Vertical displacement events and halo currents

Abstract

The need for high I3 ( -i reactor economy) and high plasma currents ( --f good confinement)under the consmint of the safety factor at the plasma boundary % > 2 c a b for vertically elongated plasma cross-sections in a tokamak reactor, which are inherently unstable to vertical displacements. Furthermore. the creation of divertor configurations allowing particle and energy exhaust with reactor relevant poloidal field coil configurationsusing superconducting coils means that the main shaping coils have to be installed outside the toroidal field coilsalso leads to elongated plasma shapes.One of the main research areas of present divertor tokamaks is the position, shape and disruption control of noncircular plasmas withas far as possiblereactor relevant poloidal field coil configurations allowing merent divertor configurations ( S N single null D N double null) with an inherently large divertor region (figure 1). Passive structures close to the plasma are necessary to slow down the vertical plasma displacement which is unstable on the fast Alfven time scale by means of induced currents in these structures (section 2). The vertical position instability, now on the slower L/R time scale of the passive structure, can then be handled by active fast position control systems, these being increasingly designed as prototypes for a nextstep device [I]. Nevertheless, loss of control can occur (section 3). resulting in a "vertical displacement event" (VDE), first observed in JET discharge #1947 in 1984 121.The dangers associated with these M E s and the intimately linked plasma disruptions are twofold. Firstly, thermal loads are exerted on target plates and wall segments coming from the thermal plasma and magnetic energies. Secondly, electromagnetic forces act on structures resulting from the liberation of the magnetic self energy of the toroidal plasma current $ , and of the interaction energy of I , , with the external quadrupole currents needed for elongated plasma crosssections [;?-SI. This interaction energy drives the vertical instability and generates vertical $3 forces on the passive structures and their supports amounting to as much as 350 tons in JET, or vessel movements of up to 2 mm [3,4]. A substantial fraction of these forces can be due to halo currents, which are caused by the flux changes during VDEs and flow outside the closed flux surfaces in the scrape-off layer (SOL) closing through plasma facing vessel components (section 4). These halo currents also exert deforming forces on the vessel itself and the plasma facing invessel components which are large enough to damage these structures [4]. Arcing due to the voltages induced during fast plasma movements is a third cause of severe damage observed 1471.This review examines results from all non-circular tokamaks with a distinct emphasis on investigations in ASDEX-Upgrade. There a major fraction of the experimental time has been dedicated srudying VDEs of SN plasmas over a large range of q-values (section 5 ) in an attempt to obtain the scaling of both the displ...

Analysis of Electromagnetic Forces in Tokamak Vacuum Chamber Due to Induced Poloidal Current After Thermal Quench

“…11 -радиального и вертикального компонентов электромагнитной нагрузки в вакуумной камере Т-15МД. Интегральные электромагнитные нагрузки в вакуумной камере определяются в соответствии с соотношением (1). Электромагнитные нагрузки на каждом проводящем элементе вакуумной камеры определяются с использованием соотношений (6) и (11).…”

“…Большой срыв в плазме токамака начинается с быстрого (порядка 0,1-1 мс) выброса из неё тепловой энергии (TQ -Thermal Quench), после чего начинается неуправляемое движение плазмы по вертикали (VDE -Vertical Displacement Event), сопровождаемое снижением плазменного тока (CQ -Current Quench) за счёт внезапно возросшего после TQ электрического сопротивления плазмы [1][2][3][4].…”

High resolution temperature and density profiles during the energy quench of density limit disruptions in Rijnhuizen tokamak project

“…Nonetheless, a detailed physical understanding of their time evolution has still not been fully accomplished. 2 Experimentally, [3][4][5][6][7][8][9][10][11] the difficulties in probing the disruption are mainly connected with its extremely short duration ͑mi-croseconds͒, unpredictable onset and 3D structure. Also, except for electron temperature and density, local plasma parameters are difficult to measure, or are not measured at all.…”

Analysis of Plasma Evolution During Vertical Plasma Disruption in Thе Modified Tokamak T-15

“…Термоядерный синтез, 2012, вып. 4 Условно явление VDE можно разделить на два типа [2] по моменту начала быстрой диссипации те-пловой энергии плазмы -теплового срыва, в результате которого происходит падение плазменного давления практически до нуля. К первому типу VDE обычно относят явление, при котором сам тепловой срыв является инициатором неуправляемого движение плазмы по вертикали, -так называемое «холод-ное» VDE, которое ещё принято называть «большой срыв» (MD -Major Disruption).…”