Module IV, Lecture 2 DNP experiments and hardware

Transcription

1 Module IV, Lecture 2 DNP experiments and hardware tunnel diodes, Gunn diodes, magnetrons, traveling-wave tubes, klystrons, gyrotrons Dr Ilya Kuprov, University of Southampton, 2013 (for all lecture notes and video records see

2 Microwave sources A tunnel diode has a very narrow and heavily doped p-n junction, resulting in a very small bandgap and the appearance of a negativeresistance region in the voltage response curve. Tunnel diodes have very small capacitances and recovery times and can operate at over 100 GHz. If a tunnel diode is biased at the centre of the negative-resistance region and coupled to a tuneable cavity, it can produce a very stable oscillator with output power in a microwatt to milliwatt range. Leo Esaki,

3 Microwave sources A Gunn diode does not have a p-n junction, it simply contains a block of n-type material, usually gallium arsenide, which has a very non-linear voltage response curve around 3.2 kv. The negative resistance area is used in a manner similar to the tunnel diode. When the diode is DC biased into the negative resistance region, spontaneous microwave frequency oscillations arise. The frequency of a Gunn diode is primarily determined by the thickness of the active region in the semiconductor, but can be tuned to some extent by varying the frequency of the resonant cavity or circuit. Typical microwave output power is about 100 mw at about 100 GHz. The efficiency is not high over 95% of the input power is dissipated as heat. J.B. Gunn,

4 Electron bunching The phenomenon of electron bunching is very useful for microwave and radio-frequency amplification. In the frame of reference that travels with the wave, the force on a charge is given by the direction of the local electric field, meaning that electrons would collect in areas where the field (and therefore the force) is zero. The phenomenon is also intuitively clear if the electric field is described in terms of the corresponding potential the electrons would collect in the regions with low potential. S.E. Tsimring, Electron Beams and Microwave Vacuum Electronics, Wiley, 2007.

5 Microwave sources A magnetron device contains a hot cathode with a large negative potential, surrounded by a circular lobed anode. A magnetic field applied parallel to the axis of the cathode causes the electrons emitted from the cathode to spiral outwards towards the anode. The circular cavities serve as resonators. The resonant microwave field causes the electrons to bunch and amplify the field. The size of the cavities determines the microwave frequency, but the precision of the frequency control is quite low that is fine for cooking purposes though.

6 Microwave amplifiers A travelling wave tube consists of an electron gun, a tube with a longitudinal magnetic field, a helix of coiled wire that provides a low-impedance transmission line for the RF that is injected and removed via directional couplers, and a beam dump. If the electrons in the beam are travelling faster than the RF (the conductive helix is there to slow down the travelling wave), they give up energy to the travelling wave and increase its amplitude. vrf Electron beam focusing is achieved by placing a series of alternating-polarity toroidal magnets along the tube. pc 2 a TWTs can amplify signals within about 300 MHz to 300 GHz frequency range. The power gain is of the order of 70 db.

7 Microwave amplifiers

8 Microwave amplifiers A klystron is a variation of thetravelingwavetubethat converts kinetic energy of an electron beam into microwave power. As the electron beam passes through the first cavity resonator ( buncher cavity ), bunching occurs under the influence of the electromagnetic field of the cavity. When bunched electrons enter the second chamber, they induce electromagnetic field of the same frequency as the input field, but with much greater amplitude. Additional cavities are sometimesputinthebeampath to improve amplification. Klystrons can amplify electromagnetic signals ranging in frequency from hundreds of MHz to hundreds of GHz to powers as high as 50 MW. Klystrons are very hard to tune. Each unit has unique performance characteristics and must be individually calibrated.

9 Microwave amplifiers A gyrotron, also known as cyclotron resonance maser, is a type of free electron maser. The device creates a strong beam of electrons with spiral trajectories moving in a strong magnetic field and passing through a resonant cavity. The bunching in this case occurs along the trajectory spiral. Gyrotron output power can reach many megawatts. The output frequency is easily tuned by changing the magnet field and the resonance frequency of the cavity.