated with a 28 GHz gyrotron at the Plasma Research Center, University of Tsukuba. The high-power detector consists of an attenuator and a linear polarized microstrip antenna with an F-class load rectifier, which is a commonly used system for radio-frequency wireless power transmission. The detector obtained the power distribution of the gyrotron output beam which showed good agreement with the infrared camera image. The rectenna array detector received 45 W RF input power with a 0.4 ms response time. The results revealed that the proposed narrow band detector is useful as an imaging sensor and power meter for high-power millimeter-wave beam output with a wide wavelength range.

Development of 28 GHz Gyrotron for Cooperative ECH Study

At the Plasma Research Center at University of Tsukuba, development of megawatt gyrotrons is being performed as a collaborative electron cyclotron heating (ECH) study with some research organizations. A 28 GHz 1 MW 1 s gyrotron has been developed to upgrade the GAMMA 10/PDX ECH systems. To improve the oscillation efficiency in high current regions, the magnetron injection gun (MIG) of the 28 GHz gyrotron has been modified. Output power of 1.25 MW has been achieved with this gyrotron. For the first step of the collaborative research between Tsukuba University and Kyushu University, the Tsukuba 28 GHz gyrotron was adapted to the Q-shu University Experiment with Steady-State Spherical Tokamak (QUEST) ECH system, and the plasma heating and current drive effect were demonstrated. We obtained successful results, including an electron cyclotron–driven plasma current of 66 kA in the QUEST plasma experiment. For the next step of the collaborative research, the design targets of a 2 MW 3 s and 0.4 MW continuous wave have been achieved in a design study of a new 28 GHz gyrotron.

Development of 154 GHz 1 MW Gyrotron for ECRH of LHD

At the Plasma Research Center (PRC) in University of Tsukuba, development of Mega-Watt Gyrotrons is performed for fusion research. Development of a new 154 GHz 1 MW gyrotron for ECRH system of Large Helical Device (LHD) has been done. The design studies of electron gun (MIG), cavity oscillator, mode converter, transmission mirrors, collector and superconducting magnet (SCM) have indicated more than 1 MW output with efficiency ~33 % for cavity oscillation mode of TE28,8. In the short pulse test of the first test tube, the maximum output power of 1.06 MW and the maximum total efficiency of 35.2 % with CPD were obtained.

Plasma-Wall Interaction Facilities in Korea

Although the research of plasma-material interaction (PMI) is rather immature comparing the recent success of Korean fusion program, there are several facilities and programs of PMI research in Korea. DiPS (Divertor Plasma Simulator)-2 is a linear device with a four-inch-LaB6 cathode at the Center for Edge Plasma Science (cEps), concentrating on the development of various diagnostics for divertor and scrape-off plasmas, and for PMI research such as tungsten and graphite related phenomena. This is modified from DiPS-1, which were for the simulations of divertor, space and processing plasmas using LaB6 and helicon plasma sources. MP2 (Multi-Purpose Plasma) is a linear device with an eight-inch-LaB6 cathode for PMI in National Fusion Research Institute (NFRI), and will be merged with molten salt (FLiNaK) experiment by using an Electron Cyclotron Resonance (ECR) plasma source. High power plasma torch facilities have been developed at the High-Enthalpy Plasma Research Center in ChonBuk National University, aiming for the development of new materials of the aerospace-, nano-, and automobile-industries, yet recently they have interest in fusion materials. Plasma Immersion Ion Implantation & Deposition (PIIID) facility has been utilized for the research of processing materials in Korean Institute of Science and Technology (KIST), and is to be used for fusion material researches with high energy ions (~70 keV). Electron beam irradiation has been tried for the research of graphite and tungsten at DanKook university. These facilities are to be utilized for the application to KSTAR, ITER and/or Korean DEMO fusion devices. Dust as the by-product of PMI in fusion device is to be characterized and removed in TReD (Transport & Removal experiment of Dust) device in Hanyang University. Plasma sources, diagnostics, and surface analyses of these programs will be explained with design philosophies and basic parameters of plasmas.

First Results and Future Research Plan of Divertor Simulation Experiments Using D-Module in the End-Cell of the GAMMA 10/PDX Tandem Mirror

In the new research plan of Plasma Research Center of the University of Tsukuba, a high heat-flux divertor simulator (E-Divertor) was proposed by using an end-mirror exit of a large tandem mirror GAMMA 10/PDX device. Experiments for characterization of end-loss plasma flux have been extensively performed at the end-mirror region of GAMMA 10/PDX and detailed behavior of end-loss particles has been investigated. In standard hot-ion mode plasmas (ne0 ~ 2×1018 m-3, Ti0 ~ 5 keV), the energy analysis of ion flux was performed by using end-loss ion energy analyzer (ELIEA). It was found that the high ion temperature (100 - 400 eV) is generated and has a liner relationship between diamagnetism in the central-cell. The ion temperature determined from the probe and calorimetric measurements gives a good agreement with the ELIEA measurement. Additional ICRF heating in the anchor-cell showed a significant increase of particle flux, which indicated an effectiveness of additional plasma heating in adjacent cells. Superimposing the ECH pulse of 380 kW, 5 ms induces a remarkable enhancement of heat flux and a peak value in the net heat-flux density more than 10 MW/m2 was attained during the ECH injection, which comes up to the heat-load level of the divertor plate of ITER. Recently a large-scale divertor simulation experimental module (D-module) was installed in the west end-cell and the first plasma irradiation experiments onto a new tungsten V-shaped target were successfully performed. A number of interesting results such as neutral compression, enhancement of recycling and impurity radiation during noble gas injection, have been observed.

Divertor Simulation Study Using the GAMMA 10 End-Mirror Cell

In GAMMA 10 which is a tandem mirror machine in Plasma Research Center at University of Tsukuba, divertor simulation studies have been planned and started. This paper describes the recent results of divertor simulation experiment in the GAMMA 10 end-mirror cell. As a part of these experiments, characteristics of end-loss plasma have been investigated by using probes and calorimeters. The diagnostics are installed at two positions, which are separated by 40 cm in the axial direction. In the results, ion temperature of the end-loss flux in GAMMA 10 is much higher than that of other divertor simulators. It is found that heat-flux density can be controlled within the range from 0.4 to 0.8 MW/m 2 by changing the ICRF power. In addition, heat flux density has strong dependence on diamagnetism in the central-cell which is time integrated in the plasma duration. While, particle-flux density is proportional to electron line-density in the central-cell. Particle and heat fluxes measured at axially different positions agree well with the calculation results for considering the influence of particle reflection phenomena. c

Development of Mega-Watt Gyrotrons for Fusion Research

At the Plasma Research Center (PRC) in University of Tsukuba, development of Mega-Watt Gyrotrons is performed for fusion research. We are developing a new 28 GHz 1 MW and a 77 GHz 1.5 MW gyrotron for ECRH system of tandem mirror GAMMA10 and Large Helical Device (LHD), respectively. In the short pulse test of 77 GHz gyrotron, the maximum output power of 1.6 MW and the maximum total efficiency of 49.4% with CPD were obtained. In the long pulse test, the pulse length extended to 5 sec. with 1 MW and 4500 sec. with 0.2 MW. The design study of 154 GHz 1MW gyrotron for LHD has been started. For each cavity oscillation mode, TE28,8, TE28,12 and TE31,8, cavity, electron gun (MIG), mode converter, collector and SCM design are being examined.

Research Plan for Divertor Simulation Studies and Its Recent Results Using the GAMMA 10 Tandem Mirror

As the new research plan of Plasma Research Center of the University of Tsukuba, we are planning to start a study of divertor simulation under the closely resemble to actual fusion plasmas environment making an advantage of the GAMMA 10 tandem mirror and to contribute the solution for realizing the divertor in future toroidal systems. In the research plan, the concepts of two divertor devices are introduced. One has an axi-symmetric divertor configuration with separatrix (A-Div.) and the other is a high heat flux divertor simulator by using an end-mirror exit of the large tandem mirror device (E-Div.). Preparative experiments have been successfully started at the end-mirror region of GAMMA 10 and detailed behavior of end-loss particles has been investigated by using newly developed diagnostic instruments. In standard hot-ion mode plasmas (ne0 ~ 2 × 1018 m-3, Ti0 ~ 5 keV), the heat flux density of 0.8 MW/m2 and the particle flux density of 4 × 1022/s·m2 were observed at 30 cm downstream of the end-mirror exit on the machine axis. It is confirmed that the heat flux density increases in proportion to the applied RF power. Superimposing the ECH pulse induces a remarkable enhancement of heat flux and a peak value in the net heat flux density of 8 MW/m2 was attained during the ECH injection, which almost comes up to the heat load level of the divertor plate of ITER. Two-dimensional visible image measurement of newly installed target plates using high-speed camera revealed a significant difference in the behavior of visible emission from plasma-material interaction. The above results give a clear prospect of generating the required performance and providing useful information for divertor studies in GAMMA 10.

Research plan for divertor simulation making use of a large tandem mirror device

We are planning to start a study of divertor simulation under the closely resemble to actual fusion plasma environment making use of the advantage of open magnetic field configuration and to contribute the solution for realizing the divertor in ITER as a future research plan of Plasma Research Center of the University of Tsukuba. In the research plan, the concepts of two divertor devices are introduced. One has an axi-symmetric divertor configuration with the separatrix which is similar to toroidal divertor of torus systems and the other is a high heat flux divertor simulator by using an end-mirror exit of the existing tandem mirror device. Development of magnetic field configuration for ensuring the MHD stability is under way and a designed example is investigated under the optimal condition for plasma production. Consideration of plasma heating scheme using Fokker–Planck simulation code was successfully performed at both axi-symmetric divertor and end-mirror regions. Preparative experiments using calorimeter, Mach probe and high-speed camera have been started at the end-mirror region and the heat flux density of the level in 1–10 MW m −2 was achieved in standard hot-ion mode plasma-confining experiments, which gives a clear prospect of generating the required heat flux density for divertor studies.

ＧＡＭＭＡ １０タンデムミラーにおけるクライオポンプシステムとその真空及びプラズマ特性改善への効果

A large-scale cryopumping system using low-temperatute (3.5 K) liquid helium (LHe) was installed in an axisymmetrized tandem mirror GAMMA 10 at the Plasma Research Center, University of Tsukuba, in order to obtain a clean vacuum condition with a larger pumping speed. This paper describes the detailed design of the system and the estimation of the improvement of the vacuum state and of the effectiveness on the plasma confinement. Four cryopumps are located in the end-mirror tanks and in the NBI tanks of the anchor regions on both sides. Performance tests of the cryopump with a set of simple pressure-balance equations have confirmed that the measured pumping speed agrees well with the designed value. Use of this pumping system has made it possible to reduce the base pressure in the plug/barrier to below 1×10-6 Pa and upgraded thermal-barrier experiments were succesfully carried out under the improved vacuum condition.

GAMMA 10 vacuum system and study of wall conditions

GAMMA 10 is an axisymmetrized tandem mirror with thermal barriers located at the Plasma Research Center, University of Tsukuba. Good vacuum conditions required for the formation and sustainment of the plug‐thermal‐barrier potentials are obtained with Ti‐deposited liquid‐nitrogen‐cooled panels with a pumping speed of 1.9×106 l/s and a careful wall conditioning with ion cyclotron resonance heating plasma shots. One characteristic of the pumping system is that the titanium gettering is used not in the plasma confinement regions but in separate vacuum regions such as the end regions. More than 300 wall conditioning shots are required to obtain good wall conditions. The progress of the wall conditioning is monitored by comparing the pressure at 40 ms after the start time of the plasma discharge for cases with and without plasma shots. Studies of gas dynamics indicate a long sticking time of hydrogen of the order of 10 ms on the wall surfaces. The speed of the plasma pumping in the central region is estimated to be 4–6.5×105 l/s. Because of the plasma pumping, the required vacuum conditions are attained without large‐pumping‐speed pumps in the central region.

Physics in Open-Ended Fusion Systems: Report on the International Symposium at the University of Tsukuba, Ibaraki, Japan 15–18 April 1980

The Symposium was organized by the Plasma Research Center, University of Tsukuba, with the support of the Fusion Research Association of Japan and the Foundation for Advancement of International Science. It was attended by about 100 participants from 11 countries, among whom 59 came from Japan, 22 from the USA, and two from the USSR. Brazil, the Federal Republic of Germany, France, India, Italy, the Netherlands, Norway, and Sweden were represented by one participant each.