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temperature and potential (among other quantities) to be equal to that of the limiter. Plasma that escaped the LCFS would do so with no preferential direction, potentially damaging instruments. By establishing an X-point and separatrix, the plasma edge is uncoupled from the vessel walls, and exhausted heat and plasma particles are preferentially diverted towards a known region of the vessel near the X-point.
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During the 1960s a number of different methods were used to try to address these problems. Generally they used a combination of several magnetic fields to cause the net magnetic field inside the device to be twisted into a helix. Ions and electrons following these lines found themselves moving to the
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that intersects with the X-point is called the separatrix, and, as all flux surfaces external to this surface are unconfined, the separatrix defines the last closed flux surface (LCFS). Formerly, the LCFS was established by inserting a material limiter into the plasma, which fixed the plasma
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In the 1980s, further research along these lines demonstrated that further advances were possible by using external current-carrying coils to make the lines not just helical, but non-symmetric as well. This led to a series of experiments using C and D-shaped plasma volumes..
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By increasing the current in one (or more) shaping coils to a high enough degree, one (or more) 'X-points' can be created. An X-point is defined as a point in space at which the poloidal field has zero magnitude. The magnetic
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travel around the torus at high velocities. However, as the circumference of a path on the outside of the plasma area is longer than one on the inside, this caused several effects that disrupted the stability of the plasma.
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Early fusion reactor designs tended to have circular cross-sections simply because they were easy to design and understand. Generally, fusion machines using a toroidal layout, like the
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In the simple case of a plasma with up-down symmetry, the plasma cross-section is defined using a combination of four parameters:
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is the study of the plasma shape in such devices, and is particularly important for next step fusion devices such as
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inside and then outside of the plasma, mixing it and suppressing some of the most obvious instabilities.
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the angle between the horizontal and the plasma last closed flux surface (LCFS) at the high field side.
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The negative triangularity tokamak: Stability limits and prospects as a fusion energy system. Medvedev
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In general (no up-down symmetry), there can be an upper-triangularity, and a lower-triangularity.
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The negative triangularity tokamak: stability limits and prospects as a fusion energy system
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devices, and therefore one can completely define the shape of the plasma by its
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240:{\displaystyle \kappa ={b \over a}}
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