Gadelmeier, F.

Hübner, K.

Kick, M.

Reimbold, S.

Rust, N.

Weller, A.

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Confinement of Neutral-Beam-Injected Fast Ions in W7-AS

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27th EPS Conference on Contr. Fusion and Plasma Phys.  Budapest, 12-16 June 2000    ECA Vol. 24B (2000) 1621-16241621Confinement of Neutral Beam Injected Fast Ions in W7-ASN. Rust, S. Reimbold, K. Hübner*), F. Gadelmeier, F.P. Penningsfeld, A. Weller,Max-Planck-Institut für Plasmaphysik, Association EURATOM-IPP, D-85748 Garching*) Kirchhoff-Institut für Physik, Universität Heidelberg, D-69120 HeidelbergABSTRACT: Detailed studies demonstrate that the decrease of fast particle densities andneutron rates with decreasing plasma density is owing to a decreased eposition of theinjected particles. The power deposited by neutral injection after the first particle lossesis determined from the initial rise as well as from the plateau value of the neutron rate.For this the neutron counters were absolutely calibrated by means of Indium activation.The results of the studies demonstrate classical confinement of fast particles in W7-AS.Losses of fast particles are only observed during strong MHD activities.In a previous paper [1] we reported discrepancies between measured fast particle densities aswell as neutron rates and the respective values expected from beam deposition calculations of theinjected power. The measured values were smaller than the expected ones for large energyrelaxation times of the injected ions, i.e. for low plasma densities.Neutron production during D-injection is a sensitive indicator for the confinement of the fastions, as they need approximately one energy relaxation time to produce their neutrons. For W7-AS this is between 10 and 60 ms. Therefore we have carried out detailed studies of the measuredneutron rates for a large number of discharges with different plasma parameters and neutralbeam powers. In order to get information about the confinement of fast ions, one has to knowtheir density deposited after the first orbit losses, i.e. the density of particles starting in therelaxation process after their injection. Therefore, the central point is an experimentaldetermination of the deposited power, which is smaller than the primary injected power,depending on injection angle and density.As already shown in our previous paper the initial rise of the local neutron yield Qi (t) producedby injected ions with energy Wi is given by the simple expressiondQ tdtS n WWmiti d iid         02	   , (1)where Si is the deposition rate (number of ions/cm3 s), nd the deuteron density of the backgroundplasma,   the fusion cross-section and md the deuteron mass. Summation over the three speciesof injected ions gives the initial rise of the total local neutron yield Q(t).It is essential to note that this initial rise depends on the parameters of the injection and theplasma density only, but not on the relaxation process of the fast ions, i.e. on their energydistribution function. Therefore it is an ideal tool for determining the deposited powerP =   Si Wi , but it requires a sensitive and well-calibrated neutron counter. We have donecareful calibrations of the two counters (BF3 and NE213) installed at W7-AS by means ofindium activation but, unfortunately, the BF3 counter is not sensitive enough and the uncertainty27th EPS CCFPF 2000;   N. Rust et al.: Confinement of Neutral Beam Injected Fast Ions in W7-AS 1622of the NE213 calibrated at large neutron rates increases with lower count rates. Therefore thevalues for deposited power Prise which we determine from the initial rise of the neutron signal arerather uncertain.A second possibility for determining the deposited power is an evaluation of the full shape ofthe neutron signal or at least of the plateau value in the stationary state of the discharge. But thisprocedure requires the knowledge of the fast ion energy distribution and needs several m asuredplasma parameters. In order to investigate a large number of discharges we used for the ionenergy distribution f (W) the so-called high-energy approximation:f WWW WSis decriti   2123232  . (2)Here  s de is the slowing down time for deuteron-electron collisions and Wcrit the so-calledcritical energy, where electrons and background ions receive the same heating power.As to be expected, the measured ratio Prise / Pplateau , where Pplateau  is the value for the depositedpower determined from the plateau, is about 1 for small neutrons rates with an error of about50%. This is a reasonable result taking into account the error margin of the input data.For 7 discharges we compared the results for the deposited power determined using thisapproximation with the values from a time-dependent 2D Fokker-Planck simulation of thedistribution function and the related neutron rate. Both results agreed within ± 20%, which isless than the error of the values for P determined from the neutron signals. Generally, a timedependent simulation of the neutron rate using the value Pplateau reproduces very well the rise ofthe neutron rate (Fig. 1). For about 2/3 of the investigated plasma discharges the simulationreproduces the full time history of the neutron rate.  For less than 1/3 we observe a lack ofneutrons in a part of the signal (Fig. 2), starting typically after half of the rise time and lastingFig. 1: Comparison between simulation and NE-213 neutron measurement fordischarge type 1. Neutral particle deposition calculated from the plateauvalue of the neutron signal.#4288301234567150 200 250 300 350 400 450time [ms]Q [1012 n /s]  experiment  simulation27th EPS CCFPF 2000;   N. Rust et al.: Confinement of Neutral Beam Injected Fast Ions in W7-AS 1623until half of the stationary state. During this time interval strong MHD activity of the plasma isobserved in those discharges, which obviously causes a loss of fast particles.In quiescent discharges where the electron temperature after the turn off of the neutral beam wasalmost kept constant using additional ECR heating the decay of the neutron signal is also wellreproduced by a numerical simulation using Pplateau (Fig. 3).Finally, there are a few discharges for which the analysis of the measured data gives inconsistentresults. It is an open question whether this is due to erroneous measurements or due to a veryspecial plasma configuration.Fig.3: Comparison between calculated and measured neutron decay afterturning off the neutral beam injection. During the decay, the electrontemperature is kept constant by ECRH heating.#3307102468450 475 500 525 550time [ms]Q [1012 n/s]BF3NE213simulation#4287001234150 200 250 300 350 400 450time [ms]Q [1012 n/s]  experiment  simulationFig. 2: Comparison between simulation and NE-213 neutron measurement fordischarge type 2. Neutral particle deposition calculated from the plateauvalue of the neutron signal. The lack of the measured neutrons isassociated with strong MHD activity.27th EPS CCFPF 2000;   N. Rust et al.: Confinement of Neutral Beam Injected Fast Ions in W7-AS 1624Fig. 4 shows the ratio of Pplateau /Pinj , where Pinj is the injected power as a function of theelectron density, i.e. the deposited fraction of the injected power, which decreases withdecreasing electron density because of rising shine through and charge-exchange losses. Theinjected power was calculated by the beam deposition code FAFNER. The solid line gives thevalues obtained with FAFNER for the case of the inner sources, which are injecting almosttangentially. For most of the discharges we used the inner co-NBI sources. The experimentalvalues are widely spread. This is mainly caused by the errors of the plasma data, which re to beused in the determination of Pplateau. In the mean the measured values are smaller than thecalculated ones. In addition some measurements were made with outer sources, which have alarger injection angle. As expected their ratio of Pplateau /Pinj is in the mean lower than for theinner sources. But this decrease is smaller than the spread of the values.From our observations we can draw three essential conclusions. Firstly, the deposited power isin tendency smaller for the examined density region than it is expected from FAFNERcalculations. This explains the formerly observed discrepancies between the measured andexpected neutron rates and fast particle densities. Secondly, the relaxation process of injectedions is a classical one in W7-AS, at least for the period of one energy relaxation time. Thisfollows from the excellent reproduction of the rise as well of the decay of the neutron signal innumerical simulations using the value of deposited power determined from the plateau. Thirdly,the fast ions are confined in W7-AS at least over one energy relaxation time as long as there areno plasma perturbations by mode activities.[1] N. Rust, e.a., Controlled Fusion and Plasma Phys., 24 (4) 1621-1624 (1997)Fig. 4: Fraction of neutral beam deposited power to the input power via the centraldensity.

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Confinement of Neutral-Beam-Injected Fast Ions in W7-AS

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