Nmr probe for efficient mas-dnp operation at ultra-low temperatures

Abstract

An NMR probe for insertion into an NMR magnet from below is disclosed that includes multiple counterflow heat exchangers to enable a wide range of NMR methods at ultra low temperatures, including MAS DNP at temperatures below 15 K. Coolant fluid is ducted from the bottom of the probe through a vacuum insulated transfer line up into the probehead, the region containing the sample and sample coil, where it splits into first and second coolant streams for cooling first and second spin-gas streams, each through a cold counterflow exchanger and at least one cool counterflow exchanger contained within the probe. The cold exchangers would each preferably comprise two straight continuously joined tubes inside a dewared tube that extends from a vacuum chamber in the base of the probe up into the probehead. These cold exchangers are denoted as Dual Inner Tube Exchangers (DITEs). The cool exchanger is preferably a coil of two parallel continuously thermally joined tubes, and it is denoted as a Parallel Tube Recuperator (PTR). The first coolant-stream in the probehead feeds into the top of DITE-1, through which the first coolant stream flows downward, providing final cooling to the first spin gas stream which is flowing upward past it into the probehead. The first coolant stream leaving the bottom end of DITE-1 then enters the cooler end of a first PTR in an evacuated chamber in the base of the probe and proceeds through the first PTR to its warmer end in counter-current to the first spin gas stream which had entered at the warmer end of first PTR, exited at its cooler end, and proceeded into the bottom end of DITE-1. An additional PTR may be series connected to the first PTR to further improve the overall effectiveness of heat transfer from first spin gas stream to the first coolant stream. In like manner, the second coolant stream cools a second spin gas stream. The exchangers are designed to permit fast MAS at temperatures below 15 K when the probe is supplied with room-temperature helium spin gas streams at commonly used pressures and a helium coolant stream at less than 170 kPa from a standard portable helium cryostat. Liquid helium usage rate is typically in the 2-12 L/hr range, depending on the spinning rate and sample temperature. Both the spin gases and the coolant fluid are fully recoverable without contamination for recycling using the standard helium recycling methods and equipment (with minor upgrades) commonly found in many NMR laboratories.