Gyrotron

G.F. Brand. Development and Applications of Frequency Tunable, Submillimeter Wave Gyrotrons , Int. J. Infrared and Millimeter Waves Vol. 16, pp. 879-887, 1995. 

Gyrotrons produce high-power microwaves up to megawatt range... 

Gyrotrons -PCTFE m [FLUORINE COMPOUNDS, ORGANIC - POLYCHLOROTRIFLUOROETHYLENE] (Vol 11) - [MICROWAVE TECHNOLGY] (Vol 16) - [MICROWAVETECHNOLGY] (Voll6)... 

Another report to mention is that of Toda et al., where they place an NMR coil directly above the microwave cavity of an X-band ESR system and shuttle the sample between the center of the microwave cavity and the NMR coil.30 They observed -19-fold enhancement with toluene and the BDPA radical at 0.40 T. Since their research program is focused on developing gyrotrons for high-field DNP, they did not further pursue X-band DNP experiments. 

Microwaves can be produced by four types of macroscopic cavity resonators that use the ballistic motion of electrons across a cavity opening the klystron, the magnetron, the traveling-wave tube (TWT), and the gyrotron. They can also be generated by field-effect transistors at low frequencies, by Gunn42 diodes, and by IMP ATT diodes. 

Gyrotrons are powerful vacuum tubes that emit pulsed or CW millimeter-wave beams (20 to 250 GHz, kW to 2 MW) by bunching electrons with cyclotron motion in a strong magnetic field. 

Transparency Windows for infrared lasers and gyrotrons, optical lenses... 

Link, G. Feher, L. Thumm, M. Ritzhaupt-Kleissl, H.J. Bohme, R. Weisengurger, A. Sintering of advanced ceramics using a 30-GHz, 10-kW, CW industrial gyrotron. IEEE Trans. Plasma Sci. 1999, 27 (2), 547-554. 

Bajaj VS, Farrar CT, Homstein MK et al (2003) Dynamic nuclear polarization at 9 T using a novel 250 GHz gyrotron microwave source. J Magn Reson 160 85-90... 

Bajaj VS, Homstein MK, Kreischer KE et al (2007) 250 GHz CW gyrotron oscillator for dynamic nuclear polarization in biological solid state NMR. J Magn Reson 189 251-279... 

Among these, gyrotrons and cyclotron resonance masers are high-frequency vacuum electronic devices that have the ability to produce sufficient power in the frequency range of 140-590 GHz for electrons (200-900 MHz for proton nuclei). The electron cyclotron resonance maser can emit the coherent radiation near the relativistic electron cyclotron frequency. The irradiation frequency is given by... 

At microwave frequencies in the range 72-145 GHz, the critical parameters for high-power transmission are the dielectric characteristics of the window material the dielectric loss factor tan 5 and the permittivity e[. (or the refractive index n = because they affect power absorption and reflection [42]. The dielectric loss factor tan 8 in low loss samples is usually measured as the decrease in the Q factor of a resonant cavity [43]. Low dielectric loss materials find application as the output windows of high-power microwave tubes. A specific case is that of windows for Gyrotron tubes operating in the 70-170 GHz frequency region with output powers in excess of 1 MW, as will be discussed later. 

Values of the dielectric loss of CVD diamond have been measured over the past 3 years as a suitable material grade for dielectric window applications was being developed [5]. For open resonant cavity measurements, samples are usually required to be of at least 30 mm in diameter and of thickness in excess of 0.87 mm depending on the measurement frequency and the accuracy required. For recent CVD diamond, values of tan 5 below 10 have been achieved. A specific example is a window 100 mm in diameter and 1.6 mm thick which exhibited a tan 8 value of 0.6 ( 0.2) 10 . This is the lowest value so far reported for CVD diamond and would enable the material to be used as output windows in Gyrotron tubes of powers in excess of 2 MW as discussed in 2.4. 

Figure 25. Example of a 1.3 MW, 140 GHz Gyrotron tube built at the Forschungzentrum Karlsruhe [66].

The Development of Chemical Vapor Deposited Diamond Gyrotron Windows... 

The detailed requirements for the use of diamond as a window material in high power Gyrotron tubes can be listed as follows. 

Table 4. Properties of candidate Gyrotron window materials at room temperature.

Sample DB 6 is probably the largest Gyrotron window ever made (named Star of FZK ) and exhibits a loss of 2 x 10 over the same temperature range as DB 5 [70]. 

Figure 28. The first high-power Gyrotron (JAERI/Toshiba, 170 GHz) fitted with a CVD diamond window. Measurements using beam powers up to 1 MW for 10 seconds have already been performed.

Figure 31. Examples of CVD diamond windows mounted onto vacuum flanges. The two at either end show 100 mm diameter windows mounted to conventional CONFLAT vacuum flanges. The one in the middle illustrates a double flange configuration designed specifically for Gyrotron tube assemblies. All these vacuum assemblies have been tested to be vacuum tight to better than 10-9 mbar I s" after thermal cycling up to 450°C.

A metal-to-diamond bonding technique has been developed to attach CVD diamond windows to vacuum flanges or other assemblies. This is required to mount the windows to the Gyrotron tubes or reactor ports via suitable waveguide sections. Figure 31 shows examples of three 100 mm diameter CVD diamond windows mounted to two different types of flanges. The two at either end of the picture are... 

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