Superkonduktor Merupakan Organik yang Sederhana

SUPER KRISTAL Metal yang disuntikkan dari molekul dapat mensuperkonduksikan.

Penemuan suatu superkonduktor hidrokarbon baru bertemperatur tinggiBottom of Form, berdasarkan pada subunit graphene, menunjukkan kelas baru pertama kali dari superkonduktor organis lebih dari beberapa dekade dan membawa potensi bagi para peneliti untuk mengembangkan variasi molekular yang tidak terhingga. Hal ini juga membantu mengarahkan bidang fisika yang mendiominasi terhadapa superkonduktifitas pada arah bidang ilmu kimiawi. 

Sebuah tim dari Jepang yang dipimpin oleh Yoshihiro Kubozono, seorang profesor ilmu kimia dan ilmu pengetahuan permukaan pada Okayama University, melaporkan bahwa kristal molekul picene planar—yang tersusun dari lima benzene terfusi—mensuperkonduksikan pada suhu 18 K saat disuntikan dengan atom potassium atau rubidium ( Nature 2009, 464, 76).

Meskipun suhu tersebut sangat relatif dingin sekali dibandingkan dengan suhu lebih dari 100 K mensuperkonduksikan temperatur (T_c) dari beberapa superkonduktor keramik, namun hal ini sebanding dengan T_c dari superkonduktor organis lainnya seperti potassium yang disuntikkan buckminsterfullerene (38 K) dan kalsium yang ter-interkalasikan dengan graphite (11 K).

Para ilmuwan melanjutkan untuk mencari superkonduktor baru bertemperatur tinggi karena mereka pikir akan menjadi bahan ideal nantinya bagi motor listrik yang efisien dan penyimpanan tenaga serta sistem distribusinya.

Dikarenakan superkonduktor bertemperatur tinggi dimulai dengan munculnya di laboratorium pada tahun 1980an, daftarnya telah meluas dari bahan perunggu oksida pertama kalinya hingga meliputi seperti persenyawaan magnesium diborida dan juga beberapa molekul organis. Sebagaimana picene yang dianggap sebagai suatu fragmen dari bahan karbon graphene berkawat kandang ayam, Kubozono menjelaskan, alkali yang disuntikkan superkonduktor acene sdapat saja menjadi suatu kelurga besar.
“Picene bukanlah molekul spesial, namun sangat umum sekali,” kata Kubozono. “Lebih lanjut kita mengharapkan adanya superkonduktor acene baru.”

Menurut beberapa teori mengenai superkonduktifitas, dengan menurunkan beberapa temperatur bahannya akan menghasilkan apa yang disebut dengan elektro pasangan Perunggu yang mengatasi repulsi mutual mereka dan selanjutnya dapat mengalir melalui bahan yang segera terjadi. Bahan organis yang mensuperkonduksikan umumnya berdasarkan pada persenyawaan aromatik, yang memiliki sistem π orbitals. Beberapa elektron mendonasikan kepada π orbital dari atom metal alkali dapat mensuperkonduksikan, dibawah kondisi tertentu.

Penulisnya beralasan bahwa dikarenakan picene menyerupai segmen dua dimensional dari graphite, hal ini kemungkinan juga mensuperkonduksikan saat disuntik. Kedua hal ini dilakukan di Inggris, sebagaimana apa yang dikatakan profesor bahan kimiawi yaitu Matthew J. Rosseinsky pada University of Liverpool dan Kosmas Prassides pada Durham University dalam sebuah pandangan mengenai laporan ini bahwa “hal ini merupakan contoh pertama kalinya dari superkonduktor molekular dimana komponen organisnya berisi hanya atom karbon dan hidrogen.”

Meskipun kemiripan picene terhadap graphite, elektronisnya menyerupai beberapa metal superkonduksi yang disuntik fullerenes, catat mereka. Meskipun elektronis tersebut adalah buktinya, mekanisme superkonduksi picene belumlah sepenuhnya menjelaskan apa-apa, laporan dari tim Kubozono. Namun hal yang penting dari struktur picene adalah menyoroti  pada saat dibandingkan dengan molekul pentacene, yang mana bersifat isomeric dengan picene, namun bergaris lurus: Metal alkali  yang terinterkalasikan dengan beberapa molekul pentacene tidaklah mensuperkonduksikan.

“Saya pikir perbedaan yang saling berbenturan ini adalah sangat menarik dan menyarankan sebuah petunjuk dalam memahami  asal muasal superkonduktifitas pada sistem hidrokarbon aromatik terinterkalasi,” kata profesor Hideo Hosono dari Tokyo Institute of Technology, yang baru-baru ini laboratoriumnya menemukan sebuah keluarga dari superkonduktor besi arsenida. Laboratorium Kubozono sekarang ini sedang mencari yang berkenaan dengan superkonduktor dengan menginterkalasikan atom metal kedalam acene lainnya.

Hosono menjelaskan bahwa banyak sekali pemain pada penelitian superkonduktifitas baru-baru ini memiliki latar belakang ilmu kimiawi. Banyak sekali dari penemuan superkonduksi di laboratoriumnya dilaporkan pertama kalinya pada jurnal ilmu kimiawi seperti  Journal of the American Chemical Society.

 
“Penelitian material pada superkonduktor secara luas telah kilakukan [pada bidang] bahan fisika terkondensasi,” kata Hosono. “Bagaimanapun juga, Saya merasa peranan ilmu kimiawi sangatlah cepat berkembang dalam mengeksplorasi superkonduktor baru.”

Sumber : www.chem-is-try.org

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Tokamak Fusion Test Reactor

Tokamak Fusion Test Reactor, 1989	Side view of the TFTR	Inside the Tokamak Fusion Test Reactor

TFTR Achievements	TFTR Parameters	TFTR Publications
The Tokamak Fusion Test Reactor (TFTR) operated at the Princeton Plasma Physics Laboratory (PPPL) from 1982 to 1997. TFTR set a number of world records, including a plasma temperature of 510 million degrees centigrade -- the highest ever produced in a laboratory, and well beyond the 100 million degrees required for commercial fusion. In addition to meeting its physics objectives, TFTR achieved all of its hardware design goals, thus making substantial contributions in many areas of fusion technology development.  In December, 1993, TFTR became the world's first magnetic fusion device to perform extensive experiments with plasmas composed of 50/50 deuterium/tritium -- the fuel mix required for practical fusion power production. Consequently, in 1994, TFTR produced a world-record 10.7 million watts of controlled fusion power, enough to meet the needs of more than 3,000 homes. These experiments also emphasized studies of behavior of alpha particles produced in the deuterium-tritium reactions. The extent to which the alpha particles pass their energy to the plasma is critical to the eventual attainment of sustained fusion.  In 1995, TFTR scientists explored a new fundamental mode of plasma confinement -- enhanced reversed shear. This new technique involves a magnetic-field configuration which substantially reduces plasma turbulence.

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How Nuclear Fusion Reactors Work

Fusion Reactors: Magnetic Confinement

There are two ways to achieve the temperatures and pressures necessary for hydrogen fusion to take place:

• Magnetic confinement uses magnetic and electric fields to heat and squeeze the hydrogen plasma. The ITER project in France is using this method.
• Inertial confinement uses laser beams or ion beams to squeeze and heat the hydrogen plasma. Scientists are studying this experimental approach at the National Ignition Facility of Lawrence Livermore Laboratory in the United States.

Let's look at magnetic confinement first. Here's how it would work:

Microwaves, electricity and neutral particle beams from accelerators heat a stream of hydrogen gas. This heating turns the gas into plasma. This plasma gets squeezed by super-conducting magnets, thereby allowing fusion to occur. The most efficient shape for the magnetically confined plasma is a donut shape (toroid).

A reactor of this shape is called a tokamak. The ITER tokamak will be a self-contained reactor whose parts are in various cassettes. These cassettes can be easily inserted and removed without having to tear down the entire reactor for maintenance. The tokamak will have a plasma toroid with a 2-meter inner radius and a 6.2-meter outer radius.

Let's take a closer look at the ITER fusion reactor to see how magnetic confinement works.

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Sekilas Tentang Tokamak dan Fusi Thermonuklir

Pendahuluan

Selama manusia mendiami bumi, selama itu pulalah manusia membutuhkan energi bagi kehidupannya. Konsumsi energi oleh manusia selalu bertambah seiring dengan bertambahnya penghuni bumi dan meningkatnya  kemampuan teknologi manusia. Dengan kemampuannya manusia selalu mencoba mencari alternatif-alternatif penyelesaian dalam persoalan keberlangsungan hidupnya termasuk dalam sektor energi.

Salah satu impian para ilmuwan dan teknologi adalah bagaimana mencontoh reaksi fusi yang terjadi di matahari, di muka bumi sebagai sumber energi. Untuk mendapatkan tipe reaksi yang berlangsung di matahari dalam suatu “mesin energi” di bumi merupakan mimpi sejak awal petualangan nuklir untuk energi. Solusi yang kelihatannya lebih mudah adalah melakukan penggabungan 2 isotop hidrogen, deutrium (inti yang mengandung 1 proton dan 1 neutron) dan tritium ( 1 proton dan 2 neutron).

Masalah yang muncul adalah bagaimana mendapatkan panas yang cukup tinggi agar deutrium dan tritium dapat bergabung (fusi), bagaimana mengontrol reaksi-reaksi fusi tersebut dalam kondisi diadaptasikan dengan eksploitasi industrial. Bagaimana panas yang telah diperoleh dalam kondisi plasma dapat dipertahankan agar reaksi terus berlangsung sambil melepaskan energi yang dapat bermanfaat?

Plasma Tokamak

Plasma merupakan campuran partikel-partikel bermuatan, maka plasma dapat dikontrol oleh medan magnet.  Medan magnet yang sesuai akan dapat digunakan untuk mengurung plasma dengan kerapatan yang cukup tinggi dan kesetabilan energi dengan waktu yang cukup panjang. Untuk pengurungan plasma dengan medan magnet yang terkemuka saat ini adalah teknik Plasma Tokamak (tokamak suatu akronim bahasa rusia dari “toroidalnya kamera ve magnetnaya katushka” = ”toroidal chamber with magnetic coil”.  Teknik ini diusulkan pertama kali oleh dua fisikawan Rusia,  keduanya pemenang hadiah nobel Andrei Sakharov dan Igor Tamm.

Konsep plasma tokamak diusulkan dalam kaitannya dengan ide untuk mengontrol reaksi fusi nuklir.  Dan kini teknik plasma tokamak merupakan satu-satunya model untuk mengusahakan terjadinya reaksi fusi termonuklir.  Reaksi fusi terjadi jika inti-inti dari unsur-unsur ringan bergabung menjadi suatu unsur yang lebih berat

Sebuah mega proyek plasma Tokamak telah direncanakan oleh gabungan Uni-Eropa Rusia, Amerika Serikat dan Jepang dan akan diputuskan di mana akan dilakukan pembangunannya pada tahun 1998.  Proyek itu bernama International Thermonuclear Experimental Reactor (ITER).  Rencananya ITER akan mengkombinasikan beberapa teknik unggul dari Tokamak yang telah dilakukan di negara-negara anggota.

Plasma Tokamak dan Reaktor Fusi

Konsep plasma tokamak diusulkan dalam kaitan dengan ide untuk mengontrol reaksi fusi nuklir. Yang paling mudah (secara teoretis) adalah reaksi antara  dua isotop hidrogen (deutrium dan tritium) seperti reaksi R1. Reaksi ini  selain membebaskan energi juga melepaskan neutron cepat dan pembentukan inti atom helium (partikel a ). Dalam suatu reaktor fusi, zona reaksi dikelilingi oleh suatu selimut dan selimut ini merupakan daerah konservasi energi (energi terbebaskan dari reaksi fusi dan energi kinetik neutron) menjadi energi panas.

Kriteria Lawson

Agar temperatur tinggi (T) dapat dipertahankan, kosentrasi (densitas) inti  n harus cukup tinggi dan kondisi ini harus bertahan selama waktu pengungkungan energi t cukup panjang. Suatu formula sederhana yang dikenal dengan Kriteria Lawson diusulkan pada tahun 1957 oleh Lawson, menunjukkan jika inti-inti yang mengalami penggabungan  adalah deutrium-tritium maka perkalian nTt harus lebih besar dari 6 . 10²¹ dengan satuan n adalah partikel per meter kubik, t dalam detik, dan T dalam keV. Kriteria Lawson inilah yang menjadi standar unjuk kerja dari suatu reaktor Fusi termonuklir. Berbagai reaktor di dunia terus menunjukkan perkembangan tiga perkalian fusi (triple fusion product).

Pembangkit plasma dalam Reaktor Fusi themonuklir

Pada umumnya  untuk membangkitkan plasma digunakan pemanasan radio frekuensi (Radio frequency heating). Sistem pemanasan ini dikenal dengan nama Ion Cyclotron Resonance Frequency ( ICRF). ICRF dioperasikan pada rentang frekuensi 23-57 MHz. Sebagai contoh yang digunakan oleh JET pemanasan ICRF terdapat 8 unit modul identik, setiap unit terdiri dari sebuah tandem amplifier, seperangkat tranmisi koaksial  tersusun  menjadi elemen-elemen. Elemen-elemen ini merupakan antena yang ditempatkan tepat pada dinding reaktor. Kedelapan generator RF pada JET memproduksi daya maksimum sebesar 32 MW. Daya tersebut yang digunakan untuk mengionisasi gas deutrium dan tritium menjadi plasma dan mempertahankan kondisi plasma. Daya bersih yang digunakan JET untuk pemanasan plasma ini sekitar 22,7 MW.

Realisasi Energi Fusi untuk Pembangkit Tenaga Listrik

Reaktor Fusi themonuklir yang dikembangkan kini di seluruh dunia masih menggunakan model Plasma Tokamak. Tidak terdapat perubahan yang berarti kecuali beberapa kajian yang selalu ditingkatkan untuk mendapatkan unjuk kerja yang semakin handal.

Energi yang dihasilkan oleh reaksi fusi akan dikonversikan menjadi beberapa bentuk energi lain seperti energi untuk neutron cepat dan partikel alpha (inti atom Helium) serta energi sisa. Energi sisa ini sebagian digunakan untuk menahan kondisi plasma tokamak pada kondisi kriteria Lawson. Sebagian lain dari sisa energi tersebut akan dikonversikan menjadi energi panas melalui selimut (blanket) di sekitar inti reaktor. Selimut ini memanfaatkan energi neutron cepat menjadi energi panas. Panas yang tersimpan pada selimut melalui sistem pertukaran panas digunakan untuk menguapkan air. Uap air ini yang digunakan untuk memutar turbin generator, sehingga energi listrik dapat dihasilkan seperti yang lazim pada cara konvensional.

Permasalahan yang masih harus mendapatkan penyelesaian serius adalah tidak mampunya sistem reaktor plasma tokamak tetap menahan keadaan plasma pada kondisi kriteria Lawson ketika sumber daya pemanasan plasma (ICRF) diputus. Di dunia hanya satu kali terjadi di Joint European Torus (JET) dan berlangsung hanya selama dua menit.

Selesai ditulis di Surabaya pada 17 November 2011

Oleh : Muhammad Nur

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Tokamak Reactor

The tokamak is the most successful device developed so far to attain the conditions for fusion. It is a toroidal device (shaped like a car tire) in which a vacuum vessel contains a plasma ring confined by twisting magnetic fields.

Note - the word tokamak is an acronym for the Russian words toroidal'naya kamera magnitnoi katushki, meaning toroidal chamber and magnetic coil. 

Tokamak configuration - The transient electric current that circulates in the primary coil of a tokamak induces a current in the plasma ring, which both heats the plasma and produces the poloidal magnetic field. The other important component is the toroidal magnetic field, which is generated by electric currents circulating in the toroidal field coil rings around the torus. In addition, the currents circulating in the position control coils generate auxiliary magnetic field components that modify the poloidal field, equilibrating the plasma ring and controlling its position. It is the combination of toroidal and poloidal magnetic fields that leads to the improved confinement of tokamak plasmas.

 Main components of the tokamak type magnetic confinement system.

Plasma heating - The most efficient way to heat a tokamak plasma is by passing through it a current induced by the primary coil. This coil is the primary circuit of a transformer in which the plasma ring constitutes the secondary circuit. It works like an electric heater, the amount of heat generated depending on the current and the resistance of the plasma. Unfortunately, the plasma resistivity decreases as the temperature rises and the heating process becomes less effective. The maximum temperature that can be achieved in tokamaks by the resistive heating (or ohmic heating) method is about 3×10⁷ K, twice the temperature in the center of the sun but less than needed to startup a reactor, about 10⁸ K. In tokamak experiments auxiliary heating is used to reach temperatures currently as high as 5×10⁸ K (more than 30 times the temperature at the sun-center). The two main methods of additional heating is by the injection of high-energy neutral particle beams and radiofrequency waves of various types.

Selesai ditulis di Surabaya pada 7 November 2011

Oleh Supriyono

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Advantages of fusion

Fusion offers significant potential advantages as a future source of energy – as just part of a varied world energy mix.

Abundant fuels

Deuterium is abundant as it can be extracted from all forms of water. If all the world’s electricity were to be provided by fusion power stations, present deuterium supplies from water would last for millions of years.

Tritium does not occur naturally and will be bred from Lithium within the machine. Therefore, once the reaction is established, even though it occurs between Deuterium and Tritium, the external fuels required are Deuterium and Lithium.

Lithium is the lightest metallic element and is plentiful in the earth’s crust. If all the world’s electricity were to be provided by fusion, known Lithium reserves would last for at least one thousand years.

The energy gained from a fusion reaction is enormous. To illustrate, 10 grams of Deuterium (which can be extracted from 500 litres of water) and 15 grams of Tritium (produced from 30 grams of Lithium) reacting in a fusion powerplant would produce enough energy for the lifetime electricity needs of an average person in an industrialised country.

Inherent safety

The fusion process in a future power station will be inherently safe. As the amount of Deuterium and Tritium in the plasma at any one time is very small (just a few grammes) and the conditions required for fusion to occur (e.g. plasma temperature and confinement) are difficult to attain, any deviation away from these conditions will result in a rapid cooling of the plasma and its termination. There are no circumstances in which the plasma fusion reaction can ‘run away’ or proceed into an uncontrollable or critical condition.

Environmental advantages

Like conventional nuclear (fission) power, fusion power stations will produce no ‘greenhouse’ gases – and will not contribute to global warming.

As fusion is a nuclear process the fusion powerplant structure will become radioactive – by the action of the energetic fusion neutrons on material surfaces. However, this activation decays rapidly and the time span before it can be re-used and handled can be minimised (to around 50 years) by careful selection of low-activation materials. In addition, unlike fission, there is no radioactive ‘waste’ product from the fusion reaction itself. The fusion byproduct is Helium – an inert and harmless gas.

Selesai ditulis di Surabaya pada 6 November 2011

Oleh Supriyono

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Fusion as a future energy source

Global demand for energy continues to grow year by year as the world population expands and society becomes more and more dependent on energy supplies. The need to find new sources of energy becomes increasingly important as environmental concerns mount over the emission of CO₂ from burning fossil fuels.

There is mounting concern that the emission of CO2 from burning fossil fuels is producing climatic change

At a European level, future energy supply was discussed in an EU Green Paper published in 2000 – ‘Towards a European strategy for the security of energy supply’, and a published in 2005. Of particular concern is the dependency Europe has on importing its energy from outside the EU (50% today and predicted to be 70% in 2030). The long term role of fusion is recognised in this report – ‘Thermonuclear fusion also bodes well for the future and could take over the reins from some existing energy sources towards the middle of the century’.

At national, European and international levels, future energy supply is becoming one of the key issues. Fusion offers a valuable alternative in future energy mix scenarios.

Selesai ditulis di Surabaya pada 5 November 2011

Oleh Supriyono

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Measuring the plasma

 Some of the techniques used for measuring the properties of plasmas

Measuring the key plasma properties is one of the most challenging aspects of fusion research. Knowledge of the important plasma parameters (temperature, density, radiation losses etc) is very important in increasing understanding of plasma behaviour and designing, with confidence, future devices. However, as the plasma is contained in a vacuum vessel and its properties are extreme (extremely low density and extremely high temperature), conventional methods of measurement are not appropriate. Thus, plasma diagnostics are normally very innovative and often measure a physical process from which information on a particular parameter can be deduced.

Measurement techniques can be categorised as active or passive. In active plasma diagnostics, the plasma is probed (via laser beams, microwaves, probes etc) – to see how the plasma responds. For instance, in inteferometers, the passage of a microwave beam through the plasma will be slowed by the presence of the plasma (compared to the passage through vacuum). This measures the refractive index of the plasma from which the density of plasma ions/electrons can be interpreted. With all active diagnostics, it must be ensured that the probing mechanism does not significantly affect the behaviour of the plasma.

With passive plasma diagnostics, radiation and particles leaving the plasma are measured – and this knowledge is used to deduce how the plasma behaves under certain conditions. For instance, during D-T operation on JET, neutron detectors measure the flux of neutrons emitted from the plasma. All wavelengths of radiated waves (visible, UV waves, X-rays etc) are also measured – often from many locations in the plasma. Then a detailed knowledge of the process which created the waves can enable a key plasma parameter to be deduced.

Selesai ditulis di Surabaya pada 4 November 2011

Oleh Supriyono

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Heating the plasma

One of the main requirements for fusion is to heat the plasma particles to very high temperatures or energies. The following methods are typically used to heat the plasma – all of them are employed on JET.

Ohmic Heating and Current Drive

Currents up to 5 million amperes are induced in the JET plasma – typically via the transformer or solenoid. As well as providing a natural pinching of the plasma column away from the walls, the current inherently heats the plasma – by energising plasma electrons and ions in a particular toroidal direction. A few megawatts of heating power is provided in this way.

Neutral Beam Heating

Beams of high energy, neutral deuterium or tritium atoms are injected into the plasma, transferring their energy to the plasma via collisions with the plasma ions. The neutral beams are produced in two distinct phases. Firstly, a beam of energetic ions is produced by applying an accelerating voltage of up to 140,000 Volts. However, a beam of charged ions will not be able to penetrate the confining magnetic field in the tokamak. Thus, the second stage ensures the accelerated beams are neutralised (i.e. the ions turned into neutral atoms) before injection into the plasma. In JET, up to 21MW of additional power is available from the NBI heating systems.

Radio-Frequency Heating

As the plasma ions and electrons are confined to rotating around the magnetic field lines in the tokamak, electromagnetic waves of a frequency matched to the ions or electrons are able to resonate – or damp its wave power into the plasma particles. As energy is transferred to the plasma at the precise location where the radio waves resonate with the ion/electron rotation, such wave heating schemes have the advantage of being localised at a particular location in the plasma.

In JET, a number of antennae in the vacuum vessel propagate waves in the frequency range of 25-55 megahertz into the core of the plasma. These waves are tuned to resonate with particular ions in the plasma – thus heating them up. This method can inject up to 20 megawatts of heating power.

Waves can also be used to drive current in the plasma – by providing a “push” to electrons travelling in one particular direction. In JET, 10 megawatts of these so-called Lower Hybrid microwaves (at 3.7 gigahertz) accelerate the plasma electrons to generate a plasma current of up to 3 megawatts.

Self Heating of Plasma

The Helium ions (or so-called alpha-particles) produced when Deuterium and Tritium fuse remain within the plasma’s magnetic trap for a time – before they are pumped away through the divertor. The neutrons (being neutral) escape the magnetic field and their capture in a future fusion powerplant will be the source of fusion power to produce electricity.

When fusion power out just equals the power required to heat and sustain plasma then a Breakeven is achieved. However, only the fusion energy contained within the Helium ions heats the Deuterium and Tritium fuel ions (by collisions) to keep the fusion reaction going. When this self-heating mechanism is sufficient to maintain the plasma temperature required for fusion the reaction becomes self-sustaining (i.e. no external plasma heating is required). This condition is referred to as Ignition. In magnetic plasma confinement of the D-T fusion reaction the condition for ignition is approximately six times more demanding (in confinement time or in plasma density) than the condition for breakeven.

Selesai ditulis di Surabaya pada 3 November 2011

Oleh Supriyono

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Magnetic plasma confinement

Since a plasma comprises charged particles : ions (positive) and electrons (negative), powerful magnetic fields can be used to isolate the plasma from the walls of the containment vessel – thus enabling the plasma to be heated to temperatures in excess of 100 million Kelvin. This isolation of the plasma reduces the conductive heat loss through the vessel and also minimises the release of impurities from the vessel walls into the plasma that would contaminate and further cool the plasma by radiation.

 Charged particles spiral along the magnetic field lines

In a magnetic field the charged plasma particles are forced to spiral along the magnetic field lines. The most promising magnetic confinement systems are toroidal (from torus: ring-shaped) and, of these, the most advanced is the Tokamak. Currently, JET is the largest Tokamak in the world although the future ITER machine will be even larger.

Other, non magnetic plasma confinement systems are being investigated – notably laser-induced inertial confinement fusion systems.

Selesai ditulis di Surabaya pada 2 November 2011

Oleh Supriyono

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Conditions for a fusion reaction

Three parameters (plasma temperature, density and confinement time) need to be simultaneously achieved for sustained fusion to occur in a plasma. The product of these is called the fusion (or triple) product and, for D-T fusion to occur, this product has to exceed a certain quantity – derived from the so-called Lawson Criterion after British scientist John Lawson who formulated it in 1955.

 A still from a typical JET experiment - View of a plasma from a CCD video camera (from behind a quartz window).

Attaining conditions to satisfy the Lawson criterion ensures the plasma exceeds Breakeven – the point where the fusion power out exceeds the power required to heat and sustain the plasma.

Temperature

Fusion reactions occur at a sufficient rate only at very high temperatures – when the positively charged plasma ions can overcome their natural repulsive forces. Typically, in JET, over 100 million Kelvin is needed for the Deuterium-Tritium reaction to occur – other fusion reactions (e.g. D-D, D-He³) require even higher temperatures.

Density

The number of fusion reactions per unit volume is roughly proportional to the square of the density. Therefore the density of fuel ions must be sufficiently large for fusion reactions to take place at the required rate. The fusion power generated is reduced if the fuel is diluted by impurity atoms or by the accumulation of Helium ions from the fusion reaction itself. As fuel ions are burnt in the fusion process they must be replaced by new fuel and the Helium products (the “ash”) must be removed.

Energy Confinement

The Energy Confinement Time is a measure of how long the energy in the plasma is retained before being lost. It is officially defined as the ratio of the thermal energy contained in the plasma and the power input required to maintain these conditions. At JET we use magnetic fields to isolate the very hot plasmas from the relatively cold vessel walls in order to retain the energy for as long as possible. A significant fraction of losses in a magnetically-confined plasma is due to radiation. The confinement time increases dramatically with plasma size (large volumes retain heat much better than small volumes)- the ultimate example being the Sun whose energy confinement time is massive.

For sustained fusion to occur, the following plasma conditions need to be maintained simultaneously.

• Plasma temperature: (T) 100-200 million Kelvin
• Energy Confinement Time: (t) 4-6 seconds
• Central Density in Plasma: (n) 1-2 x 10²⁰ particles m^-3 (approx. 1/1000 gram m^-3, i.e. one millionth of the density of air).
Note that at higher plasma densities the required confinement time will be shorter but it is very challenging to achieve higher plasma densities in realistic magnetic fields.

Selesai ditulis di Surabaya pada 14 November 2011

Oleh Supriyono

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