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1. INTRODUCTION1.7.4.1 SawtoothWhen the central value of the safety factor q ≈ rB T /RB p (B T and B p are the toroidaland poloidal magnetic field components, r and R are the minor and major radius ofthe tokamak torus, respectively) falls below unity; relaxation oscillations are normallyobserved in the core of a tokamak plasma. They appear on a number of plasma parametersbut are particularly evident in the central electron temperature T e (0) (9). Theoscillation in T e (0) exhibit a time trace with a distinctive sawtooth shape consisting ofa slow rise during which the plasma inside q = 1 heats up, followed by a rapid collapsewhen the plasma energy is redistributed from the core to the region outside q = 1.This then propagates as a heat pulse to the plasma periphery. This mechanism has theeffect of degrading the global energy confinement time.1.7.4.2 Edge Localized ModesThe transition from L-mode to H-mode in magnetic confinement systems is normally accompaniedby appearance in the H-mode phase of periodic edge instability phenomena,known as ELMs. The ELM is a relaxation oscillation triggered by an MHD instability,which leads to a fast (ms) loss of particles and energy from the plasma edge. Theunderlying cause for ELMs is the onset of MHD instability in the plasma edge whenthe edge pressure gradient exceeds a critical threshold. The subsequent loss of edgeconfinement leads to a temporary reduction of the pressure gradient, and the eventualrecovery of the pressure gradient leads to recurrence of the ELM. This cycle continuesindefinitely in a sustained H-mode discharge. The energy loss due to ELMs causes areduction of the global energy confinement time.At least three major types of ELMs have been defined (10). In a given experiment,the level of the plasma heating power, P , or more directly the net power reaching theplasma edge P edge = P in − P rad (i.e. heating power minus radiation losses inside theedge region) is a key factor in determining the ELM type. A number of types of ELMwith different amplitude, frequency and power dependences can be distinguished (seefigure 1.7):• Type I ELMs (giant ELMs) with high amplitude and low frequency, which developwhen the edge power flow significantly exceeds the H-mode threshold.12
1.7 Plasma Operational LimitsFigure 1.7: Divertor region D α intensity in a typical plasma showing the characteristicsof different types of ELMs. The figure is taken from Ref. (3).• Type II ELMs (grassy ELMs) are associated with strongly shaped tokamaks athigh edge pressure.• Type III ELMs (small ELMs)with small amplitude and high frequency appearwhen the power flow to the plasma edge is only marginally above the H-modepower threshold.Other improved regimes have also been observed besides the H-modes. Regimeswith core or Internal Barriers (ITBs), see figure 1.6, have been discovered that lead tosignificant enhancements in confinement and plasma performance. Transport barriersassociated with weak or negative shear have been observed on all of the large tokamaks:TFTR, DIII-D, JET and JT-60U. Other classes of improved confinement regimes areregimes without and with edge radiation. Examples of the former are given in reference(3) and the latter include the RI-mode of TEXTOR which will be discussed in thefollowing section.1.7.5 RI-modeThe duration of a burning fusion plasma will depend largely on the properties of theedge plasma being in contact with the wall elements. The first wall has to withstandand exhaust the α-particle heating power, and the helium ash must be removed fromthe plasma. Wall erosion will affect the lifetime of wall elements and impurities arereleased into the plasma, which then can cause fuel dilution and power loss owing to13
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9. DISCUSSIONthat the impurity dens
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9. DISCUSSIONthe plasma distributio
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REFERENCES[21] Weiland J. Collectiv
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REFERENCES[73] Neuhauser J. et al.
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PhD and MPhil Thesis Classes - UniversitÃ© Libre de Bruxelles

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