Experiments on DD Fusion Using Inertial Electrostatic Confinement

Acta Physica Universitatis Comenianae Volume LIII (2016) 7181 Experiments on D-D Fusion Using Inertial Electrostatic Confinement (IEC) Device in the Laboratory Exercises on Plasma Physics at the Comenius University* Michal Stano, Michal Raèko Department of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynska dolina F2, 842 48 Bratislava, Slovak republic [email protected] Abstract: Experiments using a small scale nuclear fusion reactor, known as fusor or Inertial Electro- static Confinement (IEC) device, are presented. Fusion of deuterium is achieved by acceleration for deuterium nuclei in a high voltage low pressure electrical discharge. Rate of fusion reaction is studied for discharge voltage of up to 32 kV and discharge power of 80 W by detection of the generated neutrons. Detection rates of up to 1.9 s1 are achieved using a moderated 6LiI(Eu) scintillation detector. Maximum total neutron flux generated by the device is estimated to 23200 neutrons per second, corresponding to 2.7´108 W of power released by the fusion reactions. Experiments at reduced discharge voltage of 20 kV are included in laboratory excercises on plasma physics held at the Comenius University. 1. Introduction Controlled nuclear fusion has been a field of intense research for more than last 50 years. The motivation for this research is to create a clean and inexhaustible source of energy powered by fusion of heavy hydrogen isotopes, deuterium and tritium. Although deuterium is relatively abundant element on Earth, representing 0.0156 % of all hydrogen at- oms, no current technology can exploit its potential [1]. In order to create conditions for nuclear fusion, the nuclei must overcome the electric repulsion and approach each other close enough to come within the range of the nuclear force. This can only be done by accelerating the nuclei to sufficient kinetic energies. Although there is no exact threshold energy for the fusion reactions to occur, reasonable reaction efficiency, expressed by the cross section of reaction, can only be achieved at energy in order of 104 eV and above, see Figure 1. The highest fusion cross-section is achieved by the reaction of deuterium with tritium 2D + 3T ® 4He (3.5 MeV) + 1n (14.1 MeV) (1) This reaction is the easiest one to be achieved at sufficient rate and therefore it is intended to be used in the fusion power plants, once they become available. However, use of radioactive tritium as a fuel and release of most of the energy in form of 14 MeV neutrons are re- garded as drawbacks of this reaction for power generation applications. *) Dedicated to Prof. V. Martiovit 75-th anniversary 72 M. STANO, M. RAÈKO Another reaction with reasonable cross-section is the D-D reaction, which proceeds in two reaction channels with nearly equal efficiency [2]: 2D + 2D ® 3He (0.8 MeV) + 1n (2.5 MeV) (2a) 2D + 2D ® 3T (1.0 MeV) + 1p (3.0 MeV) (2b) Figure 1 shows cumulative cross section of the both channels. Fig. 1. Cross-sections of fusion reactions [2] and of D-T elastic scattering [3]. 1 barn equals 1024 cm2. The required kinetic energy in the order of 104 eV to 105 eV can be obtained relatively eas- ily by accelerating deuterium ions in an electrostatic field. However, only a small part of collisions of the accelerated ions results in a fusion reactions. Even the largest cross-section of the D-T reaction is overshadowed by the elastic scattering cross section, which is higher by several orders of magnitude. Although the accelerator type fusion reactors, col- liding high energy ions into a low-energy target, are capable of achieving fusion reactions, fast redistribution of kinetic energy from high-energy ions to low-energy particles makes them unable to generate more energy by fusion than the energy invested into ion acceleration. In order to turn the nuclear fusion into a useful source of energy, the research has fo- cused on formation of a high temperature plasma with temperature in order of 108 K. The advantage of Maxwellian plasma is that the energy distribution among particles is not al- tered by their collisions. Since the necessary ion energy is achieved by the high temperature, the reaction is called thermonuclear. The most advanced thermonuclear reactors today are toroidal magnetic vessels called tokamaks and stellarators. Although the accelerator type fusion reactors are unable to serve as energy sources, they can be used as neutron generators for some other applications. These are mostly related to neutron activation analysis (NAA) [4]. Their main advantage over conventional EXPERIMENTS ON D-D FUSION USING ... 73 neutron sources is the lack of radioactive material and the possibility to control the neutron production rate. One type of the accelerator-type reactor had been developed by Farnsworth, the inven- tor of electrical television, and later improved by Hirsch [5]. The basic principle of their device, known as the fusor, is following. The reactor is composed of two electrodes placed in a vacuum vessel filled by a low pressure deuterium gas. The cathode, formed by a spherical grid, is placed in the center. The anode may be formed either by a concentric spherical grid or by the conductive vacuum vessel itself. The ions which are formed between the two electrodes in a high voltage electric discharge are accelerated towards the center by the negative high potential applied to the cathode. Depending on the grid trans- parency, the ions may pass the central area several times and, while possessing sufficient energy, some collisions may result in fusion reactions. Since the gas in the fusor is only weakly ionized, the accelerated ions collide mostly with the neutral gas molecules. The head-on collision among energetic ions are thus rather rare. Some fusors had been equipped with additional ion sources, sometimes referred to as ion guns [6]. In this way, amount of accelerated ions in the central area can be increased and also the discharge can be maintained at lower background pressure, thus enhancing the ion-ion collisions instead of ion-neutral ones. Lower pressure also reduces the charge exchange reactions between ions and neutrals and results in a longer ion life time. In addi- tion, use of additional ion sources also improves the ion energy distribution in favor of the high energy ions. Ions generated in the ionizer located at the anode pass, on their way to cathode, the full acceleration voltage. Contrary, the ions formed in the fusor chamber by electron impact ionization gain only reduced energy given by the potential drop between the position of their formation and the cathode. Recalling that electron impact ionization has the best efficiency at electron energy of about 100 eV, many ions are formed in vicin- ity of the cathode and gain too little energy to be able to induce the fusion reactions. The neutron output from a fusor varies depending on its construction and conditions of the discharge. Neutron yields from a few thousands neutrons per second (n/s) in small demonstration devices up to 109 n/s in the most advanced reactors have been achieved. The fusor build by Hirsch generated yields of up to 5´107 n/s in pure deuterium and up to 4´109 n/s in a deuterium-tritium mixture. His device was equipped with six ion guns and given neutron yields were achieved at discharge voltage of 150 kV and current of 10 mA. The idea of Farnsworth and Hirsh to isolate a non-Maxwellian plasma of fusion gases in a potential well is also known as an Inertial Electrostatic Confinement (IEC). Research related to IEC has been focusing on several fields, including theoretical and experimental studies of maximizing the neutron production efficiency, development of a grid-less potential wells and also development of IEC applications. An overview of the achievements in the field of IEC can be found in a book by Miley and Krupakar Murali [7]. One of the noteworthy recent application is development of a D-D fusion neutron source, which can be used to detect explosives by the means of NAA, intended for uncovering of anti-per- sonnel landmines [8]. Nevertheless, despite the considerable effort, IEC fusion had not develop into a practi- cal neutron source for any application. Authors are also unaware of any current commer- cial production of IEC devices. The main drawbacks of the IEC devices can be attributed to limited neutron yields covering only low range of requirements needed for NAA appli- 74 M. STANO, M. RAÈKO cations and impossibility to pack the device in a small volume in order to generate high neutron flux densities comparable with conventional radioactive sources. Another weak- ness is limited lifetime of cathode due to its continuous sputtering by energetic ions. In consequence, the only typical use of IEC fusion are the demonstration experiments. Due to a simple construction of IEC devices, these experiments are popular at universities and among amateur constructors, many of them being high school students [9]. The present paper reports on fusion experiments using a fusor constructed at the Comenius University. A high school project, on which this paper is based, was awarded a third place at the Intel ISEF 2013 international science fair. The fusor has later been adapted for a use as one of the experiments in the laboratory exercises on plasma physics held at the Comenius University. 2. The apparatus The fusor used in this work was built in a cylindrical vacuum chamber with the diame- ter of 70 mm and the height of 80 mm. The chamber was made of stainless steel and was sealed by copper and Viton seals.

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