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Experimental Results with Atomic Hydrogen Storage

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Experimental Results with Atomic Hydrogen Storage EXPERMENTAL RESULTS WITH ATOMIC HYDROGEN STORAGE BEAM SYSTEMS* Helmut Hellwig and Howard E. Bell Atomic Frequency and Time Standards Section National Bureau of Standards Boulder,Colorado 80302 USA Summary hydrogen atoms1 is an appealing design [2], [S]. For such a device, operating far below oscillation threshold, Two atomic hydrogen storage beam devices are the cavity pulling factor F1 is very favorable [l], [6]: described, one based on the detection of hydrogen atoms, the other on the detection of changes in the microwave signal due to the hydrogen resonance. Electron bombardment ionization is used for the detection of a omic hydrogen. Detector efficiencies of up to 5 X IO-‘ are measured using a method which is where QC and Q are the cavity and line quality factors, 1 based on the study of pulse decay at maser oscillation respectively.However, an efficient, low-background threshold. Quantitative measurements of the atomic hydrogen detector is essential to exploit the advantage hydrogen beam intensity as a function of source pres- given by hydrogen atom detection as compared to the sure and RF discharge power are given. The overall detection of the microwave power. efficiency of the atomic hydrogen detection in the 8 hydrogen storage beam device is estimated at about 10- . For a passive device, based on detecting the Ways to increase the efficiency are indicated. microwave signal which is transmitted through the cavity containing the hydrogen atoms, J. Viemet, c. Audoin, The second device, which is based on microwave and M. Desaintfuscien have shown [l] that the cavity detection, closely resembles a hydrogen maser oscil- pulling factor F2 is lator with a low-Q cavity below oscillation threshold. Cavity pulling can be reduced to a point where environ- mental temperature fluctuations limit the stability mainly via the second-order Doppler effect. A quartz crystal oscillator is locked to the hydrogen hyperfine transition using the dispersion of this resonance. Locking to the Here, Ly is a gain parameter [for negligible saturation dispersion feature eliminates the need for frequency the gain is equal to (1 - CY)-’ at resonance]. Information modulation in order to find line-center. The stability of on the hydrogen line can be obtained by detecting either the frequency standard was measured against crystal the microwave intensity or the dispersion signal. Since oscillators and cesium beam frequency standards; the dispersion signal contains the frequency information, stabilities of 4 X wererecorded for sampling times its use avoids the need for frequency modulation of the of 30 seconds and of 3 hours. slaved quartz crystal oscillator; however, the hydrogen flux has to be modulatedin order to subtract--in first KeyWords (for information retrieval): Atomic approximation- -the dispersive characteristic of the hydrogen beam, Atomic hydrogen generation, Dispersion, microwave cavity. Frequencystability, Frequency standard, Hydrogen atom detection, Hydrogen flux calibration, Hydrogen In the following, WC report in Part 1 our results maser, Relaxation measurements, Spinexchange. with a system employing the detection of hydrogen atoms, and in Part 2 we discuss the design and performance of Introduction a system employing microwave signal detection. As compared to the hydrogen maser oscillator, a 1. HydrogenAtom Detection passive hydrogen standard offers a significant reduction in cavity pulling [l], [2]. This, together with the 1. l Beamintensity calibration elimination of the requirement to obtain self-oscillation, offers considerable design advantages, might lead to In order to evaluate the efficiency of an atomic improved long-term stability, and possibly offers greater hydrogen detector it is necessary to know the hydrogen flexibility to evaluate the frequency bias due to wall- beam intensity. We used a hydrogen maser oscillator to collisions of the stored hydrogen atoms (wallehift)[3]. Based on certain theoretical considerations, a hydrogen storage beam tube employing the detection of l Experiments with stored beams have been done earlier; *Contribution of the National Bureau of Standards, for example, an experiment using cesium is described not subject to copyright. in Ref.[4]. 242 calibrate the intensity of the hydrogen beam. We then (d) the maser was brought far below oscillation used the beam system of the maser oscillator to evaluate threshold by detuning the microwave cavity, yz was the performance of our hydrogen atom detector.2 measured by observing the pulse decay (Fig. la); (e) the hydrogen beam intensity was reduced to a level The beam intensity at oscillation threshold of the where no spin-exchange occurred, the pulse decay was hydrogen maser [7] is given by again measured (Fig. lb). and yZsE wasobtained as the difference between this measurement and the one according to (d) above; (f) Ith was calculated using eqs (3) and (5). We used this procedure to measure the beam 22 withc E hVf8r p RV, where V isthe volume, Q the intensity as a function of the atomic hydrogen source loaded quality facfor, and r] the filling factor [7 J of the parameters. The source was a pyrex glass bottle with cavity; p is the magnetic dipole moment of the transi- a collimator opening consisting of a bundle of channels tion,h is Planck'sconstant, and y therelaxation with about 40 pm diameter each and a length of 0.6 mm. constant. One has to distinguish caPefully between the The total diameter of the bundle was 1. 3 mm. The relaxation of the population difference described by y dissociating discharge was driven by a ZOO-MHz RF 1' and the relaxation of the oscillating magnetic moment generator. Figure 2a shows that the beam intensity 4 described by y2. It can beshown that [8] depends linearly on the source pr sure at constant RF power, and reaches more than lof* atoms per second. It should be noted that typical beam intensities for 2 Yo = YlY2 hydrogen maser oscillator operation are about lo1 atoms per second. Figure 2b depicts the dependence of the beam intensity on the RF power of the discharge at where y1 and y2 are each the sum of all corresponding constant source pressure. This curve essentially shows relaxation processes. We now assume that all relaxa- the degree of dissociation into atomic hydrogen and tion processes other than hydrogen-hydrogen spin indicates a leveling off of the dissociation with higher RF exchangecan be described by y = and that for power. spin-exchange y = 2yzsE Tk!s allowstous 1SE rewrite eq (4): The atomic hydrogen detector was a commercial residual gas analyzer, i.e., an electron bombardment ionizer followed by a quadrupole mass spectrometer and 2 an electron multiplier. The ionizer emits a sheet of electrons, about 2 mm high (along the beam axis), across the hydrogen beam. For highest ionization efficiency, If y is measured, the threshold beam intensity the electron voltage is chosen to yield the maximum can be ca1:ulated from eq (3) since the constant c is ionization cross-section for hydrogen (approx. 70 V). known.3 The relaxation constants can be measured by We measured the electron multiplier gain and obtained observing the decay of the radiation intensity in the from the comparison between the detected current and maser cavity after applying a suitable, stimulating the calibrated hydrogen beam intensity (corrected for microwave pulse. The maser cavity was equipped with geometrical factors, and the presence of atoms in the two coupling loops; the pulse was applied to one, the F = 1, m = 1 level) an efficiency of 1.5 X 10-5 at an radiative decay observed at the other using a three-stage electron emission current of l mA. The efficiency in- superheterodyne receiver. The pulse was generated by creased near1 linearly with the electron current to Y 5 pulsing a frequency synthesizer whose frequency (about reach 5 X 10- at maximum emission current (3 mA). 20 MHz) was subtracted from quartz crystal controlled 1440 MHz to match the atomic hydrogen resonance 1.2Operation of thebeam frequency. Two important items must be considered: (1) When pulsed, the maser must be far below oscillation The hydrogen storage beam tube system which we threshold.(2) Only y is directlyobservable; in order used is shown in Fig. 3. A hydrogen maser oscillator to obtain y [see eq (5)4 one must also determine y was modified by adding a second output opening to the 2SE' The follow& measurement procedure was therefore storage bulb, followed by a dipole magnet as the second established: (a) the maser was made to oscillate; state selector and the atomic hydrogen detector which (b) the maser was tuned to maximum oscillation ampli- was positioned at the proper deflection angle. A beam tude; (c) the maser oscillator was brought as accurately stop was placed in the center of the bulb to prevent the as possible to oscillation threshold by introducing a direct passage of the hydrogen beam. Differential suitable inhomogeneity in the axial DC magnetic field in pumping decoupled the detector vacuum chamber from order to fulfill the conditions on which eq (3) is based; themain vacuum chamber byabout a factor of In operation, the vacuu-Toat the detector was measured to beapproximately 10 torr (m 10-8N/m2). 2Care has to be taken that identical geometries are used in the maser oecillator mode and in the particle detec- The beam intensity is always measured after the tion mode. In our experiments we always used a 4-mm aperture mentioned in footnote 2. Also, it must be diameter circular aperture at the entrance of the noted that we are only observing the difference in the storage bulb or the detector entrance, respectively, intensities of the F = 1, m = 0 and F = 0, m = 0 states. and always positioned at a distance of 45 cm from the The massresolution of the mass spectrometer was exit of the state selector magnet.
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