The study of control of "partial" detached plasma and heat transport

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The most attractive feature of the divertors has been the expected capability to create a dense and cold detached plasma with low neutral back flow to the main plasma. However, a loss of plasma pressure often only occurs along a narrow flux surface region of a few millimeters width and often occurs only near the separatrix. This partially detached plasma is produced in a narrow zone between the high and low temperature divertor regions. The partially detached plasma is a very complex phenomenon with atomic and molecular collision processes. It is difficult to control the detached and partially detached plasmas by commonly used diagnostics, such as heat loads, Langmuir probes, etc.We have developed a new way to stably maintain a detached plasma based on feedback control of the negative hydrogen ion density n_<H^-> in the linear divertor plasma simulator, TPD-SheetIV. A small amount of secondary gas puffing strongly reduced heat load of target plate Q and rapidly increased n_<H^-> in the periphery of the plasma without increased radiative and three-body recombination. The detached plasma is steadily maintained in the region of the target plate by rapidly varying gas flow ratio G so as to maximize the value of n_<H^-> and keep gas pressure P constant. The new system has achieved the goal of reducing both Q and amount of G in a detached plasma.