Recent lithium experiments in tokamak T-11M

Accepted Manuscript Recent Lithium Experiments In Tokamak T-11M S.V. Mirnov, A.M. Belov, N.T. Djigailo, A.N. Kostina, V.B. Lazarev, I.E. Lyublinski, V.M. Nesterenko, A.V. Vertkov PII: DOI: Reference:

S0022-3115(13)00040-8 http://dx.doi.org/10.1016/j.jnucmat.2013.01.032 NUMA 46863

To appear in:

Journal of Nuclear Materials

Please cite this article as: S.V. Mirnov, A.M. Belov, N.T. Djigailo, A.N. Kostina, V.B. Lazarev, I.E. Lyublinski, V.M. Nesterenko, A.V. Vertkov, Recent Lithium Experiments In Tokamak T-11M, Journal of Nuclear Materials (2013), doi: http://dx.doi.org/10.1016/j.jnucmat.2013.01.032

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O-17 Recent Lithium Experiments In Tokamak T-11M S.V.Mirnova*, A.M.Belova, N.T.Djigailoa, A.N.Kostinab, V.B.Lazareva, I.E.Lyublinskic, V.M.Nesterenkoa, A.V.Vertkovc a b c

SSC PF TRINITI 142 190 Troitsk Moscow, Russia.

NRNU MEPhI, Kashirskoye sh., 31,Moscow, 115409 Russia

JSC “Red Star”, Elektrolitnyj pr. 1A, Moscow, 113 230 Russia

Abstract. The T-11M lithium program is focused to finde solutions for technological issues of a steady-state tokamak. A recent lithium activity in T-11M was focussed on three directions: investigation of different Li-limiters, investigations of long-term effects of lithium behavior in hydrogen plasmas and development of a new kind of CPS limiters. The new socalled R limiter has been used as Li collector for investigation of lithium fluxes near the plasma boundary and chamber walls. It was shown, that the Li circulation near the limiter exceeds up to 4 times the primary lithium flux from limiters. 90% of primary flux is collected by lateral sides of limiter and only 10% arrives at the chamber wall. The deuterium glow discharge was used to test the long-term lithium degradation under the deuterium bombardment. It was shown that such degradation is small. That means that the Li-limiter can be used as Li-emitter in steady-state tokamak operation. A new vertical lithium limiter was successfully tested in T-11M. JNM Keywords: F0400 First Wall Materials, P0500 Plasma-Materials Interaction, I0100 Impurities, S1300 Surface Effects PSI-20 Keywords: Tokamak, Limiter, Liquid metals, Plasma facing components.

Corresponding author Address: TRINITI Troitsk Moscow 142 190 Russia Corresponding Author e-mail: [email protected] Presenting Author : S.V.Mirnov Presenting Author e-mail: [email protected]

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1. Introduction The main subject of T-11M lithium research program is the solution of technological problems, connected with the heat removal and erosion resistance of plasma facing components (PFC) in a steady-state tokamak reactor, which can be used as a fusion neutron source (FNS). The heat removal from the periphery of FNS plasma and PFC shielding from plasma interaction is done in our concept by the closed-loop lithium circulation near the plasma boundary. The basic technological scheme of such a circuit (Fig.1) should contain: a lithium emitter (limiter) on the plasma boundary, a lithium collector placed in shadow of the emitter and a system of lithium returning (or extraction), which allows to maintain the steadystate lithium loop circulation. Consequently the region of emitter-collector should have two characteristic zones: lithium emission-circulation and lithium collection (Fig.1). Obviously, a part of the lithium flux from the plasma into the collector should reach the tokamak walls and accumulate there. The accumulated lithium should protect the wall from the direct contact with the hot plasma during the development of plasma instabilities. The thickness of this layer will be determined by the balance between the arrival and departure of lithium flux back into the plasma column, as it happens in the current tokamaks. The heat exhaust from the plasma into PFC is carried out in this concept in general by noncoronal lithium radiation, which is a strict function of the traveling time of lithium ions between emitter and collector [2]. The idea of plasma boundary cooling by lithium radiation was successfully tested in the early T-11M [3] and the FTU [4] tokamak experiments by using a lithium capillary porous system (CPS) [2]. The calculations showed that the "energy cost" - the energy radiated by single lithium atom between its entering into the plasma and ionization up to coronal equilibrium state - can be 1-2keV, which is about 1000 times more

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effective for plasma cooling than conventional lithium evaporation [3]. The use of lithium to control plasma – PFC interaction in current fusion devices is pretty popular today, but it is unclear what will happen in long-term plasma experiments with lithium in future. Several questions seem most actually. These are: the consequences of lithium accumulation by the first wall, hydrogen accumulation by lithium surfaces, degradation of lithium emission under the long-term hydrogen ion bombardment and so on. The T-11M lithium program tried to give answers to some of these questions. The recent lithium activity in tokamak T-11M had three main directions: investigation and control of lithium transport in the tokamak limiter scrape of layer (SOL) by the use of an additional ring R-limiter, modeling of degradation of lithium emission from CPS limiter under the long-term bombardment by deuterium plasma of D2 glow discharge (DGD) and the investigation of new schemes of lithium CPS limiters, namely the vertical rail limiter.

2. T-11M tokamak experiments

The T-11M is a small classical tokamak (R/a=0.7/0.25m, Jp=70÷100kA, BT=1.2T, Δt=0.2s) with four (Fig.2) local limiters: two movable (a= 0.23-0.19m) horizontal rail limiters (lithium CPS and graphite [2-3]), one ring (R-limiter) [5] and one new vertical rail limiter. The lithium migration in the SOL region was studied by Li light emission from movable Climiter, heated up to 4000C, which could be considered as a recombination target for the incident flux of Li ions [6]. The lithium collection by the limiters was investigated by the postmortem analysis of the witness-samples located on the limiter surfaces. The analysis of samples has been performed by the chemical method, which included the immersing of a sample in hot water and determination of the amount of alkali and accordingly lithium in the

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resulting solution. A molybdenum electrode with +600V voltage and total current equal to 5A (Fig.3) was inserted in the tokamak chamber and used for excitation of D2 glow discharge (DGD) for modeling of long-term PFC exposition in deuterium like steady-state plasma. Lithium and deuterium fluxes from the limiters and the chamber wall were detected by observation of LiI,II and Dα - light emission (Fig.2) [2].

2.1. Ring R-limiter investigations

The circular R-limiter-collector was made from a stainless-steal CPS ring with radial width equal to δ=3cm (Fig. 2,3). It was placed (Fig.5 B) in the shadow of the main lithium CPS rail limiter (a=19cm) and intersected the main part of magnetic surfaces (r=22÷25cm) in the limiter SOL. The measurements of the total weight [7] of lithium, collected by the R limiter during the all experimental campaign gave some absolute estimation of total lithium flux along the magnetic field in the lithium limiter shadow. The conventional method of control of lithium longitudinal fluxes used in T-11M is based on the observation of LiI radiation by lithium recombination on C-target (movable C limiter, Fig.2), which is inserted in SOL (lithium limiter shadow) [6]. This method can give the relative radial (along small r) distribution of lithium fluxes. Fig. 4 presents two such distributions of lithium emissions for shots without (I) and with (II) the R limiter [5] during the quasi steady-state phase (50-150ms from start) of T-11M discharges. The clear visible difference of Li distributions in SOL allows us to make an attempt of an absolute calibration of the lithium flux. Namely, in the case (II) we can separate the zone of lithium emission and circulation (r=22÷19cm) [5] and the zone of Li-collection (r=27÷22cm) by the cold lateral sides of the limiters. If we suppose, that the undisturbed parts of lithium flux distributions in cases (I) and (II) were identical and

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the disturbance is caused only by lithium collection on the cold CPS surface of the R-limiter (r=25÷22cm), we can calculate by a simple integration over the radius the relation between the Li amount, which could be collected by the R limiter and the total lithium amount, which should be circulated in SOL (r=22÷19cm). This relation is equal to 0.12. If we consider that the surfaces of the cold R-limiter should be able to capture almost all of the lithium ions, we can estimate the total amount of Li, circulating in the SOL during the plasma exposure. Fig. 5 A, B presents the schemes of lithium circulation and collection around the lithium (A) and lithium plus R (B) limiters. Arrow 1 represents the primary lithium flux from the lithium limiter to plasma, arrow 2 – the secondary flux of circulated lithium plasmalimiter, arrays 3 and 3R mean lithium fluxes from plasma collected by the lateral sides of the lithium and R-limiters, arrow 4 means lithium flux to the chamber wall and arrow 5 means lithium back flux from the chamber wall to plasma. The total weight of lithium, collected by the R limiter during 1000 shots of experimental campaign of T-11M was approximately 120+/-10 mg. Approximately 100 mg lithium was collected simultaneously by the lateral sides of the main lithium limiter. (It should be pointed out, that during the previous 1000 shots experimental campaign with ordinary T-11M limiter geometry without R-limiter the lateral sides of the lithium limiter collected approximately 200mg of lithium [5]). In summary we should conclude, that the total lithium amount (1+2), injected in the T-11M plasma column during 1000 shots (~150sec) of experimental campaign was approximately 1g, the circulation part (2) was approximately 80% of it. Approximately 20% from (1+2) was the lithium, which is collected by the limiters and about the same value should be the total flux (1), which is the direct weight loss of CPS limiter. The Li flow into

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the wall of chamber (4) in the quasi-stationary phase of T-11M discharge with ordinary geometry limiter is about 10% of (1) and less than 1% of (1) in the R-limiter experiment. The knowledge of the total amount of lithium atoms injected in plasma and the total energy of radiation losses from plasma column, measured by AUXV detectors (Fig.2) allows to calculate the real radiation “energy cost” of a single lithium atom entered in T-11M plasma (“noncoronal radiation”). In our experiments it was within 800-1000eV, which is close to the theoretical limit of 1-2keV in Te interval from 20 up to 1000eV [3]. For the estimation of the role of the lithium flux from the chamber wall back into the plasma column (5 arrow Fig. 5) in T-11M, a special R-limiter experiment was performed by changing the rail CPS lithium limiter with a identical rail C-limiter (Fig.2). The CPS lithium limiter was removed in this experiment into the vacuum port and only one source of lithium remained, namely the chamber walls, coated by approximately 20g lithium during the previous experimental campaigns. The 400 shots plasma exposure showed that the intensity of the total lithium flux from the chamber wall into plasma column (5) was less, than 10% of primary lithium flux (1). The relative values of lithium fluxes in and out of the plasma column of T-11M during the operation in modes A and B (Fig. 5) are listed in Table 1. The values of all fluxes are presented as a fraction of the primary flux from the lithium limiter into the plasma. The relative measurement errors are equal to 20 - 30% by our estimates except the weak fluxes (from the wall to the plasma and back), where they can exceed 50%. All estimates are made for the main quasi steady-state phase of T-11M discharges.

2.2. Long- term effect investigations

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To test the possible long-term degradation of the rail CPS lithium limiter under the influence of prolonged deuterium bombardment (so called "lithium poisoning by deuterium bombardment") the T-11M tokamak chamber was used as a long-term irradiation test stand (Fig.3) using long-term deuterium plasma glow discharge (DGD) exposition. Series of alternated DGD exposures and conventional operation test shots of T-11M were used to observe the evolution of lithium and deuterium fluxes (LiI, Hα, Fig.2) from the lithium limiter into the plasma column (emitter properties of lithium limiter) and hydrogen (deuterium, Dα) recycling as a function of the DGD plasma exposures with different time duration. Fig 6 presents the time evolution of LiI and Dα intensities during one cycle of test operation, namely: two initial tokamak test shots after one night with a low vacuum condition (10-2Pa), lithiation procedure in HeGD with heating the lithium limiter up to 3500C and lithium cleaning, followed by four tokamak test shots, five hours of DGD, tokamak test shots again, two days with a low vacuum condition and finaly two control tokamak shots. The LiI signal showed (Fig.6) that the intensity of lithium emission of fresh lithium film (limiter surface) after its exposition during five hours in deuterium plasma declined by no more than 20% This exposure corresponded in total to approximately 1000 seconds of T-11M ordinary shots in terms of the first wall and about 10 seconds of the ordinary tokamak shot in terms of the limiter fluxes. That shows that the effect of "lithium poisoning by deuterium bombardment" is small enough and the Li-limiter can be used successfully during the steady-state tokamak operations as Li-emitter. Simultaneously iit was discovered in these experiments that Li emission is very sensitive to “lithium poisoning by residual gases” during, for example, several days of low vacuum (up to 10-2Pa). This effect should be carefully investigated in future. We can also not exclude today, that conventional decrease of lithium emissivity of a

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lithium limiter during the experimental campaign is primarily the effect of “lithium poisoning” by products of tokamak PFC erosion.

2.3. Experiments with a “vertical” limiter

To further increase the circulating flow of lithium flux it is necessary to increase the size of the lithium limiter in particular the part that crosses and intersects the radial flow of lithium ions traveling from the lithium limiter (emitter) into the tokamak wall. For this purpose a lithium CPS limiter with a vertical design was created and successfully tested in the T-11M operating mode. The new limiter allows approximately for a 2-fold increase of the "cold" collector area of the lithium limiter as compared to the horizontal rail limiter of T-11M (Fig.7A). This kind of lithium limiter can be installed in almost all tokamaks with horizontal ports of the vacuum chambers. The main misgiving about the vertical geometry of CPS limiter with liquid metal was an enhanced probability of metal splashing during tokamak disruptions. The vertical limiter was tested in a 1000 shots campaign of T-11M with approximately 30% disruptive shots at the end of the current pulse without any signs of liquid metal splashing on the low part (ring target) of the vertical limiter, as one can see in Fig.7B. That means the CPS tokamak limiter can operated in a vertical geometry. The future plans of the T-11M and “Red Star” Teams are to creat and test a so-called "longitudinal" limiter, which allows to increase the collected area more than 3 times. Both versions of the vertical and "longitudinal" limiters will be used in the future modification of T-15 tokamak.

3.Conclusions

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1. The T-11M experiments supported in general the lithium strategy toward a steady-state fusion

neutron

source

on

a

tokamak

basis.

Experiments

on

the

T-11M with a circular R-limiter are able to refine significantly the values of lithium flows near the lithium limiter. The circulation part of the Li flow, which is the main element of the lithium circulation loop, represents ≈ 80% of the total lithium flux from the lithium limiter into plasma column during quasi steady-state stage of T-11M plasma pulse. The primary lithium flux from the lithium limiter into the plasma column was approximately 20% of total flux. The relative lithium flux into the chamber wall was ≈ 10% of the primary Li flux in T-11M with ordinary limiter geometry and drops approximately up to ≈1% of primary in experiment with the circular R–limiter. The Li flux from the chamber wall back into plasma column was not more than 10% from the primary flux. 2. In our experiments the real “energy cost” of a single lithium atom entering the plasma (“noncoronal radiation”) was within 800-1000eV, which is close to the theoretical predictions 1000-2000eV in Te region from 20 up to 1000eV. 3. Successful experiments on modeling the T-11M lithium limiter long-term degradation under prolonged deuterium bombardment allow us to hope that the Li emitter will not loose its Li-emissivity during the steady-state tokamak operations. 4. The verical liquid lithium CPS limiter was manufactured and successfully tested in 1000 shots of T-11M campaign. The CPS tokamak limiter can also operated in vertical geometry.

Acknowledgments

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The authors thank the T-11M stuff for excellent experimental operations. The work was supported by ROSATOM contract № Н.4f.45.90.11.1013. References [1] S.V.Mirnov S.V. Jorn. of Nucl. Mat. 2009 v 390-391 (2009) 876. [2] V.A.Evtikhin, et al., Plasma Phys. Controlled Fus. 44 (2002) 95. [3] S.V.Mirnov, E.A. Azizov, V.A.Evtikhin et al. Plasma Phys.Contr. Fus. 48 (2006) 821. [4] M.L.Apicella et al, Plasma Phys. Controlled Fus. 54 (2012) 035001. [5] S.V.Mirnov, E.A.Azizov et al. Nucl. Fusion 51 (2011) 073044. [6] V.B.Lazarev et al, 35 EPS Conf. on Plasma Physics and Controlled Fusion, Hersonissos ECA Vol. 32, 2008 Р5– 004. [7] E.A.Azizov et al. 36 EPS Conf. on Plasma Physics and Controlled Fusion. Sofia 2009г. P5.192. Tables Table 1. The relative values of lithium fluxes in and out of the plasma column of T-11M during the operation in modes A and B (Fig. 5). Figure captions Fig. 1. The principal scheme of steady-state lithium loop circulation [1] Fig.2 Scheme of T-11M limiters and diagnostics. Fig.3 Scheme of long-term T-11M experiment. Fig. 4 The radial distributions of lithium emissions I for cases without (I) and with (II) R limiter [5] steady-state phase of discharge (LiC-on C-limiter,LiLi- on Li limiter), λ- the characteristic decay length of lithium flows in SOL. Fig.5. Schemes of lithium circulation and collection around of lithium (A) and lithium plus R limiters (B).

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Fig.6. Dynamics of Li emission and deuterium recycling after: lithiation, 5h DGD and 2days of vacuum break. ILi(Li) and Hα(Li) are LiI and deuterium lights on Li limiter. Fig.7 Views of vertical CPS limiter before A and after B 1000 shots plasma exposition.

Tables

Li fluxes in a.u. Arrow numbers

1

2

3

3R

4

1

~4

~ 0.9

-

~0.1

1

~ 3-4

~0.5

~0.5

5

in Fig. 5 A, B

A - ordinary shots with single Li limiter B - shots with Li and R limiters

≤ 0.1

< 0.05 < 0.05

Table 1.(75x50)

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Figures:

Fig.1 (75x50)

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Fig.2 (75x50)

13

Fig.3 (75x50)

14

Fig.4 (75x50)

15

Fig.5(160x75)

16

Fig.6 (160x75)

17

Fig.7 (160x75)

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