Supersonic Expansion Discharge Source

Our supersonic expansion discharge source producing a jet of nitrogen plasma.

Motivation

As part of our goal to acquire high-resolution spectra of astronomically important molecular ions with the SCRIBES experiment, we require a way to produce ions that are translationally and rotationally cold. The benefits of cold ions are numerous: their spectra are astronomically relevant, simpler and easier to assign; and, owing to their narrow velocity distribution, their spectra have reduced spectral linewidths. However, most commonly-used ionization techniques (e.g. electron impact, fast atom bombardment, electrical dicharge) produce high-temperature ions. For the SCRIBES experiment, we are coupling a continuous DC discharge with a supersonic expansion to overcome these difficulties.

Supersonic Expansions

Supersonic expansions have been used for many years to translationally and rotationally cool gas-phase species. By forcing a gas at high pressure through a small orifice into vacuum, an expansion plume is generated in which the gas molecules are accelerated faster than the local speed of sound, thus creating a supersonic expansion. In this adiabatic process, the heat energy of the gas is converted into directed translational energy, and the temperature of the gas decreases as the velocities of the individual molecules approach the same value. However, because the gas is expanding into a greater volume, the local number density, and consequently the collision rate, decrease rapidly. As a result, full thermodynamic equilibrium is lost throughout the expansion, and the degrees of freedom of the gas (vibration, rotation, and translation) can no longer be represented by a single temperature. Due to energy transfer efficiency considerations, typically in a supersonic expansion T_vib >> T_rot ≥ T_trans. Depending on the details of the gas conditions, typical T_rot and T_trans values range from 0.5 - 30 K.

Our supersonic expansion discharge source.

Molecular ions can be produced under supersonic expansion conditions by seeding a small amount of precursor gas into a larger amount of buffer gas, and striking a DC discharge as the mixture passes through the orifice. Typically, this is done by pulsing a small amount of gas through a valve upsetream of the orifice, thereby producing short pulses of expanding plasma. However, SCRIBES requires a continuous beam of ions, so we must instead utilize a continuous expansion. This creates two unique challenges: maintaining a good vacuum with a high, continuous flow of gas leaking into the chamber, and constructing a supersonic expansion source that can withstand the thermal load generated by the discharge while still producing cold ions. We solve the first challenge by pumping our system with a large Roots blower (pumping speed 3200 L/s at 20 mTorr), and the second by performing tests on a variety of source designs.

Boltzmann plot of the (6-0) P branch of the iodine B-X transition which shows a rotational temperature of 8 K.

Source Design, Testing and Evaluation

The continuous supersonic expansion discharge source that we have designed consists of a stainless steel baseplate with a small pinhole, a macor spacer, a stainless steel electrode, and a macor cap to insulate the electrode from the surrounding vacuum chamber. Gas-tight seals are made between each piece by means of high-temperature silicone o-rings. The baseplate is held at ground potential, and a negative high voltage is applied to the electrode to generate a plasma, which expands through the electrode and into the vacuum chamber. With this design, the source easily achieves a lifetime of >120 operating hours.

Having constructed a robust source, we now need to explore the design parameter space to determine how to maximize ion production while minimizing ion temperature. Many aspects of the design can be changed, including the diameters of all apertures, thicknesses of the various pieces, the geometry of the apertures (e.g. straight capillary, angled, or flared like a trumpet), the discharge voltage/current, the gas pressure, and the gas composition. As an initial performance check, we have performed laser-induced fluorescence measurements on the B³Π⁺_0,u - X¹Σ⁺_g (6-0), (8-1), (10-2), and (12-3) vibronic transitions of the iodine molecule. With these measurements, we have observed rotational temperatures as low as 8 K.

To measure the temperature distributions of ionic species in the expansion, we plan to perform cavity ringdown spectroscopy on the ν₂ fundamental band of H₃⁺ located at 3.6 μm. (for more information on our group's work with this species, check out the Hydrogen page). For this spectroscopy, a mid-infrared laser has been constructed by mixing the outputs of a 10 W frequency-doubled Nd:YVO₄ laser (532 nm) with a tunable ring dye laser (560-750 nm) in a MgO-doped periodically-poled LiNbO₃ crystal. Its 350+ μW of output power can access the entire spectral range from 2.2-4.8 μm. The ringdown measurements of H₃⁺ will yield the ions' rotational temperature, and this will act as feedback for refining the design of the supersonic source.

The DFG cavity ringdown spectrometer.

63	K. N. Crabtree, C. A. Kauffman and B. J. McCall "Performance of a Continuous Supersonic Expansion Discharge Nozzle Evaluated by Laser-Induced Fluorescence Spectroscopy" Sixty-Fourth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2009.
62	C. A. Kauffman, K. N. Crabtree and B. J. McCall "A Continuous Supersonic Expansion Discharge Nozzle for Rotationally Cold Ions" Sixty-Fourth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2009.

Supersonic Expansion Discharge Source

Motivation

Supersonic Expansions

Source Design, Testing and Evaluation

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