GB2619948A - Neutral beam injection apparatus and method - Google Patents

Abstract

Apparatus and method for generation of high energy particles is disclosed. The apparatus comprises: an ionisation chamber 10 adapted to split a source input element into a discrete stream of positively charged ions and a discrete stream of electrons; at least two electrostatic accelerators 100A & 100B adapted to accelerate each of the said stream of ions and said stream of electrons to substantially the same speed or velocity as one another; and a neutralisation zone 40 in which the said streams are brought together to form an output of neutral particles. The neutral beam may be injected into a magnetically confined fusion plasma, or used to generate thrust in, for example, a space vehicle. The electrostatic accelerators may comprise electrical conductors (102, Fig. 5) and electromagnets (104) arranged in series along the axis of acceleration. The neutralisation chamber 40 may comprise a series of annular electromagnets (42, Fig. 6).

Description

Neutral beam injection apparatus and method

Field of the invention

The present inventive concept relates to a neutral beam injection device for use in heating or as a thruster device, for example to heat magnetically confined plasmas for use in fusion reactors for electricity generation, or as a thruster which might be applied to, for example, a space vehicle.

Fusion powered electricity generation is seen as a low carbon, sustainable source of energy.

Background to the invention

Fusion energy is considered as a promising future source of energy for low carbon power generation. One category of research into fusion energy is fusion generated by plasmas confined by magnetic fields. These plasmas need to be heated up to very high temperatures, exceeding 100 million degrees, to achieve fusion conditions. -2 -

One of the main ways that such plasmas are heated to these high temperatures is by the introduction of very high energy neutrals (i.e. neutrally charged particles) into the plasma. The devices that deliver these neutrals are called neutral beam injectors or variations to that effect (e.g. NBIs). The broad concept of neutral beam injectors is established.

Neutral beam injector devices generally take the particles of the desired element to be introduced in the plasma, often deuterium, atomise and ionise them, accelerate the ions, neutralise the ions using a neutral gas background or a plasma; thus allowing the neutrals to enter the target plasma.

Such an atomisation and ionisation step can be done in various ways, including the use of suitable energy photons and high electrostatic Such an acceleration step can be done via electrostatic acceleration.

For such a neutralisation step there are various ways to achieve this. One common method is by passing the accelerated ions though a neutral gas.

For an introduction into target plasma, the neutrals can penetrate through the magnetic field and become ionised again via collisions with the target plasma particles.

Previous devices suffer from low efficiencies at the higher energies which are needed to achieve fusion at a performance that is better than previously achieved.

The inventive concept uses a combination of electrostatic, magnetic, electronic and, optionally, computational elements to ionise, accelerate, neutralise and deliver high energy neutral particles.

In certain embodiments, the invention can be used to deliver high energy neutral particles to magnetically confined plasmas to increase their temperature. Through the use and methods of this inventive concept it is possible to increase the efficiency with which particles can be accelerated to high energies.

In certain embodiments, the inventive concept can be used to provide thrust for space vehicles, such as for satellite orbital manoeuvre. -3 -

Plasmas are made of charged particles freely flowing past each other; one way to contain such arrangement is via the use of magnetic fields. This is done, for example, in magnetic confinement fusion research, where plasmas are contained in toroidal magnetic field arrangements.

One way to heat these plasmas is by the injection of neutral particles into the plasma.

The particles need to be neutral to penetrate the magnetic field. The particles need to be high energy, which in practice means high speed, in order to increase the average energy of the particles in the plasma, i.e. to heat the plasma.

In order to produce high energy neutral particles a multi-step approach is used by current methods. This begins by taking neutral particles of the desired element (often deuterium molecules), which are then atomised and positively ionised. The ions are then accelerated, using electric fields, to the desired energy. The final step before introducing these particles to the magnetically confined plasma is their neutralisation. This is achieved by passing the aforementioned ions through a (low density) neutral gas.

Collisions with the particles of the neutral gas result in the neutralisation of some of the fast-moving ions, which allows them to penetrate the magnetic field that confines the plasma to be heated.

This method becomes increasingly inefficient with higher ion energies (speeds) and therefore different methods are required to deliver high energies to fusion plasmas. One method that is being developed and is near deployment is the use of negatively charge ions. Negatively charged ions can be more readily neutralised and therefore do not suffer from the same drop in efficiency at the neutralisation stage as positively charged ions. However, it is technically more challenging to create negative ions and it is Less efficient to generate them than to generate positive ions.

The aim of the present inventive concept is to efficiently deliver high energy neutral particles. -4 -

Summary of invention

The present inventive concept provides apparatus and accompanying method for generation of high energy particles.

Apparatus for particle acceleration according to the present inventive concept comprises an ionisation chamber adapted to split a source input element into a discrete stream of positively charged ions and a discrete stream of electrons, at least two electrostatic accelerators adapted to accelerate each of the said stream of ions and said stream of electrons to substantially the same speed as one another, and a neutralisation zone in which the said streams are brought together to form an output of neutral particles.

Once the streams are brought together, the particles therein should be travelling at substantially the same velocity as one another. In other words, their speed and direction of travel should be substantially the same as one another. Preferably, within the neutralisation zone the particles are also travelling substantially co-axially as well. This improves the efficiency of neutralisation.

The ionisation chamber may comprise an enclosure having an opening adapted for introduction of a source element, a photon emitter adapted to ionise the source element, and at least two chargeable grids. The said chargeable grids can each be charged positively or negatively. The said chargeable grids can be charged in opposition to one another. The said chargeable grids can be switched between positive and negative charges and vice versa, in rapid succession. The ionisation chamber may further comprise at least two exit openings adapted for exit of electrons and positively charged ions.

The apparatus may further comprise a reservoir adapted to store quantities of the source element, the reservoir being in fluid connection with the ionisation chamber via a controllable valve.

The photon emitter is preferably adapted to provide a wavelength (corresponding to an energy) of photons appropriate to ionise the source element. The preferred wavelength varies according to the source element. -5 -

In use, the apparatus uses photons to initialise the ionisation of a quantity of the source element in a gaseous state. This can be introduced into the ionisation chamber via the said controllable valve connecting the ionisation chamber to a gas reservoir.

The amount of ionisation is subsequently increased via the oscillating acceleration of the initial charged particles. This can be achieved by an alternating of the charge on the charged grids, on either side of the chamber. The grids can then be subsequently biased at a fixed charge to accelerate the positively charged ions and the electrons out of the chamber and in opposite directions.

The operation of the ionisation chamber may be described as being "pulsed" in that a batch or puke of charged particles is generated, rather than a continuous stream. This allows for the more efficient control of the resulting beam and the reduction of instabilities during the travel of the charged particles.

The apparatus may comprise a first steering means adapted to steer each beam of charged particles from the said exit openings and into paths of equal Length. Providing paths of equal length enables efficient subsequent acceleration thereof in parallel, and in due course to efficient neutralisation of the said particles. The steering means may comprise magnets suitably arranged.

In some embodiments, the beams have a substantially parallel formation. In alternative embodiments, the paths may travel along the sides of an isosceles triangle, having a base defined by the exit openings. For example, the isosceles triangle is relatively tall, the base of the isosceles triangle being relatively short compared to the length of the two equal sides. In an example, each beam travels from its exit opening along the base, turning into the Long sides of the isosceles triangle (i.e. one beam on each side) , traveling towards the tip of the isosceles triangle, where the last steering stage into the neutralisation stage occurs.

Each electrostatic accelerator may comprise an electromagnet and an electrical conductor arranged substantially coaxially along an axis of acceleration and spaced apart from one another along that axis of acceleration. The electrical conductor can be said to form an electrostatic grid. -6 -

The electrical conductor may be charged. The electromagnet may be provided with an electrical current.

Preferably each electrostatic accelerator comprises two or more of each of an electromagnet and an electrical conductor, arranged respectively alternating and spaced apart from one another coaxially along the said axis of acceleration.

Preferably, along the said axis of acceleration in a particular direction, each successive electrical conductor is charged to a higher potential than the previous electrical conductor, in the direction of desired acceleration. In other words, the electrical conductor closest to the ionisation chamber in use is charged to the Lowest potential and the electrical. conductor furthest away from the ionisation chamber is charged to the highest potential.

The charge on the or each conductor may be reversed.

One or more of the electrostatic accelerators may comprise at Least one sensor associated with a respective electrical conductor and located in relatively closer proximity thereto, the sensor being adapted to detect passage of a pulse of charged particles through the respective electrical conductor. Each of the electrostatic accelerators may comprise a plurality of such sensors, each in close proximity to a respective electrical conductor. Optionally, each of the electrostatic accelerators may comprise substantially one such sensor for each electrical conductor.

The apparatus may comprise a control means connected to the or each electrical conductor and adapted to reverse the charge polarity of one or more said electrical conductors, the control means also being connected to the or each sensor. Thus, after passage of a puke of charged particles through an electrical conductor, the charge polarity of an electrical conductor can be reversed. In this way, a "pull" force can be exerted on a puke before the pulse passes through a particular electrical conductor and then the charge can be reversed to provide a "push" force on the puke after it has passed through the said electrical conductor. The "push" force also reduces divergence of the beam.

The electrical conductors are generally formed to provide an electrostatic grid with an opening adapted to be large enough to allow passage of charged particles. In use, 7 -charged particles are attracted or repelled by a static charge according to the relative polarity between the grid and the particles, and arranged so that instead of the charged particles colliding with the grid they pass through it.

Preferably the electrical conductors have at least an opening substantially co-axial to the said axis of acceleration.

Preferably the electrical conductors are substantially cylindrically symmetrical around the said axis of acceleration. This provides for acceleration of particles with low deviation from the axis of acceleration by providing an electric field which is substantially uniform in an axial plane. The shape of the electrical conductors reduces divergence of the said beam.

The electrical conductors may thus be narrower at one end and wider at an opposite end. The electrical conductors may be arranged so that the said narrower end is nearer the ionisation chamber and the said wider end is further away from the ionisation chamber. In other words, the electrical conductors are shaped and arranged so that they widen out in the direction of desired acceleration.

In this preferred arrangement, the shape of the electrical conductors could be described as a solid of revolution. In other words the electrical conductors can be described as being broadly frustoconical or frustoparaboloidal in shape but it is not intended that the shape of the electrical conductors strictly adheres to that shape. In one embodiment, the electrical conductors comprise a portion with substantially parabolic cross-section, with an opening in the region of the vertex to allow particles to pass through.

Preferably, the electromagnets are arranged so that the magnetic field is substantially parallel to the acceleration direction of the charged particles. This reduces the divergence of the beam and allows for the beam to stay collimated for longer. The magnetic field also introduces an E X B drift, causing the particles to gyrate around the direction of travel (in which they are being accelerated). This aids the increase in neutralisation efficiency at the neutralisation stage.

A key aspect of this inventive concept is that the acceleration stages are calculated such that the speed of the electrons and the ions is the same, as they exit their respective -8 -acceleration stages. This means that the electrostatic grids accelerate the ions and the electrons to different energies, so that their velocities match.

The apparatus may comprise a second steering means adapted to steer each beam of charged particles from the said electrostatic accelerators to the neutralisation zone.

The neutralisation zone may comprise electromagnetic rings providing a magnetic field that is parallel to the direction of travel of the charged particles. The magnetic field ensures the beams are better collimated and it also generates an E X B drift on both of the beams of charged particles. This increases the collision rate allowing for faster recombination and thus neutralisation of the said beams.

The particles may subsequently be passed through a magnetic steering stage, to divert any remaining charged particles away from an entrance into the target plasma. Neutral particles would be unaffected and continue to the said entrance.

In implementations of the invention where it is used for thrust generation, the particles exit the neutralisation stage towards the opposite direction to the desired direction of thrust. The final magnetic steering stage is used to refine this direction, by diverting the ions in the opposite direction of desired thrust A method for particle acceleration according to the present inventive concept comprises the steps of: introducing a source element into an ionisation chamber; ionising the source element to split the said source input element into a discrete stream of positively charged ions and a discrete stream of electrons; a first steering step to steer the said discrete streams to respective electrostatic accelerators; accelerating the particles of said streams to substantially the same speed as one another; a second steering step to direct the accelerated streams to a neutralisation zone in which the said streams are brought together to form an output of neutral particles.

The introducing step may be by way of introducing the source element into the ionisation chamber via a controllable valve connecting the ionisation chamber to a gas reservoir. The ionisation step may comprise a step of using photons to initialise the ionisation of a quantity of the source element in a gaseous state. -9 -

The ionisation step may comprise increasing the amount of ionisation via oscillating acceleration of the initial charged particles. This can be achieved by an afternating of the charge on the charged grids, on either side of the chamber. The grids can then be subsequently biased at a fixed charge to accelerate the positively charged ions and the electrons out of the chamber and in opposite directions.

The ionisation step may comprise forming a batch or puke of charged particles. This allows for the more efficient control of the resulting beam and the reduction of instabilities during the travel of the charged particles.

The first steering step may comprise effecting a first steering means adapted to steer each beam of charged particles.

The second steering step may comprise effecting a second steering means adapted to steer each beam of charged particles from the said electrostatic accelerators to the neutralisation zone.

The method may comprise a step of passing the neutralised stream through a magnetic steering stage, to divert any remaining charged particles away from an entrance into the target plasma. Neutral particles would be unaffected and continue to the said entrance.

Thus, the present inventive concept provides a method comprising introducing an electron and an ion beam to target, wherein particles within those beams are traveiling at substantially the same velocity as one another.

Preferably, the above methods further comprise providing an E X B drift by applying a coaxiai magnetic field in the neutralisation stage. This can enhance the interaction between ions and electrons to further enhance the efficiency of neutralisation.

A particle output energy may be of the order of 200keV per positively charged ion. The skilled reader will appreciate that this is high compared to existing neutral beam injectors. This is the energy which the electrostatic grid on the ion acceleration stage would impart on the ions (e.g. by setting the final grid ring at -200kV). If the ions were deuterium ions, D+, then their speed wouid be of the order of 4400 km/s. The electron speed would be matched, so that the electrons are travelling at 4400 km/s and this -10 -would mean that the electron energy (per electron) would be 55 eV (e.g. by setting the final grid ring on the electron acceieration stage leg at +55 V).

Detailed description of aspects of the invention

Embodiments of aspects of the present inventive concept will now be described in further detail with reference to the accompanying drawings, in which: Figure 1 shows a schematic diagram of an embodiment of the present inventive concept; Figure 2 shows an embodiment of an ionisation chamber suitable for use with the present inventive concept; Figure 3 shows an embodiment of an electromagnetic steering stage suitable for use with the present inventive concept; Figure 4 shows a schematic diagram of an embodiment of an eiectrostatic accelerator suitable for use with the present inventive concept; Figure 5 shows an embodiment of aspects of an electrostatic accelerator suitable for use with the present inventive concept; Figure 6 shows a schematic diagram of an embodiment of a neutralisation zone suitabie for use with the present inventive concept.

In Figure 1, an ionisation chamber 10 provides discrete streams of charged particles to respective first steering means 20A and 20B. The first steering means 20A and 20B are adapted to steer the said discrete streams so that they have substantially equal lengths.

In this exampie, the streams are substantially parailel to one another although it wili be appreciated that other geometries are possible. Electrostatic accelerators 100A and 100B are adapted to accept the said discrete streams and are further adapted to acceierate the constituent particles to substantialiy the same speed as each other.

Second steering means 30A and 30B are adapted to steer the said discrete streams so that they are substantiaily paraliei to one another and substantially co-axial with one another. Neutralisation chamber 40 is adapted to bring the said discrete streams together to combine them into a neutraliy-charged single stream output A target 50 is also shown, which is not within the scope of the present inventive concept.

In Figure 2, an embodiment of an ionisation chamber 10 is shown having an enclosure 12 with an opening 14 formed within it for the introduction of a neutral gas into the enclosure 12. A photon emitter 18 is provided across the enclosure 12 from the opening 14. At either side of the enclosure 12 are arranged grids 16A and 16B. The grids 16A and 16B comprise electrical conducting material with gaps formed within to provide passage of particles therethrough.

In use, the ionisation chamber 10 uses photons to initialise the ionisation of a quantity of gas of the chosen element. This is introduced into the ionisation chamber via a controllable valve (not shown) connecting the chamber 10 to a gas reservoir. Ionisation is further effected by oscillating acceleration of the initial charged particles, by alternating of the charge polarity of the grids 16A and 16B. The grids 16A and 16B are then subsequently biased at a fixed charge to accelerate the positively charged ions and the electrons outside of the chamber and in opposite directions.

This process results in a pulse of particles. This provides for a more efficient control of the resulting beams and the reduction of instabilities during the travel of the charged particles.

The discrete streams can be subsequently and separately steered, using magnetic fields in the said steering means, into two parallel directions and into the next stage, the acceleration stage.

The acceleration stage utilises the Lorentz force as briefly shown in Figure 3.

Figure 4 shows a schematic diagram of an embodiment of an electrostatic accelerator 100, in which electrostatic conductors 102 are arranged spaced apart along an axis of acceleration A and substantially coaxially thereto. The electrostatic accelerator also has annular electromagnets 104 also arranged spaced apart along the axis of acceleration A and substantially coaxially thereto. In practice, the electrostatic conductors 102 and electromagnets 104 are not generally aligned with each other along the axis of acceleration -in other words they are staggered and they alternate so that for example an electrostatic conductor 102 is arranged between two electromagnets 104.

-12 -In use, along the said axis of acceleration A in the direction of desired acceleration, each successive electrical conductor 102 is charged to a higher potential than the previous electrical conductor 102, in the direction of desired acceleration. In other words, the electrical conductor 102 closest to the ionisation chamber in use is charged to the lowest potential and the electrical conductor 102 furthest away from the ionisation chamber is charged to the highest potential. Thus, the electrical conductor 102 furthest from the ionisation chamber is at a higher magnitude of potential, so that the beam is continuously accelerated through the electromagnetic accelerator 100. At high energies, travel time between the electrical conductors can be of the order of microsecond or hundreds of nanoseconds.

In use, the polarity of one or more of the electrical conductors 102 can be reversed -for example once a pulse of particles has passed. This can be detected by suitable electronic sensors (not shown) which can be placed near one or more electrical conductor 102 after the electrical conductor in the direction of acceleration. The sensors and fast electric switching allows for the polarity of the electrical conductor 102 to be switched quickly at the correct time, just after a particle puke has passed. This allows for the electrical conductor 102 to provide a push force that also reduces the divergence of the beam.

The electromagnets 104 are arranged such that the magnetic field is parallel to the acceleration direction of the charged particles. This reduces divergence of the beam and allows for the beam to stay collimated for longer.

The magnetic field also introduces an E X B drift, causing the particles to gyrate around the direction of travel (in which they are being accelerated). This aids the increase in neutralisation efficiency at the neutralisation stage.

As an example, if the desired neutral energy is of the order of 200keV (per ion), which is high compared to existing neutral beam injectors, then this is the energy which the electrostatic grid on the ion acceleration stage would impart on the ions (e.g. by setting the final grid ring at -200kV). If the ions were deuterium ions, D+, then their speed would be of the order of 4400 km/s. The electron speed would be matched, so that the electrons are travelling at 4400 km/s and this would mean that the electron energy (per -13 -electron) would be 55 eV (e.g. by setting the final grid ring on the electron acceleration stage leg at 55 V).

Figure 5 shows an embodiment of aspects of an electrostatic accelerator, wherein a series of electrical conductors 102 and annular electromagnets 104 (to aid clarity, not all are Labelled in Figure 5) are arranged alternately and coaxially along an axis of acceleration A. Each of the electrical conductors 102 forms a substantially paraboloidal or conical shape with circular symmetry around the axis of acceleration A. Each of the electrical conductors 102 has an opening at an apex thereof and a much wider opening at a base thereof. The electrical conductors 102 are arranged with their bases in the direction of desired acceleration along the axis of acceleration A, and their apexes in the opposite direction.

Figure 6 shows a schematic diagram of an embodiment of a neutralisation zone 40 suitable for use with the present inventive concept. The neutralisation zone 40 has a series of annular electromagnets 42 arranged substantially coaxially along an axis of neutralisation X. In use, a said discrete beam of electrons 44 is brought in line with a discrete beam of ions 46 for neutralisation. Neutralisation is aided by the velocities of the particles in the said beams 44, 46 having been substantially equalised.

Claims (19)

1. -14 -Claims 1. Apparatus for particle acceleration comprising an ionisation chamber adapted to split a source input element into a discrete stream of positively charged ions and a discrete stream of electrons, at least two electrostatic accelerators adapted to accelerate each of the said stream of ions and said stream of electrons to substantially the same speed as one another, and a neutralisation zone in which the said streams are brought together to form an output of neutral particles.
2. 2. Apparatus according to claim 1, wherein the ionisation chamber comprises an enclosure having an opening adapted for introduction of a source element, a photon emitter adapted to ionise the source element, and at Least two chargeable grids.
3. 3. Apparatus according to claim 2, wherein the said chargeable grids can be charged in opposition to one another.
4. 4. Apparatus according to claim 2 or claim 3, wherein the said chargeable grids are adapted to be switched between positive and negative charges and vice versa, in rapid succession.
5. 5. Apparatus according to any preceding claim, further comprising a first steering means adapted to steer each beam of charged particles from the said exit openings into paths of substantially equal lengths.
6. 6. Apparatus according to claim 5, wherein said beams are substantially
7. 7. Apparatus according to any preceding claim, wherein each electrostatic accelerator comprises an electromagnet and an electrical conductor arranged substantially coaxially along an axis of acceleration and spaced apart from one another along that axis of acceleration.
8. 8. Apparatus according to claim 7, wherein each electrostatic accelerator comprises two or more of each of an electromagnet and an electrical conductor, arranged respectively alternating and spaced apart from one another coaxially along the said axis of acceleration.
9. 9. Apparatus according to claim 8, wherein along the said axis of acceleration in a particular direction, each successive electrical conductor is charged to a higher potential than the previous electrical conductor, in a direction of desired acceleration.
10. 10. Apparatus according to any of claims 7 to 9, wherein the or each eFectrical conductor is adapted to have its charge reversed.
11. 11. Apparatus according to any of claims 7 to 10, wherein one or more of the electrostatic accelerators comprises at least one sensor associated with a respective electricaE conductor and located in relatively doser proximity thereto, the sensor being adapted to detect passage of a pulse of charged particles through the respective electrical conductor.
12. 12. Apparatus according to claim 11, wherein the apparatus further comprises a control means connected to the or each electrical conductor and adapted to reverse the charge polarity of one or more said electrical conductors, the control means also being connected to the or each sensor.
13. 13. Apparatus according to any of claims 7 to 12, wherein the or each of the electrical conductor has at least an opening substantially co-axial to the said axis of acceEeration.
14. 14. Apparatus according to claim 13, wherein the or each eFectrical conductor is substantially cylindrically symmetrical around the said axis of acceleration.
15. 15. Apparatus according to any of claims 7 to 14, wherein the or each electromagnet is arranged so that the magnetic field is substantially parallel to the acceleration direction of the charged particles.
16. 16. Apparatus according to any preceding claim, wherein the neutralisation zone may comprise electromagnetic rings adapted to provide a magnetic field that is parallel to a direction of travel of the charged particles.
17. 17. A method for particle acceleration comprising the steps of: introducing a source element into an ionisation chamber; ionising the source element to split the said source input element into a discrete stream of positively charged ions and a discrete stream of electrons; a first steering step to steer the said discrete streams to respective -16 -electrostatic accelerators; accelerating the particles of said streams to substantially the same speed as one another; a second steering step to direct the accelerated streams to a neutralisation zone in which the said streams are brought together to form an output of neutral particles.
18. 18. A method comprising introducing an electron and an ion beam to target, wherein particles within those beams are travelling at substantially the same velocity as one another.
19. 19. A method according to claim 17 or claim 18 further comprising providing an E X B drift by applying a coaxial magnetic field in the neutralisation stage.