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Overall behavious of PFC integrated SST-1 vacuum system

Ziauddin Khan1, Dilip C Raval1, Yuvakiran Paravasu1, Pratibha Semwal1, Kalpeshkumar R Dhanani1, Siju George1,2, Mohammad Shoaib3, Arun Prakash2,3, Gattu R Babu1, Prashant Thankey1, Firozkhan S Pathan1 and Subrata Pradhan1

1Institute for Plasma Research, Near Indira Bridge, Bhat, Gandhinagar 382 428, India2Second affiliation, Address, City and Postcodes, Country3Second affiliation, Address, City and Postcodes, Country

E-mail: ziauddin@ipr.res.in

Abstract. As a part of phase-I up-gradation of Steady-state Superconducting Tokamak (SST-1), Graphite Plasma Facing Components (PFCs) have been integrated inside SST-1 vacuum vessel as a first wall (FW) during Nov 14 and May 2015. The SST-1 FW has a total surface area of the installed PFCs exposed to plasma is ~ 40 m 2 which is nearly 50% of the total surface area of stainless steel vacuum chamber (~75 m2). The volume of the vessel with the PFCs is ~ 16 m3. After the integration of PFCs, the entire vessel as well as the PFC cooling/baking circuits has been qualified with an integrated leak tightness of < 1.0 10–8

mbar l/s. The pumping system of the SST-1 vacuum vessel comprises of one number of Roots’ pump, four numbers of turbomoleculars and a cryopump. After the initial pump down, the PFCs were baked at 250 °C for nearly 200 hours employing hot nitrogen gas to remove the absorbed water vapours. Thereafter, Helium discharges cleaning were carried out towards removal of surface impurities. The pump down characteristics of SST-1 vacuum chamber and the changes in the residual gaseous impurities after the installation of the PFCs will be discussed in this paper.

1. IntroductionSST-1 Tokamak (figure 1) was successfully commissioned in 2012 [1-3] and the first plasma was achieved [4] in June 2013 with poloidal limiters having SS 304L as vessel wall material. Due to plasma wall interactions, high-Z impurities released from the vessel wall which in turn cools the plasma by radiation loss. In order to reduce this effect, in 2nd phase of SST-1 refurbishment, PFC components were installed in the system.

Plasma facing components (PFC) of SST-1 Tokamak [5,6] consists of Inboard divertor plates (IDP), Outboard divertor plates (ODP), Inboard passive stabilizers (IPS), Outboard passive stabilizers (OPS), main baffle (MBAF). Each IDP, ODP, IPS, MBAF and OPS has top and bottom modules mounted in the main vacuum vessel (VV) as shown in figure 2. All PFC’s are structurally continuous in toroidal direction. Graphite was chosen as Plasma facing material considering its good thermal properties, low atomic mass. Cu-Zr & Cu-Cr-Zr alloys plates embedded with SS 304L piping were used as back plate materials for proper heat transfer during baking and cooling operations. Approximately 3800 tiles were mounted on 132 numbers of these copper alloys back-plates. Gas-to-gas heat exchange method was adapted to heat nitrogen gas which is pressurized using dedicated gas blower system to bake the PFC components. Some of the other major parameters of SST-1 tokamak are shown in the table 1.

Figure 1. Snap shot of SST-1 machine. Figure 2. View of PFC inside the

SST-1 main vessel.

Table 1. Major SST-1 machine parameters.

Parameters ValuesMajor radius 1.1 mMinor radius 0.2 mSS surface area of VV 75 m2

Exposed surface area of PFC 40 m2

Plasma species HydrogenVolume enclosed by PFC 16 m3

Ultimate vacuum in VV ~1.0 10–8 mbarOperating pressure range 5.0 10–5 mbar (max)

All PFC components passed through temperature of 250 C for 8 hours flat top and working pressure of 4 bar under UHV conditions in validation testes. Strict metrology and QA/QC plans were structured and executed to integrate the PFC components inside the vacuum vessel.

Each and every component was tested at their functional conditions to verify its functionality and to ensure operation conformity. Minimum critical flux for any axial location in the uniformly heated tube is given by

(1)where K is the resistance co-efficient for different types of bends (ft L/d), ft is the friction factor of pipe, L is the length of tube and d is the diameter of tube.

In CST, three TMP pumping systems were mounted at the bottom CST manhole openings (B-7, B-9, and B-12) in similar configuration to VV pumping system. The total effective pumping speed of 3250 l/s (nitrogen gas) was estimated at CST due to these pumping systems. One of the TMP pumping lines of CST is equipped with RGA. The total effective pumping speed of 3250 l/s (nitrogen gas) was estimated at CST due to these pumping systems. One of the TMP pumping lines of CST is equipped

2

21 vKP

with RGA and helium leak detector to monitor online leak tightness of helium and nitrogen systems during the cool-down campaign.

2. Another section of your paperThe first paragraph after a heading is not indented (Bodytext style).

Other paragraphs are indented (BodytextIndented style).

2.1. A subsectionSome text.

2.1.1. A subsubsection. The paragraph text follows on from the subsubsection heading but should not be in italic.

3. Glow discharge cleaning and plasma break downThe gas feed system was designed and installed for catering the requirements like GDC and prefilling for short pulse plasma operation. Two piezoelectric valves were mounted at diagonally opposite locations of SST-1 machine RPs to maintain overall balance of gas distribution inside the chamber. These piezoelectric valves work at 100 VDC and start functioning above 25 VDC. Gas feed pulse shape can also be adjusted as per requirement using PCI eXtensions for Instrumentation-based control system. The schematic of SST-1 GDC system and of one of the gas feed valves scheme are shown in figure 8.

High purity grade hydrogen gas was purified to a very high purity of ∼ 99.9999% before fed into SST-1. The pressure of 1.0 × 10−5 mbar was achieved inside the VV when a rectangular pulse of 100 VDC amplitude of 10 ms was applied to the piezoelectric valves when inlet pressure to this valve was 2.0 bar (g) constantly. The pressure inside the VV was varied from 1.0 × 10−5 mbar to 5.0 × 10−5 mbar by increasing the time duration of gas puffing up to 30 ms under the similar condition.

4. ConclusionThe sentence shall be started like this

AcknowledgementThe sentence shall be started like this

References[1] Pradhan S and Team SST-1 mission 2010 J. Fusion Res. Series 9 650[2] Pradhan S, Sharma A N, Tanna V L, Khan Z, Prasad U et al. 2012 , IEEE Trans. Plasma Sci. 40

614[3] Sze S M 1969 Physics of Semiconductor Devices (New York: Wiley–Interscience)[4] Dorman L I 1975 Variations of Galactic Cosmic Rays (Moscow: Moscow State University

Press) p 103[5] Caplar R and Kulisic P 1973 Proc. Int. Conf. on Nuclear Physics (Munich) vol 1 (Amsterdam:

North-Holland/American Elsevier) p 517[6] Szytula A and Leciejewicz J 1989 Handbook on the Physics and Chemistry of Rare Earths vol

12, ed K A Gschneidner Jr and L Erwin (Amsterdam: Elsevier) p 133[7] Kuhn T 1998 Density matrix theory of coherent ultrafast dynamics Theory of Transport

Properties of Semiconductor Nanostructures (Electronic Materials vol 4) ed E Schöll (London: Chapman and Hall) chapter 6 pp 173–214