Novel Traveling Wave Tubes for CW and Pulsed DNP NMR

Massachusetts Institute Of Technology, Cambridge MA

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Abstract

? DESCRIPTION (provided by applicant): This application proposes the development of a 250 GHz traveling wave tube (TWT) oscillator (years one and two) and a 250 GHz TWT amplifier (years three and four) for utilization in dynamic nuclear polarization (DNP) NMR experiments. In DNP, a high frequency microwave source is used to irradiate electron-nuclear transitions and the high spin polarization present in the electron spin reservoir is transferred to the nuclear spi system through the hyperfine and dipolar interactions. The resulting enhancements in NMR signals have been demonstrated to exceed a factor of 400, and thus DNP NMR is now considered a major advance in NMR spectroscopy. The required microwave frequency for the electron spin system excitation is in the millimeter to terahertz regime - 263 GHz, 395 GHz and 527 GHz for g=2 electrons at 400 MHz, 600 MHz and 800 MHz 1H frequencies respectively. To date, DNP experiments have relied on gyrotron oscillators with continuous power output of 20 to 50 Watts developed for DNP magic angle spinning (MAS) experiments. Gyrotrons are now commercially available and about 30 such systems have been installed worldwide in the past five to ten years. However, gyrotrons are both costly and relatively large, the latter creating issues with siting. A TWT source will dramatically lower the cost and size of the microwave source for DNP spectrometers. If the TWT source is successfully developed, it will allow dissemination of DNP NMR techniques to hundreds of laboratories worldwide, thus making the breakthrough DNP method widely available to the biomedical research community. Extending the operation of TWTs to higher frequency and power is an area of intensive exploration in modern vacuum electron device research. Our group has an ongoing research program to investigate TWT amplifiers at 95 GHz in W-Band and have recently successfully demonstrated an innovative 95 GHz TWT in an overmoded structure. The proposed research will build upon that success in the design, fabrication and demonstration of a 20 Watt, 250 GHz TWT oscillator. The proposed device will use a novel interaction structure that allows a relatively large electron beam tunnel, thus minimizing beam interception and structure heating in continuous operation. The TWT will be used with an existing 380 MHz NMR spectrometer for DNP research, thus demonstrating experimentally the application of a TWT to DNP. Our structure concept is scalable to higher frequencies, such as 395 or 527 GHz. An amplifier is also very attractive for DNP NMR experiments, where time-domain spectroscopic techniques can produce large enhancements and in contrast to CW Cross Effect mechanisms do not exhibit a magnetic field dependence. We will build a TWT amplifier to achieve higher output power, 200 W at 250 GHz, using pulsed electron beams on the time scale of several microseconds. The amplifier can be tested with existing hardware, a large cost savings. We will also demonstrate highly efficient transmission of the output power from the TWT to the sample located in the NMR probe. Collectively, these studies will represent a complete solution of the application of the TWT to DNP NMR.