Patent Publication Number: US-6985553-B2

Title: Ultra-short ion and neutron pulse production

Description:
RELATED APPLICATIONS 
     This application claims priority of Provisional Application Ser. No. 60/350,071 filed Jan. 23, 2002, which is herein incorporated by reference. 
    
    
     GOVERNMENT RIGHTS 
     The United States Government has rights in this invention pursuant to Contract No. DE-AC03-76SF00098 between the United States Department of Energy and the University of California. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to plasma ion generators and neutron sources based on plasma ion generators, and more particularly to the production of ultra-short pulses from these ion generators and neutron sources. 
     In many applications, such as time of flight measurements, ultra-short neutron pulses (pulse width&lt;1 μs) with fast rise times or fall times are desired. These neutrons can be high energy, epithermal, thermal, or cold neutrons, and they are normally produced by a fission reactor or an accelerator-based neutron generator. When ultra-short pulses are needed, the neutron output flux can be chopped by means of a rotating mechanical chopper. 
     There are some disadvantages when these mechanical chopper schemes are used to form ultra-short neutron pulses. First, a large percentage of neutrons will be discarded and activation of material may occur. Second, when pulsed accelerator systems are employed, the mechanical chopper and the ion beam acceleration have to be properly synchronized. Ultra-short pulses cannot be formed by manipulating the plasma discharge because the rise time due to plasma buildup is typically on the order of a few μs. 
     Other neutron sources are based on ion generators. Conventional neutron tubes employ a Penning ion source and a single gap extractor. The target is a deuterium or tritium chemical embedded in a molybdenum or tungsten substrate. 
     University of California, Lawrence Berkeley National Laboratory has produced a number of compact neutron sources with a relatively high flux, particularly sources which generate neutrons using the D—D reaction instead of the D–T reaction. These sources have a variety of different geometries, including tubular, cylindrical, and spherical, and are based on plasma ion sources, particularly multicusp plasma ion sources, with single or preferably multiple beamlet extraction. These neutron sources are illustrated by copending U.S. patent applications Ser. Nos. 10/100,956; 10/100,962; and 10/100,955. 
     SUMMARY OF THE INVENTION 
     The invention is an ion source with an extraction system configured to produce ultra-short ion pulses, i.e. pulses with pulse width of about 1 μs or less and fast rise times or fall times or both, and a neutron generator based on the ion source which produces correspondingly ultra-short neutron pulses. A deuterium ion (or mixed deuterium and tritium ion or even a tritium ion) plasma is produced by RF excitation in a plasma ion generator using an RF antenna. The ion generator is preferably a multicusp plasma ion source. The single or multi-aperture extraction system of the ion source has two spaced electrodes—a plasma electrode and an extraction electrode. Although a single aperture extraction system can be used, a multi-aperture extraction system is preferred for higher ion extraction current and neutron flux. The plasma and extraction electrodes of a multiple beamlet system are typically spherical or cylindrical in shape. 
     To form a neutron generator, a neutron generating target is positioned to receive the extracted ion beam from the ion generator. The extracted ions are accelerated to energies in excess of 100 keV before impinging on the target, which becomes loaded with neutral deuterium and/or tritium atoms. Very short pulses of 2.45 MeV D—D neutrons or 14.1 MeV D-T neutrons will be produced by striking the target with ultra-short ion beam bursts. 
     To produce the ultra-short ion or neutron pulses, the apertures in the extraction system are suitably sized to prevent ion leakage, the electrodes are suitably spaced, and the extraction voltage is controlled. The ion beam current leaving the source is regulated by applying short voltage pulses of a suitable voltage on the extraction electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view of an ion source and neutron generator which can be used to produce ultra-short pulses according to the invention. 
         FIGS. 2 ,  3  are more detailed views of the extraction/acceleration system of the ion source. 
         FIGS. 4A–C  illustrate the effects of aperture size on ion extraction. 
         FIG. 5  is a cross sectional view of a simple single hole beam switching system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A. Ion Source, Neutron Source 
     As shown in  FIG. 1 , compact high flux neutron generator  10  has a plasma ion source or generator  12 , which typically is formed of a cylindrical shaped chamber. The principles of plasma ion sources are well known in the art. Preferably, ion source  12  is a magnetic cusp plasma ion source. Permanent magnets  14  are arranged in a spaced apart relationship, running longitudinally along plasma ion generator  12 , to form a magnetic cusp plasma ion source. The principles of magnetic cusp plasma ion sources are well known in the art. Conventional multicusp ion sources are illustrated by U.S. Pat. Nos. 4,793,961; 4,447,732; 5,198,677; 6,094,012, which are herein incorporated by reference. 
     Ion source  12  includes an RF antenna (induction coil)  16  for producing an ion plasma  18  from a gas which is introduced into ion source  12 . RF antenna  16  is connected to RF power supply  20  through matching network  22 . Ion source  12  may also include a filament  24  for startup. For neutron generation the plasma is preferably a deuterium ion plasma but may also be a deuterium and tritium plasma (or even a tritium plasma). 
     Ion source  12  also includes a pair of spaced electrodes, plasma electrode  26  and extraction electrode  28 , at one end thereof. Electrodes  26 ,  28  electrostatically control the passage of ions from plasma  18  out of ion source  12 . Electrodes  26 ,  28  are substantially spherical or curved in shape (e.g. they are a portion of a sphere, e.g. a hemisphere) and contain many aligned holes  30  (shown in  FIG. 2 ) over their surfaces so that ions radiate out of ion source  12 . (In the simplest embodiment, there would only be a single extraction hole  30  in electrodes  26 ,  28 .) Suitable extraction voltages are applied to electrodes  26 ,  28 , e.g. plasma electrode  26  is at 0 kV and extraction electrode  28  is at −7 kV, so that positive ions are extracted from ion source  12 . 
     The extraction system of ion source  12  includes a third electrode, suppressor electrode  32  which contains a central aperture  34  therein. Suppressor electrode  32  is at a relatively high negative voltage, e.g. −160 kV, to accelerate the extracted ion beam. The three electrode extraction/accelerator system is used to expand a high current ion beam in a relatively short distance. The spherical shapes of the plasma and extraction electrodes  26 ,  28  are such that the ion beams (or beamlets) passing through all the holes  30  in electrodes  26 ,  28  are focused close to the suppressor electrode  32 , pass through aperture  34 , cross over, and expand or diverge on the other side of suppressor electrode  32 . The diverging beam expands to a large area in a relatively short distance. Details of the extraction and acceleration system are shown in  FIGS. 2 ,  3 . 
     The plasma density on the ion source side of the plasma electrode  26  must be uniform over the entire extraction area to ensure good ion beam extraction. Plasma uniformity is obtained by positioning a spherically curved magnetic filter  36  inside ion source  12  in front of plasma electrode  26 . 
     A spherically curved target  38  is positioned so that the expanding ion beam from ion source  12  passing through electrodes  26 ,  28 ,  32  is incident thereon. Target  38  forms a portion of a spherical surface of relatively large area at a relatively short distance from ion source  12 . Target  38  is the neutron generating element, and may be water cooled. Target  38  is at a positive voltage relative to the suppressor electrode  32 , e.g. at −150 kV. 
     Ions from plasma source  12  pass through holes  30  in electrodes  26 ,  28 , and through aperture  34  in electrode  32 , and impinge on target  38 , typically with energy of 120 keV to 150 keV, producing neutrons as the result of ion induced reactions. The target  38  is loaded with D (or D/T) atoms by the beam. Titanium is not required, but is preferred for target  38  since it improves the absorption of these atoms. Target  38  may be a titanium shell or a titanium coating on another chamber wall  40 , e.g. a quartz tube. 
     Ion source  12  is positioned at one end of a sealed tube  42 , which also contains suppressor electrode  32 , and neutron generating target  38 , to form neutron generator  10 . The entire neutron generator is very compact, e.g. about 30 cm in length. 
     Because of the relatively large target area of target  38 , and the high ion current from ion source  12 , neutron flux can be generated from D—D reactions in this neutron generator as well as from D–T reactions as in a conventional neutron tube, eliminating the need for radioactive tritium. The neutrons produced, 2.45 MeV for D—D or 14.1 MeV for D–T, will go out from the end of tube  42 . 
     The neutron generator of the invention has a unique combination of high neutron production and compact size. The small size of the neutron generator is due mainly to the configuration of the extraction system, which allows one to extract a large ion beam current from a small ion source and to expand it onto a large area target. The large ion beam current is necessary for the high neutron output, because the neutron output is directly proportional to the ion beam current striking the target. The large area ion beam at the target is required to decrease the ion beam power density on the target, which would otherwise overheat the target and reduce neutron production. Compactness and high neutron output are achieved with the innovative extraction system and magnetic filter design. 
     While the invention has been described with respect to a spherical electrode geometry, an alternate embodiment can be implemented with a cylindrical geometry, i.e. electrodes  26 ,  28  are cylindrical in shape (i.e. portions of cylinders), with aligned slots  30 ; suppressor electrode  32  is cylindrical, with central slot  34 ; and target  38  is cylindrical. The ion beam then focuses down to a line and expands to impinge on the target. 
     The neutron generator of  FIG. 1  has a tubular configuration, as shown in U.S. application Ser. No. 10/100,956. Other neutron generator configurations include cylindrical, as shown in Ser. No. 10/100,962, and spherical, as shown in Ser. No. 10/100,955. All these applications are herein incorporated by reference. The principles of the invention for ultra-short pulse production apply to any configuration. 
     B. Ultra-short Pulse Production 
     Ultra-short pulses of ions or neutrons, having pulse widths of about 1 μs or less with fast rise times or fall times or both, are produced by the design of the extraction system of the ion source and by controlling the extraction voltage. The ion beam current extracted from the ion source has an ultra-short pulse width by applying corresponding ultra-short voltage pulses on the extraction electrode. The pulse width is also controlled by designing the aperture(s) in the extraction system with a diameter that is not much greater than the plasma sheath thickness in the ion source, and by spacing the electrodes of the extraction system a distance about equal to the aperture diameter. To produce ultra-short neutron pulses, a neutron generating target is struck by accelerated ultra-short ion beam bursts of suitable ions, such as D, T, or D and T. 
     In a typical ion source beam extraction system, the plasma potential is usually at a few volts above the plasma chamber potential (local ground) and the plasma electrode (the first or beam-forming electrode) is on the order of 10 volts below the local ground potential. The potential drop from the plasma potential to the plasma electrode potential occurs within a sheath region that has a thickness of about 10λ D . The Debye shielding length λ D  is given by 
         λ   D     =       kT     4   ⁢   π   ⁢           ⁢     ne   2               
 
where T is the electron temperature and n is the plasma density. For a typical plasma with electron temperature T up to 10 eV and plasma density n at about 5×10 11  cm 3 , 10λ D  is about 30 μm.
 
     Ions are accelerated from the plasma into the sheath while electrons are rejected by the sheath. However, if an aperture, on the plasma electrode is much larger than the sheath thickness, the sheath will “wrap around” the aperture, allowing the plasma to flow through the aperture without rejecting the electrons, i.e. the plasma simply leaks out of the aperture, preventing sharp narrow pulses from being formed. 
     This situation is shown in  FIG. 4A . The extraction system has a plasma electrode  50  and a spaced extraction electrode  52 . A bias supply  54  is connected between electrodes  50 ,  52 . A forward bias (electrode  52  is negative with respect to electrode  50 ) is applied for (positive) ion extraction and a reverse bias (electrode  52  is positive with respect to electrode  50 ) is applied to stop positive ions and for electron (and negative ion) extraction. Electrodes  50 ,  52  include one (or more) aligned apertures  56 ,  58  respectively. 
     Plasma sheath  60  is adjacent to plasma electrode  50  and has a thickness t of about 30 μm. When the diameter d of aperture  56  in plasma electrode  50  is much greater than the plasma sheath thickness, i.e. d&gt;&gt;t, plasma leaks through aperture  56  around electrode  50 . When a forward biased voltage is applied to extraction electrode  52 , ions are accelerated and electrons are repelled, as shown in  FIG. 4A . When a reverse biased voltage is applied to electrode  52 , ions are repelled and electrons are accelerated, as shown in  FIG. 4B . An electrode cloud  62  can build up between electrodes  50 ,  52  which can short out the electrodes. 
     If the diameter of aperture  56  (and  58 ) is made smaller than the sheath thickness t, then the sheath  60  can cover the aperture, even in the reverse biased condition, as shown in  FIG. 4C . Thus for micron sized apertures, most electrons cannot escape, even for a reverse bias voltage. Therefore, because of the ability to control ion extraction, micron sized apertures are preferred in the extractor system electrodes for producing ultra-short pulse widths. A multiple aperture multiple beamlet extraction system is thus preferred for the ion sources. 
     To control the ion flow to produce good beam optics, the distance x between the plasma electrode  50  and the extraction electrode  52  must have approximately the same dimension as the aperture diameter d, i.e. an aspect ratio x/d of about 1. The potential required to repel ions at the extraction electrode is slightly above the plasma potential. Thus the voltage difference between the electrodes is about 20 V. The minimum required voltage gradient is 0.6 MV/m. In the forward bias case, the extraction electrode can be biased at local ground potential or some negative potential depending on the current density and beam optics design. 
     This biasing effect has been experimentally demonstrated, using a single aperture setup as shown in  FIG. 5 . Experiments showed that ion as well as electron beams can be switched on and off using a biasing electrode  73  that stops the charged particles from exiting ion source  70 . Biasing electrode  73  is part of a switchable extraction aperture system  77  that has two conducting electrodes  71 ,  73  separated by insulator layer  72 . Electrode  71  is the plasma electrode and electrode  73  is the extraction electrode. System  77  is followed by insulator layer  74  and faraday cup  75 . An aperture  76  is formed in the electrode and insulator layers. 
     Electrode  71  is biased negatively (about 30 V) with respect to the chamber wall. Electrode  73  is used to stop the flow of ions by applying a positive bias with respect to the ion source chamber. Using argon as the working gas, a plasma discharge was produced with a discharge power of 40 W. The gas pressure inside the source was 2 mTorr. The source is biased at 30 V to allow the ions to be extracted, and the current is measured with the Faraday cup at ground potential. Electrode  71  is also biased with respect to the source to prevent back streaming electrons when the beam is switched on, and to avoid electron extraction when the beam is switched off. The beam energy at the Faraday cup is equal to the source potential plus the plasma potential. Because the discharge power is so low, the plasma potential is almost negligible. Thus, to read ion beam current at the Faraday cup, electrode  73  has to be biased equal to or less than the source. Experimentally, electrode  73  is first set at ground potential, which allows the ions to be extracted. The Faraday cup reads 23 nA. When electrode  73  is biased at 31 V, i.e. 1 V more positive than the source potential, the Faraday cup reading drops down to zero. 
     Thus, by providing a micro-channel biasing system with a fast voltage switch, the invention enables one to generate ion and neutron beams with very short duration, about 1 μs or less and fast rise time and/or fall time. These ultra-short ion and neutron pulses can be used for a variety of applications, including neutron interrogation of nuclear materials and induction linacs. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention which is intended to be limited only by the scope of the appended claims.