Dense plasma focus
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{{redirect|Plasma gun|the science-fiction weapon|plasma rifle}}
A '''dense plasma focus''' (DPF) is a [[Plasma (physics)|plasma]] machine that produces, by [[electromagnetic]] acceleration and compression, short-lived plasma that is so hot and dense that it becomes a copious multi-radiation source. It was invented in the early 1960s by J.W. Mather and also independently by N.V. Filippov. The electromagnetic compression of a plasma is called a "[[Pinch (plasma physics)|pinch]]". The plasma focus is similar to the ''high-intensity plasma gun device'' (HIPGD) (or just ''plasma gun''), which ejects plasma in the form of a [[plasmoid]], without pinching it.
==Applications==
Intense bursts of [[X-ray]]s and charged particles are emitted, as are [[nuclear fusion]] neutrons, when operated in [[deuterium]]. There is ongoing research that demonstrates potential applications as a soft X-ray source for next-generation [[microelectronics]] [[lithography]], [[surface micromachining]], pulsed X-ray and [[neutron]] source for medical and security inspection applications and materials modification, among others.
Other applications include simulation of nuclear explosions (for testing of the electronic equipment) and a short and intense neutron source useful for non-contact discovery or inspection of nuclear materials (uranium, plutonium).
== Positive characteristics ==
An important characteristic of the dense plasma focus is that the [[energy density]] of the focused plasma is practically a constant over the whole range of machines, from sub-kilojoule machines to megajoule machines, when these machines are tuned for optimal operation. This means that a small table-top-sized plasma focus machine produces essentially the same plasma characteristics (temperature and density) as the largest plasma focus. Of course the larger machine will produce the larger volume of focused plasma with a corresponding longer lifetime and more radiation yield.
Even the smallest plasma focus has essentially the same dynamic characteristics as larger machines, producing the same plasma characteristics and the same radiation products and radiation characteristics. This is due to the [[Plasma scaling|scalability of plasma]] phenomena.
See also [[plasmoid]], the self-contained magnetic plasma ball that may be produced by a dense plasma focus.
==How it works==
[[Image:DPFfig1 .jpg|thumb|left|The basic configuration of the Mather type]]
[[Image:DPFfig2 .jpg|thumb|right|Typical design, focusing signatures (on voltage and current waveforms, 3 microsec from start to voltage spike) and the radial implosion and breakup dynamics shown in a 6-frame nitrogen laser shadowgraphic sequence over a period of about 40 ns.]]
The charged bank of [[electrical capacitor]]s (also called a Marx bank or [[Marx generator]]) is switched onto the anode. The gas breaks down. A rapidly rising [[electric current]] flows across the backwall [[electrical insulator]], axisymmetrically, as depicted by the path (labeled 1) as shown in the Fig 1. The axisymmetric sheath of plasma current lifts off the insulator due to the interaction of the current with its own magnetic field (''J''×''B'' force). The plasma sheath is accelerated axially, to position 2, and then to position 3, ending the axial phase of the device.
The whole process proceeds at many times the [[speed of sound]] in the ambient gas. As the current sheath continues to move axially, the portion in contact with the anode slides across the face of the anode, axisymmetrically. When the imploding front of the [[shock wave]] coalesces onto the axis, a reflected shock front emanates from the axis until it meets the driving current sheath which then forms the axisymmetric boundary of the 'pinched' or focused hot plasma column.
The dense plasma column (akin to the [[Z-pinch]]) rapidly "[[Pinch (plasma physics)|pinch]]es" and undergoes instabilities and breaks up. The intense electromagnetic and particle bursts, collectively referred to as 'multi-radiation' occur during the dense plasma and breakup phases. These critical phases last typically tens of [[nanoseconds]] for a small (kJ, 100 kA) focus to around a [[microsecond]] for a large (MJ, several MA) focus.
The whole process, including axial and radial phases, may last, for the Mather DPF, a few microseconds (for a small focus) to 10 microseconds (for a large focus). A Filippov focus has a very short axial phase compared to a Mather focus.
[[Image:PF Filippov.jpeg|thumb|right|Nikolay Filippov and his Plasma Focus at Kurchatov Institute of Atomic Energy (Moscow, Russia)]]
==Design parameters==
The fact that the plasma energy density is constant throughout the range of plasma focus devices, from big to small, is related to the value of a design parameter that needs to be kept at a certain value if the plasma focus is to operate efficiently. The critical 'speed' design parameter is <math>{{I \over a} \over \sqrt{p}}</math><!-- removed the confusing duplication without math markup as (I/a)/p^1/2-->, or the current linear density divided by the square root of the mass density of the fill gas.
For example for neutron-optimised operation in deuterium the value of this critical parameter, experimentally observed over a range of machines from kilojoules to hundreds of kilojoules, is: 90 (kA/cm)/(Torr)<sup>1/2</sup> (780 kA/(m·Pa<sup>1/2</sup>)) with a remarkably small deviation of 10% over such a large range of sizes of machines.
Thus if we have a peak current of 180 kA we require an anode radius of 1 cm with a deuterium fill pressure of 4 torrs. The length of the anode has then to be matched to the risetime of the capacitor current in order to allow an average axial transit speed of the current sheath of just over 5cm/microsec. Thus a capacitor risetime of 3 microsecond requires a matched anode length of 16 cm.
The above example of peak current of 180 kA rising in 3 µs, anode radius and length of respectively 1 and 16 cm are close to the design parameters of the UNU/ICTP PFF (United Nations University/International Centre for Theoretical Physics Plasma Fusion Facility)[http://eprints.ictp.it/31/]. This small table-top device was designed as a low-cost integrated experimental system for training and transfer to initiate/strengthen experimental plasma research in developing countries [http://eprints.ictp.it/273/].
==Current research==
There is now a network (coordinated by the Asian African Association for Plasma Training, [[AAAPT]]) of 10 such identical DPF machines, operating in some 8 countries producing postgraduate students and research papers in machine optimization and diagnostics (soft x-rays, neutrons, electron and ion beams), applications (microlithography, micromachining, materials modification and fabrication, imaging and medical, astrophysical simulation) and modeling and computation.
This DPF network was organised by S. Lee from 1986, taking advantage of the fact that even a small DPF can be used to study all the plasma phenomena that a big DPF has access to. A simulation package, the Lee Model [http://www.google.com/search?sourceid=navclient&ie=UTF-8&rls=SUNA,SUNA:2006-17,SUNA:en&q=lee+model+plasma+focus], has been developed for this network but is applicable to all plasma focus devices. The code typically produces excellent agreement between computed and measured results, [http://www.intimal.edu.my/school/fas/UFLF/Papers/listofpapers.htm] and is available for downloading as an Universal Plasma Focus Laboratory Facility.[http://www.intimal.edu.my/school/fas/UFLF/]The Institute for Plasma Focus Studies [http://www.kirkbyites.net/IPFS/] was founded on 25 February 2008 to promote correct and innovative use of the Lee Model code and to encourage the application of plasma focus numerical experiments.
The International Centre for Dense Magnetised Plasmas (ICDMP) in Warsaw. Poland, operates several plasma focus machines for an international research and training programme. Among these machines is one with energy capacity of 1 MJ making it one of the largest plasma focus device in the world.
In Argentina there is an Interinstitutional Program for Plasma Focus Research since 1996, coordinated by a National Laboratory of Dense Magnetized Plasmas (www.pladema.net) in Tandil, Buenos Aires. The Program also cooperates with the Chilean Nuclear Energy Commission, and networks the Argentine National Energy Commission, the Scientific Council of Buenos Aires, the University of Center, the University of Mar del Plata, The University of Rosario, and the Institute of Plasma Physics of the University of Buenos Aires. The program operates 6 Plasma Focus Devices, developing applications, in particular ultrashort tomography and substance detection by neutron pulsed interrogation. Chile currently operates the facility SPEED-2, the largest Plasma Focus facility of the southern hemisphere. PLADEMA also contributed during the last decade with several mathematical models of Plasma Focus. The thermodynamic model was able to develop for the first time design maps combining geometrical and operational parameters, showing that there is always an optimum gun length and charging pressure which maximize the neutronic emissions. Currently there is a complete finite-elements code validated against numerous experiments, which can be used confidently as a design tool for Plasma Focus.
=== DPF for nuclear fusion power ===
Several groups have claimed the DPF could prove viable for [[fusion power]], even producing temperatures high enough for [[Aneutronic fusion|p+B11 fusion]] and that the powerful magnetic field can reduce electron-ion collisions and thus reduce [[bremsstrahlung]] losses; in contrast, the high magnetic field is theorised to aggravate [[cyclotron radiation]] losses. Another advantage claimed is the capability of [[Fusion power#Subsystems|direct conversion]] of the energy of the fusion products into electricity, with an efficiency potentially above 70%. So far only limited experiments and computer simulations have been done to investigate the capability of DPF for fusion power. [[Eric Lerner]]'s approach to fusion power using the DPF, termed "Focus Fusion", was explained in 2007 at Google's Tech Talks. <ref>[http://video.google.com/videoplay?docid=-1518007279479871760&q=Google+tech+talks+lerner&pr=goog-sl Focus Fusion: The Fastest Route to Cheap, Clean Energy<!-- Bot generated title -->]</ref>
==History==
*1958: Hannes Alfvén: Proceedings of the Second International Conference on Peaceful Uses of Atomic Energy (United Nations), 31, 3
*1960: H Alfven, L Lindberg and P Mitlid, "[http://www.iop.org/EJ/abstract/0368-3281/1/3/302/ Experiments with plasma rings]" (1961) ''Journal of Nuclear Energy''. Part C, Plasma Physics, Accelerators, Thermonuclear Research, Volume 1, Issue 3, pp. 116-120
*1960: Lindberg, L., E. Witalis and C. T. Jacobsen, "Experiments with plasma rings" (1960) ''Nature'' 185:452.
*1961: Hannes Alfvén: Plasma Ring Experiment in "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1961ApJ...133.1049A On the Origin of Cosmic Magnetic Fields]" (1961) ''Astrophysical Journal'', vol. 133, p.1049
*1961: Lindberg, L. & Jacobsen, C., "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1961ApJ...133.1043L&db_key=AST&data_type=HTML&format=&high=42ca922c9c03926 On the Amplification of the Poloidal Magnetic Flux in a Plasma]" (1961) ''Astrophysical Journal'', vol. 133, p.1043
*1962: Filippov. N.V., et al, "Dense, High-Temperature Plasma in a Noncylindrical 2-pinch Compression" (1962) 'Nuclear Fusion Supplement'. Pt. 2, 577
*1969: Buckwald, Robert Allen, "Dense Plasma Focus Formation by Disk Symmetry" (1969) [[Thesis]], [[Ohio State University]].
==Notes==
{{reflist}}
==External links==
* [http://ckplee.myplace.nie.edu.sg/plasmaphysics/ Plasma Radiation Source Lab at the National Institute of Education in Singapore]
* [http://www.icdmp.pl/pf1000.html Plasma Focus Laboratory, International Centre for Dense Magnetised Plasmas, Warsaw, Poland]
* Paper by Leopoldo Soto ([http://www.cchen.cl/index.php?option=com_content&task=category§ionid=11&id=111&Itemid=71 Chilean Nuclear Energy Commission, Thermonucluar Plasma Department]): [http://stacks.iop.org/PPCF/47/A361 New trends and future perspectives on plasma focus research]
* [http://www.focusfusion.org/ Focus Fusion Society]
* Abdus Salam ICTP Plasma Focus Laboratory. [http://mlab.ictp.it/plasma/pfd.html]
* Numerical Simulation Package: Universal Plasma Focus Laboratory Facility at INTI-UC. [http://www.intimal.edu.my/school/fas/UFLF/]
* [http://www.pladema.net Dense Plasma Focus Network in Argentina]
* Institute for Plasma Focus Studies (IPFS). [http://www.kirkbyites.net/IPFS/]
* Research papers published in 2008 by IPFS staff. [http://www.intimal.edu.my/school/fas/UFLF/Papers/listofpapers.htm]
{{fusion methods}}
{{Nuclear Technology}}
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