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The above-made observations are summarised and further developed with respect to the physical background.

The interaction zone in the nozzle vicinity is initially formed outside the separatrix in the SOL. The gas valve is opened, a neutral gas front arrives at the plasma edge with a delay time (latency) given by the nozzle-plasma distance and the respective neutrals velocity. A disc-shaped interaction zone is formed with about 10 cm diameter according to the angular jet divergence. The zone is of about 3.5 cm radial extension. The arriving atoms are ionised within an ionisation mean free path and the density rises, which causes a decrease in penetration depth of subsequently arriving neutrals. In parallel, the temper- ature is dropping in this narrow layer by atomic processes like radiation losses. The fast influx of energy along magnetic field lines due to the electron heat conduction counteracts the energy loss. If the density rise is strong and the temperature decrease small, then the pressure in the interaction zone may increase and it starts to expand along field lines as it is the case in cryogenic hydrogen pellet injections. The expansion speed is weakly or even independent on reservoir pressure as depicted by Figure VI.4. The same independence applies to the thermal plasma energy or the impurity ionisation energy. Also an impurity mass dependency is absent. At a first glance, at least the latter is astonishing due to the inverse square root mass dependency of the ion velocity. We focus on this issue below.

Pressure gradient driven B-parallel transport:

Assuming that the particles are travelling along magnetic field lines, the distance between the measurement positions isL= 5.8 m. Several later examples substantiate this assump- tion (albeit only in the initial response sequence). We consultXUV data from three shut downs of comparable Eth content but different gases and infer the toroidal cooling front

expansion velocity by vhel =τtor/ L: vAr hel = 25.2kms (#24420) vNe hel = 27.6kms (#24491) vHe hel = 36.3kms (#24398)

The velocities do not differ dramatically from each other. The question is, what is the ionisation degree. In case of no ionisation, one expects a mass dependent relation (vatom ∝ √Matom

−1

). Contrarily, ionisation produces an electron pressure rise. We as- sume that it is more significant than the temperature decrease, because we regard the very beginning of the shut down scenario, in which the energy supply into the zone is well sustained (temperature is sufficient for providing ionisation energy). And we assume that the pressure in the interaction zone interior is much higher than the background pres- sure. As a consequence, theB-parallel impurity flow is driven by the electron pressurepe.

The impurity ions evolve with sound velocity helically inB-parallel direction. The sound velocity cs in a plasma of electrons and ions is given as [4]

cs = r

γpe + pi

ρ (VI.6)

We claim that impurities are majorities such that the contribution of hydrogen is negli- gible. The number of chargesnz of the charge stage z and temperature Tz determine the

sound velocity of the ion-electron fluid as

cs = s

γnzTz + (nz· z)Te nzmz

, (VI.7)

where we presuppose Debye shielding. With Tz = Te = T, which is fulfilled in a dense

plasma of short collision times, nz cancels and we write cs =

s

γ(1 +z)T mz

(VI.8) The factor (1+z) originates from the dominant electron pressure contribution (z > 1). This simple model explains well the unexpected weak variation of toroidal expansion with impurity mass.

An equivalent temperature is obtained by

Teq =

mz

(1 + z)γ · v 2

VI.3. Toroidal and radial 85

where vhel = cs. For neon and argon, z might be strongly dependent on the distance

between nozzle and ion. With the adiabatic coefficient ofγ = 5/3 for these high collision- ality conditions, the equivalent temperatures are listed as

TAr eq = 158(1+z)1 eV, TNe eq = 93(1+z)1 eV, THe eq = 33 (1+z)1 eV,

for the three cases from above. Certainly, the composition of ionisation stages and hence- forth, the effective charge stage z, is a function of the temperature T and density nz.

Therefore, it varies in space and time and is, in principle, unknown. Yet, we may still consider qualitative numbers. For helium averaged along field lines around the torus, it should clearly range in between He1+ _{and He}2+_{. For neon and argon, the effective local}

charge might strongly increase with the toroidal distance from the nozzle, with the spatial average along field lines well above the one of helium. Argon may be of highest, neon of intermittent charge state. Reasonable numbers for the charge state inserted give a plausi- ble average of the temperature in the interaction zone. Low temperatures in the edge are qualitatively consistent with the radiation curves shown in Figure II.4. The discussion above shows also that impurity transport is necessarily due to ion sound propagation. The neutrals outside the plasma can not directly reach the toroidally opposed side. They will be reflected from the plasma by elastic or charge exchange collisions. Even those starting about tangentially will hit the plasma after wall reflection and will be finally ionised in the nozzle vicinity after a few wall-plasma reflection cycles.