INTERLACED MULTI-ENERGY BETATRON WITH ADJUSTABLE PULSE REPETITION FREQUENCY

• US 20150043717A1
• Filed: 08/07/2013
• Published: 02/12/2015
• Est. Priority Date: 08/07/2013
• Status: Active Grant

• Current Assignee: Varex Imaging Corporation
• Original Assignee: Varian Medical Systems Incorporated (Siemens AG)
• Inventors: Chen, Gongyin, Drubka, Robert
• Granted Patent: US 9,338,875 B2
• Time in Patent Office: Days
• US Class Current: 378/101
• CPC Class Codes: H05H 11/04 Biased betatrons

Abstract

Variable pulse frequency during an output session of a betatron device and adjustable energy from pulse to pulse are provided. A different bias magnetic field may be used for different cycles of an output session, thereby providing different pulse energies. In one example, the bias field can be switched from a positive value to zero, with energy stored in a storage device when the bias field is zero. The bias field can also be used to expand electrons from a stable orbit when the bias field is decreased. For variable pulse frequency, when a current in the swing coils decreases to zero, the swing coils can be disengaged from a storage device for an adjustable time before re-engaging for a next cycle, thereby adjusting the frequency. In addition, radiation dose output can be adjusted by varying a length of time for the injection of electrons into a betatron.

Claims

1. A method of operating a betatron device having swing coils for accelerating electrons and having bias coils to provide electron pulses at different energies, the method comprising:

operating the swing coils to generate a swing component of a total magnetic field at the electron orbit over a plurality of cycles, the electron orbit being a circular path in which accelerated electrons travel;

operating the bias coils to generate a bias component of the total magnetic field at the electron orbit;

operating the bias coils such that the bias magnetic component has a constant first magnitude during a first acceleration portion of a first cycle of the plurality of cycles;

accelerating, with the swing coils, a first packet of electrons with a first swing in flux subject to the total magnetic field during the first acceleration portion of the first cycle to have a first energy;

expanding an electron orbit of the first packet of electrons at an end of the first acceleration portion;

operating the bias coils such that the bias magnetic component has a constant second magnitude during a second acceleration portion of a second cycle of the plurality of cycles, the second magnitude being different than the first magnitude; and

accelerating, with the swing coils, a second packet of electrons with a second swing in flux subject to the total magnetic field during the second acceleration portion of the second cycle to have a second energy, the second energy being different than the first energy.

2. The method of claim 1, wherein expanding the electron orbit of the first packet of electrons includes:

decreasing a current in the bias coils at the end of the first acceleration portion of the first cycle such that;

the total magnetic field at the electron orbit is reduced,

the first packet of electrons expand from a stable orbit, and

the first packet of electrons hits a metal target to produce x-ray radiation at the first energy.

3. The method of claim 1, wherein the second magnitude is less than the first magnitude, and the second energy is less than the first energy.

4. The method of claim 1, further comprising:

operating the swing coils and the bias coils such that the cycles with the first accelerated electron energy and the second accelerated electron energy alternate.

5. The method of claim 1, further comprising:

at the end of the first acceleration portion;

disconnecting the bias coils from a bias power supply, and

connecting the bias coils to a bias storage device, thereby causing the first packet of electrons to expand from a stable orbit;

when a current in the bias coils decreases to zero, disconnecting the bias coils from the bias storage device; and

keeping the bias coils disengaged from the bias storage device for a first adjustable delay time.

6. The method of claim 5, further comprising:

connecting backwound coils to the bias storage device at the end of the first acceleration portion, the backwound coils connected in series with the bias coils and producing an opposite flux within the electron orbit.

7. The method of claim 5, wherein the first adjustable delay time is until an end of the second acceleration portion of the second cycle, wherein the constant second magnitude of the bias magnetic component is zero during the second acceleration portion of the second cycle, the method further comprising:

at the end of the second acceleration portion of the second cycle, connecting the bias coils to the bias storage device to expand the second packet of electrons from a stable orbit; and

after the current in the bias coils reaches a peak reverse value and then decreases to zero and in advance of a third cycle, increasing a forward current in the bias coils with the bias storage device;

when the bias storage device is depleted and the forward current reaches a peak;

disconnecting the bias coils from the bias storage device, and

connecting the bias coils to the bias power supply, and

operating the bias coils with the power supply such that the bias magnetic component has a constant third magnitude during a third acceleration portion of the third cycle.

8. The method of claim 7, further comprising:

when the current in the bias coils reaches a peak reverse value and then decreases to zero, disconnecting the bias coils from the bias storage device;

keeping the bias coils disengaged from the bias storage device for a second adjustable delay time; and

connecting the bias coils to the bias storage device to increase a forward current in the bias coils in advance of the third cycle.

9. A betatron device for providing electron pulses at different energies, the betatron device comprising:

a vacuum enclosure for electrons to orbit during acceleration portions of respective cycles;

magnetic cores that conduct and contain magnetic flux and that shape magnetic fields at the electron orbit;

swing coils configured to generate a swing in magnetic flux within the electron orbit and a swing component of a total magnetic field at the electron orbit over a plurality of cycles of a single output session;

bias coils configured to generate a bias component of the total magnetic field at the electron orbit during the acceleration portions of the respective cycles;

a bias storage device selectively coupled to the bias coils via at least a bias storage switch; and

a bias power supply selectively coupled to the bias coils and the bias storage device via at least a bias power switch;

10. The betatron device of claim 9, wherein the bias storage switch and the bias power supply switch are configured to:

provide a first steady bias coil current from the bias power supply to the bias coils such that the bias magnetic component has a constant first magnitude during a first acceleration portion of a first cycle of the respective cycles; and

provide a second steady bias coil current to the bias coils such that the bias magnetic component has a constant second magnitude during a first acceleration portion of a first cycle of the respective cycles, the second magnitude being different than the first magnitude.

11. The betatron device of claim 10, wherein the constant second magnitude is zero.

12. The betatron device of claim 10, wherein the bias storage switch and the bias power supply switch are configured to:

disconnect the bias coil from the bias power supply and transfer energy from the bias coils to the bias storage device at the end of the first acceleration portion, thereby expanding an orbit of first electrons.

13. The betatron device of claim 12, wherein the bias storage switch and the bias power supply switch are configured to:

disengage the bias coils from the bias storage device when the bias coil current reduces to zero and keep the bias coil current at zero for the second acceleration portion;

connect the bias coils to the bias storage device to increase the reverse current in the bias coil to expand an orbit of second electrons at the end of the second accelerating portion;

disconnect the bias coils from the bias storage device when the reverse current reaches a peak value and reduces back to zero;

after a variable period of delay time, connect the bias coils to the bias storage device to increase a forward current in advance of the next cycle, and

disconnect the bias coils from the bias storage device when a forward current reaches a peak value and connect the bias coils to the bias power supply to sustain a steady current.

14. The betatron device of claim 9 wherein:

the bias power supply has a first terminal and a second terminal;

the bias power switch has a first end coupled with the first terminal;

the bias coils are coupled with a second end of the bias power switch;

the bias storage switch having a first end coupled with the second end of the bias power switch, the bias power switch being in parallel with the bias coils; and

the bias storage device coupled in series with the bias storage switch and coupled in parallel with the bias coils.

15. The betatron device of claim 9, further comprising:

backwound coils connected in series with the bias coils and producing an opposite flux within the electron orbit.

16. A method of operating a betatron device having a swing coil for accelerating electrons to provide electron pulses at adjustable cycle rates, the method comprising:

operating the swing coils to generate a swing component of a total magnetic field at the electron orbit over a plurality of cycles, the electron orbit being a circular path in which accelerated electrons travel;

accelerating, with the swing coils, a first packet of electrons with a first swing in flux subject to the total magnetic field during a first acceleration portion of a first cycle of the plurality of cycles;

expanding an electron orbit of the first packet of electrons at an end of the first acceleration portion;

after the first acceleration portion of the first cycle, storing energy of the swing component in a swing storage device as the swing component decreases;

when a current in the swing coils decreases to zero, disengaging the swing coils from the swing storage device for a first adjustable delay time; and

after the first adjustable delay time, engaging the swing coils with the swing storage device for a next cycle.

17. The method of claim 16, wherein the swing component reaches a maximum at the end of the first acceleration portion.

18. The method of claim 16, wherein expanding the electron orbit of the first packet of electrons includes:

operating bias coils at the end of the first acceleration portion of the first cycle such that;

the total magnetic field at the electron orbit is reduced,

the first packet of electrons expand from a stable orbit, and

the first packet of electrons hits a metal target to produce x-ray radiation.

19. The method of claim 16, further comprising:

after acceleration portions of each of the plurality of cycles, storing energy of the swing component in the swing storage device as the swing component decreases;

expanding an electron orbit of a current packet of electrons at an end of a current acceleration portion of a current cycle;

when the current in the swing coils decreases to zero, disengaging the swing coils from the swing storage device for the first adjustable delay time; and

after the first adjustable delay time, engaging the swing coils with the swing storage device for the next cycle.

20. The method of claim 16, further comprising:

while the swing coils are disengaged from the swing storage device, charging the swing storage device with a power supply such that the swing component has a same amplitude for each of the cycles.

21. The method of claim 16, wherein the swing storage device is one or more capacitors.

22. The method of claim 21, wherein the swing coils are engaged and disengaged from the one or more capacitors using a switch, wherein the switch is open between cycles and closed during a cycle.

23. The method of claim 22, wherein operating the swing coils during the first cycle includes:

during a first phase of the first cycle, increasing a first current polarity in the swing coils using the one or more capacitors charged to have a first charge polarity;

during a second phase of the first cycle, charging the one or more capacitors to have a second charge polarity using the first current polarity in the swing coils;

during a third phase of the first cycle, increasing a second current polarity in the swing coils using the one or more capacitors charged to have the second charge polarity;

during a fourth phase of the cycle, charging the one or more capacitors to have the first charge polarity using the second current polarity in the swing coils.

24. The method of claim 16, wherein the first charge polarity is a negative charge polarity and the first current polarity is a negative charge polarity.

25. The method of claim 16, further comprising:

operating bias coils to generate a bias component of the total magnetic field at the electron orbit during the first acceleration portion of the first cycle;

after the first acceleration portion of the first cycle;

disconnecting the bias coils from a bias power supply, and

storing energy of the bias component in a bias storage device as the bias component decreases;

when a current in the bias coils decreases to zero, disengaging the bias coils from the bias storage device for a second adjustable delay time; and

after the second adjustable delay time, engaging the bias coils with the bias storage device.

26. The method of claim 25, further comprising:

when the bias component is not to be used for the next cycle;

engaging the bias coils with bias storage device at an end of a next acceleration portion of the next cycle to expand a second packet of electrons from a stable orbit, the second adjustable delay time being longer than the first adjustable delay time when the bias component is not needed for the next cycle, and

storing energy of the bias component in the bias storage device; and

when the bias component is to be used for the next cycle;

in advance of a next acceleration portion of the next cycle;

engaging the bias coils with the bias storage device after the second adjustable delay time, and

subsequently disengaging the bias coils from the bias storage device and reconnecting the bias coils to the bias power supply;

at the end of the next acceleration portion of the next cycle;

disconnecting the bias coils from the bias power supply,

engaging the bias coils to the bias storage device to expand the next packet of electrons from a stable orbit for x-ray generation, and

storing energy of the bias component in the bias storage device.

27. A betatron device for providing electron pulses at adjustable cycle rates, the betatron device comprising:

a vacuum enclosure for electrons to orbit during acceleration portions of respective cycles;

magnetic cores that conduct and contain magnetic flux and that shape magnetic fields at the electron orbit;

swing coils configured to generate a swing in magnetic flux within the electron orbit and a swing component of a total magnetic field at the electron orbit over a plurality of cycles of a single output session;

a swing storage device selectively coupled to the swing coils via at least a swing storage switch;

a swing power switch configured to selectively couple a swing power supply to the swing coils and the swing storage device,

wherein the swing storage switch is configured to engage the swing coils with the swing storage device during a cycle and to disengage the swing coils from the swing storage device for an adjustable delay time between cycles, and

wherein the first power switch is configured to disconnect the swing power supply from the swing coils and the swing storage device during a cycle.

28. The betatron device of claim 27, wherein the swing storage switch is configured to disengage the swing coils from the swing storage device when a current in the swing coils is zero and the swing storage device is in a specified charge state.

29. The betatron device of claim 28, wherein the specified charge state is a peak negative charge.

30. The betatron device of claim 27, wherein the swing power switch is configured to connect the swing power supply to the swing storage device for at least a portion of the time between cycles to charge the swing storage device.

31. The betatron device of claim 27, wherein:

the swing power supply has a first terminal and a second terminal;

the swing power switch has a first end coupled with the first terminal;

the swing storage device is coupled with a second end of the swing power switch;

the swing storage switch has a first end coupled with the second end of the power switch, the swing storage switch being in parallel with the swing storage device;

the swing coils are coupled in series with the swing storage switch and coupled in parallel with the swing storage device.

32. The betatron device of claim 27, further comprising:

bias coils configured to generate a bias component of the total magnetic field at the electron orbit during the acceleration portions of the respective cycles;

a bias storage device selectively coupled to the bias coils via at least a bias storage switch; and

a bias power switch configured to selectively couple a bias power supply to the bias coils and the bias storage device,

wherein the bias storage switch and the bias power supply switch are configured to;

provide a first steady bias coil current from the bias power supply to the bias coils such that the bias magnetic component has a constant first magnitude during a first acceleration portion of a first cycle of the respective cycles; and

provide a second steady bias coil current to the bias coils such that the bias magnetic component has a constant second magnitude during a first acceleration portion of a first cycle of the respective cycles, the second magnitude being different than the first magnitude.

33. The betatron device of claim 27, further comprising:

an electron gun that injects electrons into the vacuum enclosure for acceleration during each cycle; and

a metal target that produces x-ray radiation upon impact of electrons that are expanded from the electron orbit.

34. The betatron device of claim 27, wherein a first delay time between a first pair of cycles is different than a second delay between a second pair of cycles.

35. A method of controlling an amount of x-ray radiation dose emitted by a betatron device, the method comprising:

adjusting a length of time of an injection pulse of electrons into a betatron device;

accelerating a respective packet of electrons during an acceleration portion of each of a plurality of cycles while the electrons are in an electron orbit;

expanding the electron orbit for the respective packets of electrons at an end of the acceleration portion of each cycle; and

hitting a metal target with the expanded electrons to produce x-ray radiation, wherein the length of time of the injection pulse is adjusted based on a desired amount of dose output for the radiation beam.