Abstract:
A method of producing an ion beam in a tandem accelerator, the method includes the acts of insulating, in a cavity containing an insulating gas under pressure, a beam tube having first and second ends and a terminal situated between the first and second ends; seating, using a load lock valve, the insulating gas under pressure; generating an operating voltage using a first voltage source situated outside the cavity when the operating voltage is less than or equal to a threshold value; generating the operating voltage using a second voltage source situated inside the cavity when the operating voltage is greater than the threshold value; coupling the terminal to the first or second voltage sources; generating, using a particle source, an ion beam; and accelerating the ion beam in a first and second parts of the beam tube.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to ion accelerators and, more particularly, to tandem accelerators having a low-energy static voltage injector and an extended lower voltage limit, and a method of operation thereof. 
   BACKGROUND OF THE INVENTION 
   Tandem accelerators typically use a charging system and are used to produce beams of charged particles having a very high energy such as, for example, 10 MeV. These beams can then be used for various applications, such as, for example, basic nuclear physics studies, cancer therapy, radioisotope production, and materials analysis. Unfortunately, conventional tandem generators cannot be properly controlled by their voltage feedback systems when the terminal voltage (i.e., the voltage at a terminal or terminal cell inside the tandem accelerator) falls below a certain voltage level such as 5-10% of the maximum operating voltage. For example, the voltage cannot be regulated below approximately 0.3 MV for 3 MV conventional tandem generator, and for a 25 MV generator, voltage regulation is lost around approximately 1-2 MV. 
   Typically, a Tandem Van de Graff-type accelerator, or more simply, a Tandem has a column having a low-energy or first beam tube (or column) and a high-energy or second beam tube (or column), a stripper cell, beam steering means, a terminal cell (e.g., including an electrode) which is disposed between the high- and low-energy beam tubes, and a charging system, all of which are contained within a pressure vessel which provides proper pressure (e.g., either pressure or vacuum) for proper electrical isolation. The charging system typically comprises a moving belt or a chain comprising an insulating material which is used to convey a charge to the terminal via, for example, a “pickoff pulley,” as disclosed in, for example, U.S. Pat. No. 3,353,107, entitled “High Voltage Particle Accelerators Using Charge Transfer Processes,” to Van de Graaff (the &#39;107 patent) which is incorporated herein by reference in its entirety. Van de Graaff discloses a power source comprising a single insulating belt which is located within a pressure vessel. As mentioned above, this system is difficult to regulate below a certain voltage value and is therefore not suitable for certain uses. 
   In Tandems, as well as other electrostatic accelerators, a generating Voltmeter (GVM) is typically used to measure the voltage on the terminal. The GVM comprises a rotating and stationary vane assembly in close proximity with each other, with the rotating vane assembly facing the terminal. The GVM generates a signal that is proportional to the voltage on the terminal. The terminal voltage is controlled by either the feedback from a corona discharge between the terminal and sharp needles, known as corona points, or feedback from the accelerated beam itself as it is deflected and its beam current recorded by slits. This feedback is then used to control the terminal voltage. 
   Although there have been attempts to use other power sources in tandem accelerators, such as disclosed in U.S. Pat. Nos. Re 34,575 (the &#39;575 patent) and 6,414,327, entitled “Electrostatic Ion Accelerator” and “Method and Apparatus for Ion Beam Generation,” respectively, both to Klinkowstein, which are incorporated herein by reference in their entirety. Similarly to the &#39;107 patent, these two patents disclose power supplies located within a pressure vessel used to house the beam tubes. Further, the solid state rectifier-type power supply disclosed by the &#39;575 patent is subject to “sag” which is attributable to AC impedance of capacitors in earlier stages and a voltage ripple effect which is amplified by the number of stages used. Accordingly, it is difficult to control the voltage at the terminal of the &#39;575 patent. 
   Another type of tandem accelerator is disclosed in U.S. Pat. No. 5,293,134, entitled “Tandem Accelerator,” to Holmes, which is incorporated herein by reference in its entirety, and which discloses using vacuum rather than an insulating gas within a pressure vessel. Further, Holmes discloses integrating accelerating and electric stress shields which are used as capacitors and are different from the beam tube which is taught by the present invention. 
   Further, it is difficult to maintain a corona discharge current, which is necessary to control the terminal voltage, in the absence of a beam, at terminal voltages which are lower than a threshold value in conventional tandems. 
   Accordingly, there is a need for an apparatus capable of providing stable low-voltage operation for a tandem accelerator which can be accurately and easily controlled when the terminal voltage is below the aforementioned threshold level. 
   Further, there is a need for a tandem accelerator which can provide a moderately low-energy beam having an energy which is suitable for high-resolution analytical techniques such as, for example, Nuclear Reaction Analysis (NRA). 
   SUMMARY OF THE INVENTION 
   Therefore, it is an object of the present invention to solve the above-noted and other problems of conventional tandem accelerators and to provide an apparatus and a method for providing a low-energy output beam which is suitable for providing enhanced analytical resolution in various techniques such as, for example, NRA. 
   In one embodiment, an ion accelerator apparatus includes a negative ion-source for producing a negatively-charged ion beam, a beam tube having opposing first and second ends, a vacuum chamber extending between the first and second ends, and adapted to receive the ion beam at one of the first or second ends. A high-voltage terminal cell (hereinafter terminal or terminal cell) at positive voltage is situated between the first and second ends of the beam tube and receiving a high voltage. A pressure vessel defining at least part of a cavity contains the terminal and an insulating gas under pressure. While within the beam tube, the ion beam travels in the vacuum of the vacuum chamber. A high voltage power source is coupled to the terminal and is situated outside of the cavity. 
   The ion accelerator apparatus further includes a stripping canal located adjacent to or in the terminal and changes the charge of the ion beam by stripping off electrons such that the ion beam is further accelerated in the second part of the beam tube. 
   The high voltage power supply generates a high voltage (e.g., positive) with respect to ground potential. The apparatus may further include a load-lock valve for scaling the insulating gas under pressure and which is situated between an external high-voltage power supply and the terminal. A drift detector may be configured to determine drift of the accelerated and deflected ion beam, and a controller may be configured to adjust the voltage of the high-voltage terminal based upon the determined drift of the ion beam from a reference value, which may be predetermined. Of course, if desired, one embodiment may include a feedback loop for feeding back a signal (e.g. an error signal which is a result of a comparison of a measured signal with a reference value) from one or more locations either within or outside of the beam tube, to the high-voltage power supply. 
   Further, in a method for producing an ion beam in a tandem accelerator having a cavity, the method including the acts of controlling an atmosphere in the cavity filled at least in part with an insulating gas; producing, using an ion generator, an ion beam; receiving the ion beam by a beam tube having first and second parts and which is located at least in part within the cavity; transmitting a predetermined high voltage from a high-voltage generator located outside of the cavity; receiving the ion beam by the beam tube and accelerating the ion beam in the first part of the beam tube; receiving the ion beam by a terminal cell which is situated between the first and second parts of the beam tube and changing a charge state of the ion beam; and accelerating the ion beam in the second part of the beam tube. 
   The method may further include the acts of stripping the ion beam in a stripping canal so as to change the charge of the ion beam after the ion beam has been accelerated in the first part of the beam tube such that the ion beam can be accelerated by the second part of the beam tube; subjecting the ion beam to a magnetic defection field; determining, using a drift detector, drift of the ion beam; and providing the determined drift information to a controller (which, for example, can include a phase locked loop (PLL)). 
   The method may further include the act of adjusting, by the controller, the voltage of the terminal cell using the determined drift information. The method may further include the acts of situating a load-lock valve between the high-voltage generator and the terminal; passing an output voltage from the high-voltage generator to the terminal via the load-lock valve; sealing the insulating gas in the cavity using the load-lock valve; and maintaining a predetermined pressure within the cavity using a pressure maintaining device, such as, for example, a pump, a pressurized gas, and the like. 
   According to another aspect of the present invention, a method of producing an ion beam in a tandem accelerator is disclosed, the method includes the acts of insulating, in a cavity containing an insulating gas under pressure, a beam tube having first and second ends, a vacuum chamber, and a terminal situated between the first and second ends; sealing, using a load lock valve, the insulating gas under pressure; generating an operating voltage using a first voltage source situated outside the cavity when the operating voltage is less than or equal to a threshold value; generating the operating voltage using a second voltage source situated inside the cavity when the operating voltage is greater than the threshold value; coupling the terminal to the first or second voltage sources; generating, using a particle source, an ion beam; receiving the ion beam at the first end of the beam tube; accelerating the ion beam in a first part of the beam tube that is situated between the first end of the beam tube and the terminal; changing the charge state of the ion beam; and accelerating the ion beam in a second part of the beam tube which is situated between the terminal and the second end of the beam tube. 
   According to the method, the coupling step may include the acts of inserting a voltage injector through the load lock valve when the operating voltage is less than or equal to the threshold value; and withdrawing the voltage injector from the load lock valve when the operating voltage is greater than the threshold value without the need to remove the insulating gas while inserting or withdrawing the voltage injector. Further, the method may include the acts of determining drift of the ion beam after the ion beam has been accelerated, by the terminal, in the first and second parts of the beam tube and deflected through a fixed and stable magnetic field; selecting a desired voltage based upon the determined drift as compared to a reference value; and setting an output voltage setting of the first voltage source in accordance with the selected desired voltage. 
   The ion accelerator may further include a beam tube having a plurality of beam tube parts and a high-voltage terminal situated between the beam tube parts, the beam tube is situated within a cavity having a desired pressure, and a high-voltage power supply is coupled to the high-voltage terminal and situated outside of the cavity, the beam tube includes a vacuum chamber in which the ion beam travels when it is in the beam tube, and a controller which can determine the drift, with respect to a reference value, of the ion beam after it is deflected by a magnetic field, and adjust a voltage setting of the high-voltage power supply based upon the determination. The beam tube includes a voltage divider network which provides a uniform gradient along the beam tube with both ends of the beam tube, farthest away from the terminal, being at ground potential. 
   The beam tube may include a voltage grading resistor chain, insulating rings, metal electrodes, protective spark gaps, as is known in the art, for maintaining a proper voltage gradient. 
   The voltage provided by the high-voltage source may be precisely controlled before it is provided to the terminal even at low voltage values such as below 5% of the nominal operating voltage, thus obviating the need for a conventional feedback system which measures the voltage at the terminal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: 
       FIG. 1  is a block diagram illustrating a tandem generator according to the present invention; 
       FIG. 2  is a block diagram illustrating the tandem generator of  FIG. 1  with the voltage injector withdrawn; and 
       FIG. 3  is flow chart illustrating a process according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the drawings the same reference numerals have been used to indicate corresponding features. 
   Preferred embodiments of the present invention will now be described in detail with reference to the drawings. For the sake of clarity, certain features of the invention will not be discussed when they would be apparent to those with skill in the art. 
   The present invention includes a solid state power supply (e.g., a power supply using solid state components such as, for example coils, capacitors, diodes, etc.) to generate an electrical charge in addition to another power source which can include an electrostatic induction source such as a Van de Graaff-type power supply which uses an electromechanical means such as, for example, a belt or a chain (e.g., a Pelletron chain) to generate an electrical charge. The solid state power supply may include for example, Glassman High Voltage, Inc, model LT100P20 (0-100 kV, 0-25 mA) positive high voltage supply. 
   A block diagram of a tandem generator according to the present invention is shown in  FIG. 1 . A tandem generator  100  includes an accelerator tank  102 , a beam tube  104 , a high-voltage terminal cell  120 , a load-lock valve  110 , a high voltage power supply  112 , a charging system  130 , a stripping cell  134 , and an optional controller  126 . A beam steering and/or focusing device  132  conditions the ion beam  136  prior to entering the beam tube  104 . 
   The accelerator tank  102  has first and second opposing ends  102 A and  102 B, respectively, and forms at least part of a cavity  128  which can maintain a desired atmosphere for proper electrical and/or thermal insulation. Accordingly, an insulating gas  138  such as, for example, sulfur hexafluoride (SF 6 ) can be maintained at a desired temperature and/or pressure within the cavity by the accelerator tank  102 . A conventional ion generator  122  provides a beam of negative ions, as desired. The beam conditioning  132  can include, for example, a 90 degree magnet so that ions of the proper charge/mass ratio are injected into the beam tube, as well as beam alignment devices. In the present embodiment, it will be assumed that negatively-charged ions are provided by the ion generator  122 . The ion generator  122  can be optionally controlled by the controller  126 . 
   The beam tube  104  is fully (as shown) or at least partially contained within the cavity  128  and has first and second ends  104 A and  104 B, respectively. The beam tube  104  can include a plurality of tubes, and in the present example, includes a first beam tube  106  and a second beam tube  108 . 
   One or more optional beam steering and/or focusing devices  132  are positioned at various locations, as desired, and steer, focus, and/or otherwise condition the ion beam  136  between the ion generator  122  and the entrance  104 A or downstream of the accelerated beam. Suitable beam-focusing devices include electrostatic lenses such as, for example, Einzel lenses or magnetic lenses such as quadrupoles. 
   The terminal cell  120  is located between the first and second ends  104 A,  104 B of the beam tube  104  and, in the present example, is situated between the first beam tube  106  and the second beam tube  108  which accelerate the ion beam received from the ion generator  122 . For example, the terminal cell  120  is located at the center of the beam tube  104 . The terminal cell  120  may include the stripping cell  134  which can change a charge of the ion beam by, for example, using a stripper foil, or a gas (e.g., N 2 ), which is suitable for the type of ion produced by the ion generator  122  and which can remove electrons from the ion beam  136  (e.g., H −  generated by the ion generator  122 ) so as to produce a positively charged ion beam. For example, if the ion beam  136  generated by the ion generator  122  is a negative ion beam, the stripper cell can convert it into a positive ion beam after the ion beam passes therethrough. In other words, the stripper cell changes the charge of the ion beam. For ions heavier than H, there is a distribution of charge states that occurs after the beam passes through the stripping cell  134 . 
   The charging system  130  is mounted in the interior of the cavity  128  and may include a chain-type charging system such as a Pelletron™-type charging system (manufactured by the National Electrostatics Corp., Middletown, Wis.). The charging system  130  provides a charge to the terminal cell  120  and includes a drive pulley  116 , a pickoff pulley  114 , and a mechanical force-transmitting device such as, for example, a chain  118  having metal pellets and nylon links. Although the Pelletron-type charging system is shown  130  for the user&#39;s convenience, in the preferred embodiment, it can be present and not operating. Accordingly, existing tandem accelerators using charge-tensioning devices such as the Pelletron-type charging system can be modified to benefit from the features and advantages of the present invention. As the Pelletron-type charging system is well known in the art, for the sake of clarity, a further description thereof will be omitted. 
   The high-voltage power supply  112  is located outside of the cavity  128  and generates a desired direct current (DC) voltage. In one embodiment, the high-voltage power supply  112  may be configured to provide voltage in the range of, for example, from 100 kV to 200 kV. The high-voltage power supply  112  is coupled to the terminal via the load-lock valve  110  such that the high-voltage potential produced by the high-voltage power supply  112  is supplied directly to the terminal cell  120 . The terminal end of the high voltage cable has a spring device so that positive contact can be made with the terminal over a wide range of operating temperatures and vibration conditions. 
   The load-lock valve is attached to the accelerator tank  102  and is radially disposed from the terminal cell  120 . The load-lock valve  110  passes one or more high-voltage leads  140  (e.g., voltage injectors) which couple the high-voltage power supply  112  to the terminal cell  120 , and provides a suitable air-tight seal such that the insulating gas (e.g., SF 6 ) contained within the cavity  128  is maintained at a desired pressure. Suitable load-lock valves include the capability of withstanding a pressure differential of 100 PSI, an o-ring static seal and a captivating device to prevent accidental withdrawal. 
   A pressure control system may be provided to maintain a proper pressure and temperature within the accelerator tank  102  and can include pressure sensors, pumps, high-pressure tanks, heat exchangers etc. As shown in  FIG. 1 , an optional controller  126  can control the overall operation of the tandem accelerator and may be incorporated into, for example the high-voltage power supply  112 , if desired. 
   An optional feedback system can include optional sensors  124  which are located at various positions either inside of or outside of the cavity  128  and which relay various operating parameters to, for example, the optional controller  126 , the high-voltage power supply  112 , and/or a user. For example, the sensors  124  can include voltage, current, temperature, pressure, etc., sensors, as desired, and provide corresponding information to the controller  126 , the high-voltage power supply  112 , and/or the user, as desired. 
   In the present embodiment, an optional conditioning cell  146  is located downstream of the second beam tube  108  and receives the ion beam  136  which was accelerated by the terminal voltage. The conditioning cell  146  can include a Faraday cup  147 , beam profile monitor  149 , other sensors  140  such as, for example, a deflection magnet  144  and position slits  142  located downstream of the deflection magnet  144 , etc. The Faraday cup  147  determines the beam current of the ion beam  136  and relays corresponding information to the controller  126 . The beam profile monitor  149  evaluates the position and focus of the ion beam  136  and relays corresponding information to the controller  126 . After passing through the retracted Faraday cup  147  and the beam profile monitor  149 , the ion beam  136  passes through the deflection magnet  144  which delivers the ion beam  136  to any of a number of beam lines. The position slits  142  are located on one or more of the beam lines. The magnetic field of the deflection magnet  144  is selected to deflect the beam of a given energy into a desired beam line. 
   For example, for ions heavier than H, there are multiple charge states that are generated in the stripper cell  134  and these ions are then further accelerated. Only ions having a certain charge state and energy are properly deflected in the deflection magnet  144  and can pass through a corresponding pair of slits of the position slits  142 . This deflection is then measured by the sensors  140  which transmit information based upon the deflection to the controller  126 , the high-voltage power supply  112 , and/or a user. The controller  126  may then determine proper voltage and may control the high-voltage power supply  112  or the charging system  130 , depending upon which is being used, accordingly. Although not shown, a nuclear reaction can be used to initially calibrate the voltage on the terminal cell  120 , and the deflection magnetic  144  can then be adjusted to deliver this beam to a detection system (not shown). Any change in the beam energy will deflect the beam over the position slits  142  located on the beam line. Accordingly, the deflection of the ion beam  136  can be used to adjust the voltage of the terminal cell  120  by adjusting the voltage of, for example, the high-voltage power supply  112 . 
   It is also envisioned that the magnetic field  144  can be optionally controlled by the controller  126  and that the controller  126  can optionally determine the initial energy of the ion beam  136  before it enters the beam tube  104 . 
   An advantage of the present invention is that regulated voltages provided to the central terminal  120  can go below the typical feedback limit that is required for conventional feedback systems to work in tandem accelerators such as, for example, the 3 MV Tandem Van de Graaff accelerator (manufactured by the National Electrostatics Corp.). Accordingly, the tandem accelerator according to the present invention can be used to provide enhanced analytical resolution in techniques such as, for example, Nuclear Reaction Analysis (NRA), which can require beam energies &lt;500 keV (or terminal voltages &lt;250 kV). 
     FIG. 2  is a block diagram illustrating the tandem generator of  FIG. 1  with the voltage injector withdrawn. When operating the tandem generator  100  using a terminal voltage that is above a threshold value of about 0.3 MV, the one or more high voltage leads  140  are withdrawn from the cavity  128  via the load-lock valve  110 . Accordingly, the high-voltage power supply  112  is no longer coupled to the terminal cell  120 . This prevents arcing when the charging system  130  produces voltages which are greater than the threshold value. When operating at terminal voltages which less than or equal to the threshold value, then the one or more high voltage leads  140  are inserted at least in part within from the cavity  128  so that the high-voltage power supply  112  is coupled to the terminal cell  120  (as shown in  FIG. 1 ). 
   A brief summary of the operation of the tandem accelerator according to the present invention when using the high-voltage power supply  112  will now be given. In operation, the ion generator  122  outputs an ion beam  136  which is coupled to the beam tube  104  adjacent to an end of the first beam tube  106 . The beam can be conditioned using optional conditioning means such as, for example, a beam focusing and/or steering device  132 , a pre-analyzing magnet, etc. The ion beam  136  is then accelerated in the first beam tube  106  and thereafter enters the terminal cell  120 , which is charged by the high-voltage power supply  112  to a desired voltage. An advantage of the present invention is that the voltage can be precisely set and regulated even at reduced voltages, using, for example, a voltage regulator within the high-voltage power supply  112  itself before it is output to the terminal  120 . Thus, a conventional feedback system which measures voltage at the terminal  120  and the corona discharge as described earlier, and then attempts to properly adjust voltage using, for example, the corona points, is not required by the present invention. Further, the present tandem generator may be operated at reduced voltages by virtue of having the external voltage source or power supply  112  charge the central terminal  120  to a desired voltage, which may be less than 5% of the nominal operating voltage of the tandem generator and yet the reduced voltage remains regulated and allows proper operation of the tandem generator. In conventional tandem generator, the central terminal  120  is charged by the charging system  130  where the voltage on the central terminal cannot easily be regulated below certain levels, such as below 5% of the nominal operating voltage. 
   Referring back to  FIG. 1 , at the terminal cell  120 , the polarity of the ion beam can be changed by the stripping cell  134 . Thus, for example, if the ion beam generated by the ion generator  122  is a negative ion beam  136 , the polarity of the ion beam  136  is then changed to a positive ion beam. Thereafter, the ion beam  136  enters the second beam tube  108 , where it is further accelerated. Thereafter, the ion beam  136  can be further conditioned (e.g., focused and/or steered) by, for example, the conditioning cell  146 , and the energy of the ion beam  136  can be optionally determined and provided to the controller  126 , which can control the high-voltage power supply  112  to adjust output voltage correspondingly. Accordingly, a precise and accurate ion beam is generated for a desired purpose such as, for example, low-energy NRA or other purposes. 
   According to the present invention, assuming that the ion beam  136  includes H −  ions having an initial energy (e.g., the extraction voltage-prior to entering the first beam tube  104 ) of 50 keV, and if the terminal cell  120  is charged to 100 kV by the high-voltage power supply  112 , after passing through the first beam tube  104 , the ion beam  136  would have an energy of about 50 keV+100 keV. Then, assuming that a stripping gas (e.g., Nitrogen) is used to change the polarity of the ion beam  136  so that it now includes H +  ions, the ion beam  136  can then be accelerated by the second beam tube  108  and would have an energy of 50 keV+100 keV+100 keV=250 KeV (i.e., an energy equal to an initial energy, the energy provided by the acceleration voltage within the terminal cell  120  while the beam  136  is in the first beam tube  106  and the energy provided by acceleration voltage within the terminal cell  120  while the beam exits the second beam tube  108 ) upon exiting the second beam tube  108 . Thus, the final energy E f  of the ion beam would be equal to the post-stripping charge state q of each ion, the extraction voltage (e.g., of the ion generator) V i , and the acceleration voltage V t  and as is shown Equation (1) below.
 
 E   f   =V   i +( q+ 1) V   t  
 
   As previously mentioned, by using a regulated power supply for the high-voltage controller  112 , the voltage transmitted to the terminal can be precisely controlled and regulated, even at reduced voltage below 5% of the nominal operating voltage of the tandem generator, at the high-voltage power supply (e.g., a highly accurate and stable switching power supply) without having to measure and control a voltage at the terminal using a conventional feedback system. 
   A flow chart illustrating a process according to the present invention is shown in  FIG. 3 . Process  300  begins at step  302  where it is determined (e.g., by the controller  126 ) whether the operating voltage (e.g., the desired voltage at the terminal cell  120 ) is greater than a predetermined threshold value. If it is determined that the operating voltage is greater than the threshold value, the process continues to step  304 . However if it determined that the operating voltage is less than or equal to the threshold value, the process continues to step  308 . 
   In step  304 , the one or more high voltage leads (or voltage injectors)  140  are withdrawn from the cavity  128  via the load-lock valve  110  so that the high-voltage power supply  112  is not coupled to the terminal cell  120 . This can prevent arcing when operating the tandem generator at voltages which greater than the threshold value. The process then continues to step  306  where, for example, the charging system  130  is operated and charges the terminal cell  120  to the operating voltage. 
   In step  308 , the one or more high voltage leads  140  are positioned at least in part within the cavity  128  (e.g., via the load-lock valve  110 ) so that the high-voltage power supply  112  is coupled to the terminal cell  120 . The process then continues to step  310  where the power supply  130  (e.g., the electrostatic voltage source) is optionally disabled (e.g., by not enabling power to the power supply  130 . The process then continues to step  312  where the high-voltage power supply  112  is controlled so as to produce the operating voltage which is supplied to the terminal cell  120 . 
   Thus, by using an external high-voltage power supply to supply operating voltages which are less than a threshold value, existing tandem accelerators using electrostatic induction systems can be modified to benefit from the features and advantages of the present invention. 
   A further advantage of the present invention is that by applying an external voltage source to the terminal inside the tandem accelerator, the use of costly and complex chain and/or belt charging systems can be avoided. Further, the voltage provided by the external high-voltage power supply can be graded using a conventional voltage-divider network, which for the sake of clarity is not shown. Accordingly, a proper voltage gradient would be present in the beam tube during operation. 
   The high-voltage power supply can include a highly accurate and stable source such as those which are used for electron or ion-beam lithography. 
   Thus, according to the present invention, a low-cost, stable, and easily regulated tandem accelerator that can be operated at low voltage levels is provided. 
   Certain additional advantages and features of this invention may be apparent to those skilled in the art upon studying the disclosure, or may be experienced by persons employing the novel system and method of the present invention, chief of which is that a more stable and easily regulated tandem accelerator which can operate at low voltage levels is provided. Another advantage of the present invention is that conventional tandem accelerators can be easily modified to incorporate the features and advantages of the present invention. 
   While the invention has been described with a limited number of embodiments, it will be appreciated that changes may be made without departing from the scope of the original claimed invention, and it is intended that all matter contained in the foregoing specification and drawings be taken as illustrative and not in an exclusive sense.