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
1
Institute for Plasma Research, Near Indira Bridge, Bhat, Gandhinagar 382 428, India
Second affiliation, Address, City and Postcodes, Country
3
Second affiliation, Address, City and Postcodes, Country
2
E-mail: [email protected]
Abstract. As a part of phase-I up-gradation of Steady-state Superconducting Tokamak (SST1), 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 m2 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. Introduction
SST-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-togas 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
Major radius
Minor radius
SS surface area of VV
Exposed surface area of PFC
Plasma species
Volume enclosed by PFC
Ultimate vacuum in VV
Operating pressure range
Values
1.1 m
0.2 m
75 m2
40 m2
Hydrogen
16 m3
~1.0  10–8 mbar
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
P  K v 2 
2
(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, B9, 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
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 paper
The first paragraph after a heading is not indented (Bodytext style).
Other paragraphs are indented (BodytextIndented style).
2.1. A subsection
Some 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 down
The 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. Conclusion
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Acknowledgement
The sentence shall be started like this
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