ELSEVIER Journal of Nuclear Materials 241-243 (1997) 760-764 Retention of Ne and N 2 in the closed and pumped TdeV divertor with attached and detached plasmas N. Richard *'~, B. Terreault 2, E. Haddad l, j. Gunn 2, G. Abel 2, S. Chiu 5, J.-L. Gauvreau 3, H.H. Mai 2, W.W. Zuzak 2 Centre Canadien de Fusion Magndtique, 1804 Boulel ard LioneI-Boulet. Varennes, Qua.. Canada J3X ISI Abstract We have studied the retention of a recycling impurity, neon, and a wall-pumped impurity, nitrogen, in the closed and cryopumped divertor of the TdeV tokamak, under a variety of heating, puffing, pumping and biasing conditions. Retention times were deduced from either the decay time or the rise time of the plasma impurity content, measured spectroscopically, following either a short puff or a steady injection. For both neon and nitrogen, the compression ratio increases rapidly with the main plasma density. Retention is not degraded by plasma detachment. In density scans, other conditions being kept constant, the compression ratio for neon is found to grow with the divertor neutral pressure as ~ p15. However, by puffing, pumping or biasing, retention and PD can be varied quite independently of each other. Keywords: TdeV; Particle fuelling; Particle confinement; Detached plasma; Biasing 1. Introduction High radiation levels will be necessary for power re- moval in long-pulse ignited tokamaks. The bulk of this power must be radiated near the plasma boundary or in the divertor region in order to retain high central temperatures. Encouraging results, using neon seeding, were obtained in TEXTOR [1]. The crucial point is to demonstrate high radiation levels (> 80% of input power) together with high confinement and low Zen. [2]. Calculations indicate that it may be possible to radiate sufficient power close to the separatrix in ITER, using argon injection [3], but this requires narrowly specified plasma parameters. If impuri- ties could be retained well enough in the divertor, to radiate therein, main plasma optimization could be decou- pled from power exhaust. Retention of non-recycling im- purities, ionized close to the divertor plates, is predicted [4] to be excellent. However, recycling noble gases are most ~' Corresponding author. Tel.: + 1-514 562 8733; fax: + 1-415 652 8625; e-mail: richard@ccfm.ireq.ca. MPB Technologies, Inc. 21NRS-t~nergie and Matrriaux. 3 IREQ, Hydro-Qu&bec. often used as radiators because they are easy to inject in a controlled manner. For these impurities, retention is thought to be determined by the location of ionization [4], therefore the geometry of each machine is important. TdeV provides a useful comparison since it is characterized by a narrow plasma fan, due to triplet coils, and a large closed plenum on the outboard side, where the neutrals can be thermal- ized (see Fig. 1). An unsettled question is the effect of detachment on retention: the location of the ionization front would be crucial, and retention could be seriously deteriorated by detachment. In this paper we study divertor retention of a totally recycling impurity (Ne), and a partly recycling impurity (N2), for both attached and detached plasmas [5]. TdeV (R = 0.87 m, a = 0.26 m, B. v = 1.8 T) was operated in the upper single-null mode with the ion VB drift pointing away from the X-point. The plasma current was 190 kA, and the feedback-controlled density was varied from 3 to 6 × 10 I'~ m 3. The L-mode plasmas were additionally heated by 700 kW of lower hybrid waves [6]. The divertor plenum was pumped by 6 cryosorption units giving effective pumping speeds of 6 m~/s for D 2, 3.1 m~/s for Ne and 3.6 m~/s tbr N 2 (calibrated with gases at 300 K). The slanted graphite plates (hatched in Fig. I), that separate the 0022-3115/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0022-3115(96)00602-2