In order to provide frequency standards for the Deep Space Network (DSN) which are more stable than present-day hydrogen masers, a research task was established under the Advanced Systems Program of the TDA to develop a Hg-199(+) trapped ion frequency standard. The first closed-loop operation of this kind is described. Mercury-199 ions are confined in an RF trap and are state-selected through the use of optical pumping with 194 nm UV light from a Hg-202 discharge lamp. Absorption of microwave radiation at the hyperfine frequency (40.5 GHz) is signaled by atomic fluorescence of the UV light. The frequency of a 40.5 GHz oscillator is locked to a 1.6 Hz wide atomic absorption line of the trapped ions. The measured Allan variance of this locked oscillator is currently gamma sub y (pi) = 4.4 x 10 to the minus 12th/square root of pi for 20 is less than pi is less than 320 seconds, which is better stability than the best commercial cesium standards by almost a factor of 2. This initial result was achieved without magnetic shielding and without regulation of ion number

Dick, G. J.

Maleki, L.

Prestage, J. D.

English

NASA Technical Reports Server

TDA Progress Report 42-92
N88-18776
7G
October--December1987
The JPL Trapped Mercury Ion Frequency Standard
J. D. Prestage, G. J. Dick, and L. Maleki
Communications Systems Research Section
In order to provide frequency standards for the DSN which are more stable than present-
day hydrogen masers, a research task was established under the Advanced Systems Pro-
gram of the TDA to develop a 199Hg_ trapped ion frequency standard. This article de-
scribes the first closed-loop operation of this standard.
Mercury 199 ions are confined in an RF trap and are state-selected through the use of
optical pumping with 194-nm UV light from a 2°2Hg discharge lamp. Absorption of
microwave radiation at the hyperfine frequency (40.5 GHz) is signaled by atomic fluo-
rescence of the UV light. The frequency of a 40.5-GHz oscillator is locked to a ].6-Hz-
wide atomic absorption line of the trapped ions. The measured Allan variance of this
locked oscillator is currently ay(r) = 4.4 × ]O-12/x/T for 20 < _ < 320 seconds, which is
better stability than the best commercial cesium standards by almost a factor of 2. This ini-
tial result was achieved without magnetic shieMing and without regulation of ion number.
I. Introduction
Recently, there has been much activity directed toward the
development of trapped ion frequency standards. This has
occurred because ions confined in an RF quadrupole trap are
subjected to v.ery small perturbations of their atomic energy
levels and to weak forces which equalize any population dif-
ferences among the ground-state hyperfine levels. Potentially,
the largest source of frequency fluctuation for such a standard
stems from the motion of the atoms within the trap via the
second-order doppler or relativistic time dilation effect. To
minimize this perturbation, heavy ions are preferable to light
ions, since for a given energy a heavy ion will have a smaller
velocity. For this reason and for other reasons discussed later
in this article, 199Hg+ ions have been used for most trapped
ion frequency standard work.
Only two parameters are needed to describe the short-term
stability of a passive atomic frequency standard. One is the
line Q, which equals f/_f where fis the resonant frequency of
the reference atom and Af is the width of the atomic reso-
nance. For 199Hg+, fis 40.5 GHz and Afis as small as 0.1 Hz;
thus, trapped ion standards have line Q's which are orders of
magnitude higher than other microwave atomic frequency
standards.
The other parameter that determines stability is the signal-
to-noise ratio (SNR) achieved in measuring the atomic reso-
nance. The short-term stability is inversely proportional to the
product of Q and SNR. At present, four groups are developing
trapped Hg + ion clocks: Hewlett-Packard in Palo Alto, Cali-
fornia [1]; the National Bureau of Standards in Boulder,
Coloracto [2] ; l'Universite Paris-Sud in Orsay, France [3] ; and
JPL/NASA. It should be noted that F. G. Major first proposed
the use of trapped 199Hg+ ions as a frequency standard while
working for NASA at the Goddard Space Flight Center in
1969 [4].
13
https://ntrs.nasa.gov/search.jsp?R=19880009392 2020-03-20T08:22:58+00:00Z
II. Ion Trapping
The electrode structure and time-varying voltages used to
trap ions are shown in Fig. 1. The electric potential inside the
trap when no ions are present is
and
2 m
¢,_0z
8e 2 I:2o 4eU o
m2_22_ 4 m_ 2
V T =
(U 0 + V 0 cos [2t) (r 2 - 2z 2)
_2
where, for the present work, I2 = (21r) 500 kHz, Uo = 20 V,
Vo = 660 V, _z = (ro2+ 2z_), and r0 = the inside radius of ring
electrode = 1.9 cm = x/_'z o.
A charged particle moving in the inhomogeneous oscillatory
electric field of the ion trap feels a net force (averaged over
one cycle of _2) toward the region of the weaker field provided
that the amplitude of its motion at frequency _2 is small com-
pared with its distance from the center of the trap. The motion
under these conditions is a combination of a fast oscillation
at frequency I2 (micromotion) and a slower frequency co, as
shown in Fig. 2.
The action of the RF field in trapping ions is described by
the electric pseudopotential energy [5]:
e 2 V2 (r 2 + 4z 2)
_=
rnfZ2_ 4
where m and e are the ion's mass and charge, respectively. The
dc potential energy
eU o (r 2 _ 2z 2)
edc = _2
is added to the pseudopotential, giving the total potential
energy for an ion in the trap:
me0 2 r 2 me0 2 z 2
r
4_r- 2 + 2
where
2e 2 Vg 2eU o
tO 2 =
rn_2
Under the conditions listed earlier, ---19 electron volts of
kinetic energy is required for a 199Hg+ ion at the trap center
to reach one of the trapping electrodes.
Our trap is inside a vacuum chamber with a pressure of
---1-2 × 10-8 torr. By heating a powder of isotopically en-
riched mercuric oxide (HgO) to about 100°C, a vapor of
neutral 199Hg fills the vacuum chamber to about 10 -s torr
partial pressure. Electrons from an LaB 6 single-crystal fila-
ment are injected into the trap (---30 btA, 300 V), ionizing
some of the neutral 199Hg inside the trap electrode structure.
The resulting ion cloud is much hotter than the room-
temperature neutral vapor in part because ionization of the
vapor takes place throughout the trap. It has been found
experimentally that the average kinetic energy of the ions is
about 10 percent of the well depth. The resulting 2 eV of
kinetic energy would produce a fractional second-order dop-
pler shift of about 10 -11 . To reduce this shift, the vacuum
system is filled to about 10 -6 torr of '*He. The 199Hg+ ions
collide with these room-temperature helium atoms and are
cooled to just above room temperature.
III. Magnetic Levels and State Selection
The magnetic structure of the ground-state hyperfine levels
of 199Hg+ is shown in Fig. 3. The energy difference between
the (F = 0, m F = 0) and (F = 1, m F = 0) levels is used to define
the standard frequency, approximately fHg+ = 40.507347997
GHz. The measured frequency,f, depends quadratically on the
magnetic field at the position of the ion cloud, f = fHg+ + 97B2
(Hz/G 2). For comparison, the field dependence for hydrogen
atoms isf=f H + 2750B 2 (Hz/G2).
The ions are state selected by use of optical pumping with
light from a 2°2Hg discharge lamp. The energy levels of 2°2Hg÷
and 199Hg+ are compared in Fig. 4. Ultraviolet light of wave-
length 194.2 nm (26.4 eV) from the 2°2Hg lamp, when col-
lected and focused onto the 199Hg+ ions, will excite the tran-
sition 2S1/_ (F = 1, m_.) _ 2P1/2 .
The 2P1/2 state decays after 2 ns lifetime to either the
2S1/2 (F = O, m F = 0) or the 2S1/2 (F = 1, mF) state, thereby
scattering a 194-nm photon. Since the transition 2S1/2 (F = O,
m F = O) _ 2PI/2 is not resonant with the light from the
2°2Hg lamp, the ions are pumped out of the 2S1/2 (F = 1,
mF) states into _he 2S1/2 (F = O, m F = 0) state, at which time
.14
the ions stop scattering UV light. A flux of about 3 × 1012
photons per second per square centimeter passing through the
ion cloud will depopulate the 2S1/2 (F = 1, mF) levels in about
1/2 second. An oscillating magnetic field (strength 10 -6 G)at
a frequency of 40.507347997 GHz will transfer the atoms
from the 2S1/2 (F =0, mF =0) state to the 2S1/2 (F =1,
m F = 0) state in about 1 second. The ions will then scatter UV
light until they are pumped back into the nonfluorescing
2Sll: (F = 0, m F = 0) state.
IV. UV Optical System
Figure 5 shows the optical system used to collect and focus
UV light from the 2°2Hg lamp onto the ion cloud. The lamp is
excited with 15-20 watts of RF power (160 MHz), creating a
very bright discharge in the quartz cell containing the 2°2Hg
vapor with about 30 millitorr of argon buffer gas. The useful
light from the 2°2Hg lamp is from the 194.2-nm transition in
2°2Hg+, as shown in Fig. 4.
However, the brightest wavelengths produced in the lamp
are from transitions in the neutral mercury atom. Any light
detected at wavelengths other than 194 nm will degrade the
SNR of the measured atomic resonance. There are three ways
we suppress the detection of light with wavelengths different
from 194 nm:
(I) The photomultiplier tube (PMT) is sensitive only to
light that has wavelengths between 160 and 320 nm
with peak sensitivities of 12 to 15 percent at 210 nm.
The brightest line coming from the lamp in this band-
width is 254 nm and is 200 times brighter than the
194-nm line.
(2) The ellipsoidal collection mirror has a thin-film dielec-
tric coating which maximizes reflection at 194 nm
while keeping the reflectivity at 254 nm at 10 percent,
with lower reflectivities for longer wavelengths. The
entrance window to the trap region is coated to be
98 percent reflective at 254 nm while being 90 percent
transmitting at 194 nm.
(3) All light collected by the detection optics-stray scat-
tered light plus fluorescent light from the atoms-is
filtered with a 194-nm bandpass filter with a peak
transmission of 30 to 40 percent and a bandwidth of
45 rim. The solid angle subtended by the collection
optics around the trap center is 5 percent of the total
47r solid angle. The total collection efficiency of this
detection system is equal to the solid angle multiplied
by the bandpass filter loss multiplied by the PMT sen-
sitivity, or 0.05 × 0.35 × 0.12 = 2 × 10-3.
The atomic fluorescence and stray scattered light can be
seen in Fig. 6. The scan is triggered by the start of the elec-
tron pulse, which forms the ions and causes some increase in
detected light. After about 1 second the electron pulse is
switched off, the ions are pumped into the 2S1/2 (F = 0,
m F = 0) level, and only stray light is collected. At 2 seconds,
the 40.5-GHz radiation is switched on, transferring some of
the atoms into the 2S1/2 (F = 1, m F = 0) state, where they
scatter light as discussed earlier. Finally, at about 3 seconds,
the microwaves are switched off and the atomic fluorescence
dies away with a time constant of about 1/5 second.
V. Closed-Loop Operation
The sequence of operations used to carry out a measure-
ment of the 2S1/2 (F=O, m F =0) _ 2S1/2 (F = 1, m t = O)
frequency is shown in Fig. 7. By repeating this sequence as the
frequency of the microwaves is stepped in 0.2-Hz increments,
we measure the resonance curve shown in Fig. 8. For the
0.5-second square microwave pulse used in this measurement,
the smallest linewidth theoretically attainable is 1.6 Hz, which
corresponds to Q = 2.5 X 10 l°.
Locking the 40.5-GHz oscillator to the 1.6-Hz-wide reso-
nance line is done by stepping the oscillator 0.8 Hz to either
side of the resonance and adjusting the center frequency of
this -+0.8-Hz step to null the difference in fluorescence rates.
More precisely, suppose the center frequency of the 40.5-GHz
oscillator, Ft, is within one linewidth of the resonance. Three
measurements of the fluorescence are made on alternate sides of
Ft: C1 at F i + 0.8 Hz, C2 at F i - 0.8 Hz, and C3 at F i + 0.8 Hz.
The oscillator center frequency is then changed to
=F.+Fi+l t
(0.8 Hz/T) (C1 + C3 - 2C2)
2(SIGNAL)
where SIGNAL is the height of the fluorescence above back-
ground and T is the loop time constant in units of measure-
ment cycle time. The "second difference" (C1 + C3-2C2)
is used because it is insensitive to linear drifts in lamp inten-
sity. If the first difference (C1 - C2) had been used to change
Fi, a linear drift in lamp intensity would have forced a linear
drift in the 40.5-GHz oscillator's frequency away from the
atomic line center. The sequence of center frequencies obtained
in this closed-loop operation is shown in Fig. 9. The measure-
ment shown lasted just over 2 hours. A single measurement
cycle-consisting of an electron pulse to load ions in the trap,
a waiting period while ions are optically pumped, lamp off while
microwaves drive the transition, and lamp on and counter on to
monitor fluorescence-lasts about 2.5 seconds. The loop time
constant, T, is 5 measurement cycles, i.e., about 12.5 seconds.
15
The Allan variance derived from the sequence of frequen-
cies (F i) is shown in Fig. 10. The reference frequency for the
40.5-GHz oscillator is provided by a hydrogen maser (SAO-21
in the Frequency Standards Laboratory). For times that are
long compared to the loop time constant, the Allan variance
falls as 4.4 × 10-12/x/_ -. This short-term stability is nearly a
factor of 2 better than that of the best commercial cesium
standards (8.5 × 10-12/x/_). These initial results were obtained
without shielding the 0.8 G ambient magnetic field in the trap.
The residual field dependence at 0.8 G is 157 Hz/G. A fre-
quency stability of 2 × 10 -13 at this field sensitivity requires
magnetic field fluctuations smaller than 60 /aG over the
320 seconds required to reduce statistical error to 2 × 10-13.
For this reason, we have not pushed the Allan variance data
beyond 320 seconds in this first test.
Vl. Conclusion and Summary
In its first closed-loop operation, the frequency stability of
the trapped 199Hg+ frequency standard has been measured to
be
4.4 X 10-12
°r(r) -
for 20 < _"< 320 seconds.
Many improvements are under way to increase the short-
and long-term stability of this standard, including increased
fluorescence collection efficiency, shielding the ambient mag-
netic field, and designs for traps which could store one hun-
dred times the present ion number.
Acknowledgments
We wish to thank T. Tucker for mercury lamp construction and for assisting in the
development of the 40.5-GHz oscillator.
References
[1] L. S. Cutler, R. P. Giffard, P. J. Wheeler, and G. M. Winkler, "Initial Operational
Experience with a Mercury Ion Storage Frequency Standard," in Proceedings of the
41st Annual Symposium on Frequency Control, pp. 12-19, May 1987.
[2] D. J. Wineland, W. M. Itano, J. C. Bergquist, and F. L. Walls, "Proposed Stored
2°1rig+ Ion Frequency Standards," in Proceedings of the 35th Annual Symposium
on Frequency Control, pp. 602-611, May 1981.
[3] M. Jardino, M. Desaintfuscien, and F. Plumelle, "Prospects for a Mercury Ion Fre-
quency Standard," Journal de Physique, vol. C8, p. 327, 1981.
[4] F. G. Major, "Remarks on the Application of RF Spectroscopy of Stored Ions to
Frequency Standards," in Proceedings of the FAA Conference on Utilization of
Time and Frequency in Collision Avoidance Systems, pp. 125-126, 1968.
[5] H. G. Dehmelt, "Radio Frequency Spectroscopy of Stored Ions," Adv. At. Mol.
Phys., vol. 3, pp. 153-154, 1967.
16
•-e U0 + V0 cos I"Zt
Fig. 1. Electrode structure and voltages used in this work
,X, y
"_" 2 2
(.9
E
w
0 2000 4000 6000 8000 10,000
MAGNETIC FIELD, G
Fig. 3. Energy levels of the ground state of 199Hg + versus the
magnetic field, showing the first-order field-independent
transition used in this work
I I I I I I I I I I I I I I I 1 I I I I I I I I
_ MASS 199
_ TRAP FREQUENCY 500kHz
RFVOLTAGE 650V
I I I I I I I I I I I I I I I I I I I I I 1 I I
TIME
Fig. 2. One-dimensional motion for ion in combined RF and
dc trapping fields
70 MHz I 3.7 Ghlz
._2 GHz
202Hg + 199Hg+
(l=o) (l=h)
2P½
2_
F'=0 F'=I
m'F=0 m'F=-10+l
M=0 M= -1 0 +1
F=0 F=I
199Hg+
Fig. 4. Ground and lowest optically excited states of 202Hg + and
199Hg+ (left); a depiction of the manner in which light from
202Hg + pumps 199Hg + ions into the 2S1/2 (F = O,m F = 0) level
(right)
17
FILAMENT
ELECTRON
END CAP
ELE
UV TRANSMITTING
Wl
I 'I
FILAMENT
FEEDTHROUGHS- :UUM
CANISTER
HNELT
ELECTRODE
DISCHARGE LAMP
ELECTRODE
ELLIPSOIDAL
MIRROR
INTERFERENCE
RESCENCE
COLLECTION
OPTICS
=MT PHOTOCATHODE
Fig. 5. UV light of 194 nm is scattered by the trapped 199Hg + ions after the absorption
of microwave radiation near 40.5 GHz
%
I-
z
o
z
13
12
11
10'
9
81
7
6
S
4
3
2
1
ol
I I f I
ELECTRON
- PULSE
- (MAKES IONS)
MICROWAVES
i -
I I I I I
1 2 3 4 5
TI ME, sec
I
6
Fig. 6. Light scattered from the Ions upon application of
microwave radiation (40.5 GHz) is detected together with stray
light from electrodes and windows
ELECTRON
GUN
MICROWAVES
OFF
MEASUREMENT
CYCLE -_ 2.5 sec
ON
LAMP OF F _
I--I I-L_COUNTER OFF
Fig. 7. Sequence of operations used to determine whether the
40.5 GHz is correctly tuned to the atomic resonance (two cycles
are shown)
18
3.5
3.0
%
x 2.5
z
2.0
0
_ 1.5
u.
1.0
I I I I I I 1 I
,,1.-.1.6 Hz
m
0.5 I I I I I I I I I
-10 -8 -6 -4 -2 0 2 4 6 8 10
FREQUENCY OFFSET FROM 40.507 GHz, Hz
Fig. 8. Atomic fluorescence as the frequency of the 40.5-GHz
oscillator is swept through the atomic resonance
==
>-
z
uJ
o.,
u.
1.5 I I I I I I I I I I
1.0--
0.5--
0--
i._.k...,_= j= _lj ._j,_ldJL=jl__lL,ka.a. bJJ==..I._L._lL" a,
-0.5 _Plr_l[w'_llr "wm_'w'T'll|l"lm1_ • ""_r,lr,----IF _-_r w._
-1.O --
-1.5 I I I
0 300 600 900
I I I I I I I
1200 1500 1800 2100 2400 2700 3000 3300
MEASUREMENT NUMBER
Fig. 9. Frequency deviation of the 40.5-GHz oscillator
while servoed to the 1.6-Hz-wide atomic resonance (each
measurement lasts 2.5 seconds)
10-11
10_12
>
<
10-13
I I I 1 I I
= .
LOOP TIME
CONSTANT
I
I
I I I I I I I
2 4 8 16 32 64 128
NUMBER OF MEASUREMENTS AVERAGED TOGETHER
Fig 10. Allan variance of the sequence of center frequencies
shown In Fig. 9; multiply by 2.5 seconds to convert the
horizontal axis to time
19


The JPL trapped mercury ion frequency standard

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