Acta Physica Universitatis Comenianae Volume LIII (2016) 71–81

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 s–1 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´10–8 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 deute- rium 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 ac- celerating 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 radio- active 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. Martišovitš 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 10–24 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-sec- tion 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 tempera- ture, 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 re- lated 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 neu- tron 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 be- tween 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 po- tential 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. In the center, a spherical cathode was suspended on a high voltage feedthrough. Diameter of the cathode was 2 cm and it was made of a single piece of a tungsten wire with diameter of 800 µm. Prior to use the cathode was cleaned an- odically in 13 % solution of NaOH. The high voltage feedthrough was selfmade and it was composed of a central stainless steel rod of 2 mm diameter, a tube made of alumina ceram- ics, and an aluminum head on the air side which limited the surface electric field strength. All parts were sealed together using a Torr SealTM epoxy. The fusor was evacuated using a turbomolecular pump (TMP) backed by a rotary vane pump. The TMP was connected to the fusor chamber using a throttle valve, since the TMP had to operate at a lower pressure than the fusor. Deuterium gas was released into the fusor continuously using a fine leak valve. The pressure was monitored using a Penning gauge and a thermocouple gauge. One viewport was placed on the vacuum vessel to allow visual control of the discharge. The viewport was shielded by a lead glass to prevent the X-ray emissions. High voltage was supplied by a switched mode Spellman SL-150 supply, operating at voltage of up to 20 kV and current of up to 7.5 mA. Deuterium gas was generated by electrolysis of D2Oina Polymer Electrolyte Membrane (PEM) electrolyser and subsequently dried in a silica gel moisture trap. Neutrons were detected using a 6LiI(Eu) crystal mounted on the VA-S-50 universal scintillation probe. The detection was based on the absorption of thermal neutrons by the 6Li nuclei 6Li + 1n ® 3T (2.7 MeV) + 4He (2.1 MeV) (3) which generates a scintillation event. The detector was surrounded by a polyethylene sphere with a diameter of 10 cm called Bonner sphere, which acts as a moderator. The Bonner sphere thermalizes the neutrons prior to detection. A schematic view and a photography of the experimental setup are shown in Figures 2 and 3, respectively. EXPERIMENTS ON D-D FUSION USING ... 75

Fig. 2. Schematic view of the experimental setup. Bubblers contain silicone oil.

Fig. 3. Photo of the experimental setup. 76 M. STANO, M. RAÈKO

3. Results Discharge in the fusor was usually ignited at a voltage of about 5 kV and a pressure of about 15 Pa. The selfsustained discharge was than maintained in the constant current mode by means of the power supply. Voltage of the discharge depends on the pressure, as shown in the Figure 4. Pressure in the fusor was regulated by means of a fine leak valve. In this way, the voltage could be raised up to 20 kV, the maximum provided by the power supply. The power of the discharge had to be limited to 80 W in order to not overheat the cathode. This corresponds to 4 mA of the discharge current at the voltage of 20 kV. An overheated cathode would have increased the electron emission and electron current at the expense of ion current. This would have decreased the rate of fusion reactions. Conditions of the discharge and the cathode were visually monitored through the viewport. Stability of the discharge was good enough to perform experiments with duration exceeding one hour at stable discharge parameters. The discharge is shown in Figure 5.

Fig. 4. Dependence of discharge voltage on deuterium pressure at two values of constant current.

Thefusionreactionwasprovedbydetectionof neutrons. Reaction (2a) generates fast neutrons with energy of 2.5 MeV. These neutrons cannot be detected directly, because the reaction (3) proceeds only with thermal neutrons. Fast neutrons can be detected only with detector surrounded by a moderator. Figure 6 shows the detection count rates measured in three different situations. The first value was measured with the detector being moderated and the fusor discharge switched off. This corresponds to the combination of the detector noise and the back- ground neutrons. The next two measurements were done under the identical discharge pa- rameters. The middle one was done with the detector being not moderated whilst the last EXPERIMENTS ON D-D FUSION USING ... 77

Fig. 5. Photo of the discharge at voltage of 20 kV and current of 6 mA.

Fig. 6. Proof of fast neutrons by moderator removal experiment. Error bars correspond to 3s. one was done using the moderated detector. It can be seen that the neutrons generated in the fusor significantly raised the neutron count of the last measurement in comparison with the background and the unmoderated measurement. The measurement with unmoderated detector did not result in increased detection rate compared to the back- 78M. STANO, M. RAÈKO

ground level. This also proves that there is no other interaction between the fusor and the detector, such as an electromagnetic interference, than fast neutrons generated by the fusor, thermalized by the moderator and detected by the detector. The standard deviation s for all of the measurements was calculated, assuming the Poisson distribution, as a square root of the total counts detected. Although the achieved count rates were relatively low and a considerable time was necessary to produce good statistics, the results shown in the Fig. 6 can be reproduced during a lesson of the undergraduate laboratory exercise. A similar experiment was also performed at lower discharge voltages. Dependence of the detection rate on the discharge voltage at a fixed discharge power is shown in the Figure 7. It had been observed that the fusion rate depends on the voltage exponentially. In the studied range, the fusion rate doubles with each additional 2.6 kV of applied discharge voltage.

Fig. 7. Neutron detection rate as a function of discharge voltage at constant discharge power of 80 W. Error bars correspond to 3s.

The minimum discharge voltage, where the fusion could be observed, depended largely on detector sensitivity and length of the measurement. With the present setup, the fusion was observed down to 11 kV of discharge voltage. Each measurement of the mod- erator removal experiment exceeded one hour in this case. The results are shown in the Figure 8. A few experiments were also performed at elevated discharge voltage of up to 32 kV using the Tesla BS222A linear power supply. Although the stable discharge conditions could be maintained up to the indicated voltage, these experiments were not regarded as suitable for laboratory exercises due to the high-voltage and the X-ray safety concerns. EXPERIMENTS ON D-D FUSION USING ... 79

The neutron detection rate at the voltage of 32 kV and a current of 2.5 mA was measured to be 1.93 s–1 above the background level. In order to accurately determine the total neutron flux from the fusor, the sensitivity of the neutron detector would have had to be known. Unfortunately, no calibrated neutron detector, nor calibrated neutron source were available. A rough estimate of the total neu- tron production had been done based on the solid angle subtended by the area of the detec-

Fig. 8. Moderator removal experiment at 11 kV of discharge voltage. Error bars correspond to 3s. tor (the LiI crystal) at the center of the fusor. Accuracy of this estimate is believed to be better than a factor of 5. The purpose of this estimate was to get a sense of the related fu- sion parameters, namely the amount of neutrons generated per a Joule of the input energy and the power of the fusion reactions Pfuz in watts. Estimated values of these properties are given in the Table 1 together with the discharge parameters and the detection rates mea- sured above the background level for each situation. All measurements were done at the constant discharge power of 80 W. The estimated values of the total neutron production are in a good agreement with other fusor experiments performed under similar conditions and presented at forums of www.fusor.net [9]. For example, the experiments by W. Heil and N. Zudor were done with a fusor of the similar size. Using a calibrated detector, they observed a total neutron production of 5000 s–1 at discharge voltage of 20 kV and a current of 5 mA. Both, the neutron production and the discharge conditions are in close agreement with our results. Unfortunately, no work had been found in the peer-reviewed literature which would have been performed under similar conditions to the ones presented in this work. Most of the work related to the IEC and published in peer-reviewed literature was done using significantly higher discharge voltages resulting in a neutron production sev- eral orders of magnitude higher than the one presented in this work. Although the absolute 80 M. STANO, M. RAÈKO

scale of the total neutron production and related parameters given in the Table 1 were esti- mated, their relative variations were not affected by the estimate.

Table 1. Measured and estimated fusion parameters at various conditions of the discharge. Measured parameters: discharge voltage, discharge current, rate of detections above the background level. Indi- cated uncertainty of the detection rate corresponds to 3s. Estimated parameters: total generated neu- tron flux, amount of generated neutrons per Joule of input energy, power of fusion reactions in Watts. True values of the estimated parameters are believed to deviate from the estimated values by less than a factor of 5.

Measurement Estimate U (kV) I (mA) Detection rate Total neutron n/J P (W) (s–1) production (s–1) fuz 11.0±0.1 7.2±0.1 0.020±0.011 240 3 2.8´10–10 20.0±0.1 4.0±0.1 0.402±0.063 4830 62 5.9´10–9 32.0±1.0 2.5±0.5 1.93±0.13 23200 290 2.7´10–8

4. Conclusions Experiments on fusion of deuterium using the IEC device were included into labora- tory exercises on plasma physics held at the Comenius University, Department of Experi- mental Physics. The fusion reaction can be proved with a good statistics by the moderator removal experiment performed at 20 kV of discharge voltage within one lesson of the lab- oratory exercise. These experiments help students to understand what are the conditions necessary for the fusion of heavy hydrogen isotopes, reasons of poor efficiency of the IEC fusion devices, and the requirement of a high-temperature Maxwellian plasma needed to turn the fusion into a useful source of energy. In addition, the fusor experiments allow the students to get the basic practical experience on the neutron physics. The fusor acts as a source of monoenergetic 2.5 MeV neutrons and therefore detection of thermal neutrons, insensitivity of detector to high energy neutrons, and neutron moderation can be clearly demonstrated.

Acknowledgements This work was supported by the Enel company, the Pontis foundation and the Slovak grant agency Vega, project nr. 1/0514/12. Authors thank doc. M. Florek and RNDr. I. Szarka for helpful advices in the field of neutron detection and for providing the neutron detector for this work. This work was presented at the 20th Conference of Slovak Physicist [10].

References [1] G. McCraken, P. Scott: Fusion the Energy of Universe. Elsevier, San Diego 2005. [2] H. S. Gosch, G. M. Hale: Improved Formulas for Fusion Cross Sections and Thermal Reactivities. Nucl. Fusion 32 (1992) 611–631. EXPERIMENTS ON D-D FUSION USING ... 81

[3] A. A. Harms, K. F. Schoepf, G. H. Miley, D. R. Kingdon: Principles of Fusion Energy. World Scientific Publishing Co., Singapore 2000. [4] E. H. Seabury, B. W. Blackburn, D. L. Chichester, C. J. Wharton, A. J. Caffrey: Comparison of DD, DT and Cf-252 Neutron Excitation of Light and Medium Mass Nuclei for Field PGNAA Applications. Nucl.Instr.andMeth.inPhys.Res.B261(2007) 839–844. [5] R. L. Hirsch: Inertial-Electrostatic Confinement of Ionized Fusion Gases. J. Appl. Phys. 38 (1967) 4522. [6] T. Takamatsu, K. Masuda, T. Kyunai, H. Toku, K. Yoshikawa: Inertial Electrostatic Confinement Fusion Device with an Ion Source Using a Magnetron Discharge, Nucl. Fusion 46 (2006) 142–148. [7] G. H. Miley, S. Krupakar Murali: Inertial Electrostatic Confinement (IEC) Fusion Fundamentals and Applications. Springer, New York 2014. [8] K.Yoshikawa,K.Masuda,T.Takamatsu,S.Shiroya,T.Misawa,E.Hotta,M.Ohnishi,K.Yamauchi, H. Osawa, Y. Takahashi: Research and Development of a Compact Discharge-driven D-D Fusion Neutron Source for Explosive Detection. Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 299–302. [9] www.fusor.net/board/ [10] M. Raèko, M. Stano: D-D Fusion Neutron Generation in the Farnsworth Fusor. In: Proc. 20th Conference of Slovak Physicists, Bratislava (2013) 77–78.