News and Events


U.S. Burning Plasma Organization eNews
July 31, 2014 (Issue 86)

 

USBPO Mission Statement: Advance the scientific understanding of burning plasmas and ensure the greatest benefit from a burning plasma experiment by coordinating relevant U.S. fusion research with broad community participation.

 

CONTENTS

Announcements  
Topical Group Research Highlight
L.R. Baylor, et al.
Disruption Mitigation System Developments and Design for ITER
ITPA Update
Schedule of Burning Plasma Events
Image of the Month
C.M. Cooper
Flow, Grow, Dynamo
Contact and Contribution Information

Announcements

BPO Leadership Rotation

Remember to vote in the election of new members to serve on the BPO Council. Information concerning the online voting system was emailed to BPO members and voting closes on August 8, 2014.

The following Topical Groups are soliciting nominees for new Deputy Leaders:

Transport and Confinement

Diagnostics

Integrated Scenarios

MHD & Macroscopic Plasma Physics

Modeling & Simulation

and nominations may be emailed to the Topical Group Leaders, Chuck Greenfield or Amanda Hubbard.



First Announcement of the 19th MHD Stability Control Workshop

This is the first announcement of the 19th MHD Stability Control Workshop. The workshop will take place on November 3–5 in Auburn, AL following the APS–DPP Annual meeting in New Orleans. The theme this year is “Fundamental understanding of plasma response to 3D fields & MHD stability control.” Information concerning the venue, agenda, and other items will be made available on the workshop website at https://fusion.gat.com/conferences/mhd14/index.html



Provisional Announcement: 27th Meeting of ITPA Topical Group on Diagnostics

Dates: 3–7 November 2015

Location: ITER Organization, St Paul Lez Durance, France

Format: Progress Meeting in EU + Progress Meeting in IO + ITPA



★ The “Director’s Corner” Article will Return Next Month


Topical Group Research Highlight

Research Highlights are selected by the leaders of the BPO Topical Groups on a rotating basis. The BPO Fusion Engineering Science Topical Group informs the members of the BPO of ongoing efforts in fusion technology and the relationship to research in other areas of the fusion program (leaders are Russ Doerner and David Rasmussen). Limiting the thermal and mechanical impact of disruption events is an important issue for Tokamaks and it has been determined that a system to mitigate the effects is required on ITER for investment protection. Technology and operational techniques are being developed and tested that can rapidly thermally quench the discharge, control the timing of the current quench to reduce forces on the vessel and internal components and to suppress or dissipate runaway electrons. The required response time for these mitigation tools could be less than 10 ms, which presents a significant challenge in the design and placement of the disruption mitigation hardware.

Disruption Mitigation System Developments and Design for ITER


L.R. Baylor1a, C. Barbier1, N.D. Bull1, J.R. Carmichael1, S.K. Combs1, M. N. Ericson1, M.S. Lyttle1, S.J. Meitner1, D.A. Rasmussen1, S. Maruyama2, and G. Kiss2
1Oak Ridge National Laboratory, Oak Ridge, TN, USA
2ITER Organization, CS 90 046, 13067 St Paul Lez Durance Cedex, France
aE-mail: BaylorLR@ornl.gov

Tokamak current disruptions are known to have the potential to cause damage to machine components. Disruptions present a challenge for ITER due to the intense heat flux, large forces from halo currents, and potential first wall damage from multi-MeV runaway electrons [1]. The ITER safety relevant components, such as the vacuum vessel, have been designed to handle the expected electromagnetic loads and plasma-facing components are designed to handle the thermal loads, but excessive surface erosion is a concern. In order to extend the lifetime of plasma-facing components, reduce forces and suppress or dissipate runaway electrons, a disruption mitigation system (DMS) is being designed for ITER.

Injecting large quantities of material into the plasma during the disruption will reduce the plasma thermal energy and increase its resistivity and electron density, mitigating the effects of the disruption. Technology has been developed to inject sufficient material deep into the plasma for a rapid plasma shutdown and runaway electron collisional suppression, which is estimated to require up to 100 kPa–m3 of deuterium or neon injected within 20 ms [2]. Both fast gas valves and shattered pellet injection technology are being developed at Oak Ridge National Laboratory for use on ITER as part of a system to provide the necessary mitigation of disruptions. The shattered cryogenic pellet concept has been shown on DIII-D [3] to lead to deeper penetration and higher assimilation than massive gas injection of the same quantity. Massive gas injection has been employed on JET (with its ITER-like wall) to reliably mitigate disruptions [4]. These promising results have motivated the development for ITER of both shattered pellet injectors for simultaneous injection of multiple large solid cryogenic pellets (> 1024 atoms each) and high flow-rate fast-opening gas valves for injection of gas. The preliminary design of the disruption mitigation system using both of these technologies is now underway.

Figure 1

Figure 1: A 3-barrel shattered pellet injector prototype before installation in a guard vacuum chamber.

The shattered pellet injection (SPI) technique utilizes a pipe-gun type injector that forms a large cryogenic pellet in-situ in the barrel. The pellets are accelerated by a high pressure gas burst down a delivery tube. At the end of the tube they strike surfaces that are optimized to produce a spray of solid fragments, mixed with gas and liquid, at speeds approaching the sound speed of the propellant gas. A three barrel SPI prototype shown in Fig. 1 has been fabricated and is undergoing testing at ORNL. A bent guide tube, located in an ITER port plug shield block, will be employed to shatter the pellets. Fig. 2 shows the resulting plume of solid fragments resulting from a 16 × 20 mm cylindrical neon pellet after impact in such a tube. The resulting angular dispersion of the material is less than 20 degrees. Neon is the primary candidate material for injection by this method, but mixtures with deuterium can also be used to reduce the mass and increase the speed for a faster response time. In order to fire neon pellets at a temperature low enough to prevent the vapor from affecting the plasma, a technique for forming a deuterium outer shell of the pellet was successfully used to enable the pellet to be sheared away from the barrel when fired [5].

Figure 2

Figure 2: 16 mm neon pellet entering the shatter tube from the right at 400 m/s with the resulting spray shown exiting the tube about 1 ms after the pellet entered the tube.

The time required to form a large pellet is on the order of 15 minutes with super critical helium cooling as will be employed on ITER. The pellet can remain in the barrel, ready to fire, indefinitely if kept cold enough to maintain a low vapor pressure.

The massive gas injection (MGI) capability will use an eddy current operated fast opening valve; a prototype will undergo testing in the near future. Shown in Fig. 3, it employs the same operating principle as the Juelich valve [6] used on JET, but uses VESPEL (a tritium compatible polyimide made by Dupont) as the valve plug material that sits on a stainless steel valve seat. This valve has a unique dual coil design, with currents in the opposite direction to minimize torque on the flyer plate when used in the ITER background magnetic field environment. The 25 mm VESPEL seat concept has been tested in a test fixture at over 4000 cycles and found to maintain a leak rate of less than 10-8 Pa–m3/s. Future tests at higher operating pressure and temperature are underway to further qualify the seat design and fully characterize the valve.

The location of the DMS injectors will affect the time scale of the material injection into the plasma during the mitigation process. Calculations using the SonicFOAM computational fluid dynamics code have been performed to quantify the time scale for injection from the MGI gas valves as a function of distance from the plasma and gas species. This result, shown in Fig. 4, indicates that to achieve the desired 20 ms time scale for 2 kPa–m3 of gas, the injectors must be located as close as possible to the plasma and will need to have a quantity of gas in excess of the specified amount, resulting in some excess material injected after the 20 ms time scale. If the injectors are located inside of the port plugs, the desired response time can be met; however this design choice does not provide any ability for maintenance of the injectors and has very little room for redundancy at each of the injection locations.

Figure 3

Figure 3: ITER DMS MGI valve design featuring a hat-shaped flyer plate and bellow isolation of the closing gas from the migration gas.

The requirements for the injectors to have sufficient time response and yet be reliable and maintainable presents a design challenge. Determination that the time response for injection in ITER is indeed needed to be 20 ms or less is a high priority topic. Modeling and simulation are underway to more accurately determine the design requirements for the ITER DMS.

Figure 4

Figure 4: Time scale of mass flow into the torus for neon and deuterium gas at different injection distances through a 28 mm orifice valve and guide tube. Thermal mitigation and runaway electron time scales specified for injection are shown in the vertical lines.

This work was supported by the Oak Ridge National Laboratory managed by UT–Battelle, LLC for the U.S. Department of Energy under Contract No. DE–AC05–00OR22725.

References

[1] ITER Physics Basis Chapter 4, Nucl. Fusion 47, S203 (2007)

[2] M. Lehnen, et al., PSI Conference 2014, submitted for publication

[3] N. Commaux, et al., Nucl. Fusion 50, 112001 (2010)

[4] M. Lehnen, et al., Nucl. Fusion 53, 093007 (2013)

[5] S.K. Combs, et al., IEEE Trans. Plasma Sci. 38, 400 (2010)

[6] S.A. Bozhenkov, et al., Rev. Sci. Instrum. 78, 033503 (2007)

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ITPA Update

More information concerning the ITPA may be found at the Official ITPA Website.

Diagnostics Topical Group
  27th Meeting, ITER Organization, St. Paul Lez Durance, France, November 3–7, 2014
 
Energetic Particles Topical Group
  13th Meeting, Padova, Italy, October 21–23, 2014 (Joint with MHD, Disruptions & Control Topical Group Meeting)
 
Integrated Operation Scenarios Topical Group
  Meeting at CEA/DSM/IRFM (Cadarache), St. Paul Lez Durance, France, October 20–23, 2014
 
MHD, Disruptions, and Control Topical Group
  24th Meeting, Padova, Italy, October 21–23, 2014 (Joint with Energetic Particles Topical Group Meeting)
 
Pedestal and Edge Physics Topical Group
  27th Meeting, ITER Organization, St. Paul Lez Durance, France, October 20–22, 2014 (Joint with Transport & Confinement Topical Group Meeting)
 
Scrape-off Layer and Divertor Topical Group
  Meeting in Prague, Czech Republic, October 20–22
 
Transport and Confinement Topical Group
  ITER Organization, St. Paul Lez Durance, France, October 20–22, 2014 (Joint with Pedestal and Edge Physics Topical Group Meeting)

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Schedule of Burning Plasma Events

2014
NSTX-U First Plasma —
 
August 25–29, 7th ITER International School, Aix-en-Provence, France
 
September 8–11, 19th Joint EU-US Transport Task Force Meeting (TTF), Culham, United Kingdom
 
October 13–18, 25th IAEA Fusion Energy Conference (FEC 2014), St. Petersburg, Russia
 
October 20–22, ITPA: Meetings of the Pedestal & Edge Physics and Transport & Confinement Topical Groups, ITER Organization, St. Paul Lez Durance, France
 
October 20–22, ITPA: Meetings of the Pedestal & Edge Physics and Transport & Confinement Topical Groups, ITER Organization, St. Paul Lez Durance, France
 
October 20–22, ITPA: Scrape-off Layer and Divertor Topical Group Meeting, Prague, Czech Republic
 
October 20–23, ITPA: Integrated Operating Scenarios Topical Group Meeting, CEA/DSM/IRFM (Cadarache), St. Paul Lez Durance, France
 
October 21–23, ITPA: Joint Meeting of the Energetic Particles and MHD, Disruptions & Control Topical Groups, Padova, Italy
 
October 27–31, 56th APS Division of Plasma Physics Conference, New Orleans, LA
 
November 3–5, 19th Workshop on MHD Stability Control, Auburn, AL
 
November 3–7, ITPA: 27th Meeting of ITPA Topical Group on Diagnostics, ITER Organization, St. Paul Lez Durance, France
 
2015
W7-X First Plasma —
 
January, Due date for report concerning the ten-year strategic plan of the Fusion Energy Sciences division of the US Department of Energy.
 
2016
— 10th Anniversary of USBPO Formation —
 
2017
JET DT-campaign —
 
2019
JT60-SA First Plasma —

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Image of the Month

Image of the Month

Flow, Grow, Dynamo

The Madison Plasma Dynamo Experiment (MPDX) is a novel, versatile, basic plasma research device designed to investigate flow driven magnetohydrodynamic (MHD) instabilities and other high-β phenomena with astrophysically and computationally relevant parameters. The center of this month’s Image shows the 3-meter diameter MPDX vessel (complete with two Helmholtz coils, approximately 180 probe access ports, water cooling lines, several LaB6 cathodes, and a two-dimensional probe drive) alongside postdoctoral researcher C. Cooper. The vessel is lined with 36 rings of alternately oriented 4000 G samarium cobalt magnets which create an axisymmetric multicusp field containing ≈ 14 m3 of well confined, nearly magnetic field free plasma that ishighly ionized (> 50%). The background part of the Image provides a view of the MPDX north pole from the south pole during a 100 kW helium plasma discharge with ne = 5 × 1011 cm−3, Te = 13 eV (left) and in vacuum (right). Four of the thermally emissive LaB6 stirring cathodes and 2 molybdenum anodes which create and stir the plasma can be seen. Alumina tiles protect the magnets from the 1 mm wide plasma footprint. At present, 12 lanthanum hexaboride (LaB6) cathodes and 16 molybdenum anodes are inserted into the vessel and biased up to 500 V, drawing 40 A each cathode, ionizing a low pressure Ar or He fill gas and heating it. Up to 100 kW of electron cyclotron heating (ECH) power is planned for additional electron heating. The LaB6 cathodes are positioned in the magnetized edge to drive toroidal rotation through J⃗ × B⃗ torques that propagate into the unmagnetized core plasma. Dynamo studies on MPDX require a high magnetic Reynolds number Rm > 1000, and an adjustable fluid Reynolds number 10 < Re < 1000, in the regime where the kinetic energy of the flow exceeds the magnetic energy (M2A = (v/vA)2 > 1). The MPDX is located at University of Wisconsin, Madison and is run by Principal Investigator Cary Forest.

Contributed by C. Cooper, Department of Physics, University of Wisconsin-Madison, Madison, WI 53706 C. M. Cooper, et al, Phys. Plasmas 21, 013505 (2014)

MPDX: http://plasma.physics.wisc.edu/mpdx


Contact and Contribution Information

This newsletter provides a monthly update on U.S. Burning Plasma Organization activities. The USBPO operates under the auspices of the U.S. Department of Energy, Fusion Energy Sciences (FES) division. All comments, including suggestions for content, may be sent to the Editor. Correspondence may also be submitted through the USBPO Website Feedback Form.

Become a member of the U.S. Burning Plasma Organization by signing up for a topical group.

Editor: David Pace (pacedc@fusion.gat.com)

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