JPS6364064B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a hydrogen maser resonator having ultrahigh stable frequency characteristics used in a hydrogen maser frequency standard.

Although the frequency accuracy of hydrogen maser frequency standards is inferior to that of cesium (Cs) atomic beam frequency standards,
Its frequency stability is the best among currently used frequency standards. Therefore, it has become an essential device as a highly stable frequency signal source for VLBI (Very Long Baseline Interferometer) and tracking of deep space satellites.

In the cutting-edge technology field mentioned above, the higher the frequency stability of the hydrogen maser frequency standard (hereinafter simply referred to as hydrogen maser), the higher the measurement accuracy.
There is a strong demand for improved frequency stability.

Below, an overview of such a hydrogen maser and the problem of frequency stability will be explained.

Figure 1 schematically shows the main body of the hydrogen maser, where 1 is a discharge tube to which hydrogen molecules are supplied, 2 is a high-frequency oscillator for discharging to dissociate the hydrogen molecules in the discharge tube to form hydrogen atoms, 3 is a level selection magnet for selecting atoms with a high energy level from dissociated hydrogen atoms; 4 is a hydrogen storage sphere for storing injected hydrogen atoms; 5 is a cavity resonator; 6 1 is a resonator cylinder, 7 is a resonator support constant temperature bath equipped with a temperature control heater 7a on the outer periphery, 8 is a solenoid coil cylinder that provides a static magnetic field, 9 is a magnetic shield for blocking the influence of external magnetic fields, 10 is a It is a vacuum bell jar. Note that 11 is a post for finely adjusting the resonance frequency, 12 is a loop for taking out the oscillation output within the cavity resonator 5, and 13 is an ion pump.

In the hydrogen beam system of the hydrogen maser with such a structure, the level selection magnet 3 selects the energy levels of hydrogen atoms such as F=0, m _F =0, and F=1.
The hydrogen atom at m _F = -1 diverges, F = 1, m _F
=0, the atoms at m _F =+1 are focused in the hydrogen storage sphere 4.

The hydrogen atoms injected into the hydrogen storage sphere 4 repeatedly collide with the Teflon membrane wall on the inner surface of the hydrogen storage sphere 4, and drop to about 1
It remains in the hydrogen storage sphere 4 for nearly a second, and the cavity resonator 5
Excited by electromagnetic waves within.

The energy level used as the standard frequency is the frequency that transitions from the F = 2, m _F = 0 state to F = 1, m _F = 0, and is approximately 1420405752 Hz (standard frequency f ₀ ).
It is.

Now, if the resonant frequency fc of the cavity resonator 5 is adjusted to the vicinity of this standard frequency f ₀ , the atoms at F = 1 and m _F = 0 in the hydrogen storage sphere 4 will be in the cavity resonator 5. It is excited by an electromagnetic wave close to f ₀ and transitions to the lower energy level F = 0, m _F = 0.
The hydrogen maser causes self-oscillation at the maser oscillation frequency fm due to the electromagnetic waves it radiates at this time. The output of this maser oscillation frequency fm is taken out from the loop 12.

By the way, if the resonant frequency fc of the cavity resonator 5 and the standard frequency _f0 of hydrogen atoms are even slightly different,
The maser oscillation frequency fm is shifted from the standard frequency _f0 .

It is known that the frequency relationship of this shift is fm−f ₀ =Qc/Ql(fc−f ₀ ) (1).

Here, Qc is the Q value of the cavity resonator, and Ql is the Q value of the resonance line. (If the resonance spectrum frequency band is △f, it is expressed as Ql = fm / △f.) Now, the resonance frequency When fc fluctuates, the maser oscillation frequency f _n changes according to equation (1), and the frequency stability of the maser output is impaired.

For example, when Qc/Ql=5×10 ^{5 ,} in order for the variation △fm/f ₀ of the maser oscillation frequency fm to be about 1×10 ¹⁴ , it is necessary to control the variation of the resonant frequency fc to within about 0.3 Hz. be.

The resonance frequency fc of the cavity resonator 5 in which the hydrogen storage bulb 4 is installed is the resonance frequency of only the cavity resonator 5.
It is the sum of fc ₀ and the frequency shift f _B caused by the hydrogen storage sphere 4.

In other words, fc=fc+f _B ...(2) Also, the resonant frequency fc ₀ of the cavity resonator 5 is TE ₀₁₁
When the mode is a cylindrical resonator, it is derived from the following equation.

However, C: Speed of light D: Diameter of the resonator cylinder L: Height of the resonator cylinder The frequency shift f _B due to the hydrogen storage sphere 4 is given by the following formula, assuming that the equivalent dielectric constant of the hydrogen storage sphere 4 is εr. .

f _B =-1/2fc ₀ (εr-1) ...(4) Now, assuming that the resonator and the hydrogen storage bulb are at the same temperature and their temperature fluctuations are the same, the temperature change rate of the resonant frequency fc is as follows. The formula becomes

∂fc／∂T＝∂fc ₀ /∂T＋∂f _B /∂T ……(5) However, ∂fc ₀ /∂T: Fluctuation of resonant frequency due to resonator temperature fluctuation when there is no hydrogen storage sphere (vacuum (middle) ∂f _B / ∂T: Change in resonance frequency shift due to temperature fluctuation of the hydrogen storage sphere. Also, assuming that the inner height of the cavity resonator 5 is approximately equal to the height L of the resonator cylinder 6, ∂fc ₀ /∂T is given by the following equation from equation (3).

∂fc ₀ /∂T=-(C/2) ² α/fc ₀ (5.95/D ² +1/
L ² )...(6) However, α: Temperature expansion coefficient of the resonator cylinder Also, ∂f _B /∂T is given by the following equation from equation (4).

∂f _B /∂T=-1/2fc ₀ ∂εr/∂T ...(7) Now, the temperature fluctuation of the resonator is △Tc (t), and the temperature fluctuation of the hydrogen storage sphere is △T _B (t). When resonance frequency fc
The fluctuation of △fc _T is given by the following formula.

△fc _T = ∂fc ₀ / ∂T△Tc (t) + ∂f/∂T△T _B t = - (C/2) ² α/fc (5.95/D ² +1/L ² ) △Tc (
t) -1/2fc ₀ ∂εr／∂T△T(t) ...(8) However, △Tc: Temperature fluctuation of the resonator cylinder △T: Temperature fluctuation of the hydrogen storage sphere By the way, now regarding the resonator cylinder 6, When made of aluminum or copper, the resonance frequency is low due to its large coefficient of thermal expansion α.
The temperature fluctuation of fc ∂fc ₀ /∂T is more than -30KHz/℃, and in order to obtain frequency stability of 1×10 ^-14 , it is necessary to control the temperature to 1×10 ⁵ ℃ or less, which is extremely difficult to achieve. .

Therefore, the resonator cylinder 6 is made of quartz or crystal glass, which has a small coefficient of thermal expansion α, and is made of silver paste coated or vapor-deposited on the inner surface.

However, even in this case, the temperature fluctuation ∂fc ₀ /∂T of the resonance frequency remains approximately -1 KHz/℃, and the frequency shift fluctuation due to the temperature fluctuation of the hydrogen storage bulb 4 ∂f _B /
The influence of ∂T also remains.

This invention was made to solve this problem by setting the resonance frequency so that the variation in the resonant frequency caused by the temperature change of the cavity resonator cancels out the variation in the frequency shift caused by the temperature change of the hydrogen storage bulb. Therefore, it is an object of the present invention to provide a resonator for a hydrogen maser in which temperature fluctuations in resonance frequency are effectively suppressed.

For this reason, the cavity resonator system in which the hydrogen storage sphere is installed has a thermally integrated structure, and the resonator cylinder is made of crystal glass with a negative coefficient of thermal expansion.

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 2 is a cross-sectional view showing an outline of the hydrogen maser resonator of the present invention, and 6 indicates a resonator cylinder made of crystal glass with a low coefficient of thermal expansion. Silver paste is applied to the inner and outer surfaces of the resonator cylinder 6 to make it conductive and to be a good thermal conductor. Reference numerals 15 and 16 are aluminum end plates formed to cover the top and bottom of the resonator cylinder 6, and the lower end plate 16 includes a post 11 for fine tuning the resonant frequency and a post 11 for outputting electromagnetic waves of the resonator. The loop 12 is arranged in such a manner that the loop 12 of Also,
A donut-shaped disc spring 17 is placed on the upper end plate 15, and several screws 18 are used to press and fix the resonator cylinder 6 and the upper end plate 15 to the lower end plate 16. Then, the disc spring 17 is urged downward.

Reference numerals 19 and 20 refer to ring members made of an elastic material that constitute thermal connection means between the silver thin film layer formed on the outer surface of the resonator cylinder 6 and the end plates 15 and 16, and 7 refer to a resonator support thermostat (housing). ), 4 is a quartz hydrogen storage bulb whose inner surface is treated with Teflon.

Since the hydrogen maser resonator of the present invention is constructed as described above, the end plates 15 and 16 and the resonator cylinder 6 are first thermally integrated by the ring members 19 and 20, and therefore the resonator The temperature difference between the cylinder 6 and the hydrogen storage bulb 4 will not be large.

The temperature change in the resonant frequency fc in such a resonator system is expressed as ΔTc(t)ΔT _B (t) in equation (8). Since the hydrogen storage sphere 4 is made of quartz, the second term in equation (8) is ∂εr/∂T>0, and according to actual measurements, the value of ∂f _B /∂T is −500 to −1000Hz/ It is ℃.

On the other hand, if the temperature expansion coefficient α of the resonator cylinder 6 forming the cavity resonator 5 is a negative value, then
Since ∂fc ₀ /∂T is a positive value from the equation, the first and second terms of equation (8) vary so as to cancel each other out with respect to temperature fluctuations.

Therefore, in the hydrogen maser resonator of the present invention, by appropriately selecting the value of the temperature expansion coefficient α of the resonator cylinder 6, the resonant frequency fc can be made extremely stable against temperature changes, and temperature control is possible. By controlling the temperature of the system at 1×10 ³ °C, the frequency fluctuation of the resonator system can be set to approximately the required value (0.3 Hz).

Therefore, temperature control becomes easy and servo/
Since the gain is lower, aging drift in the temperature control system can also be ignored.

Incidentally, coarse adjustment of the resonant frequency is performed by removing the inner surface of the end plate 15, and fine adjustment of the resonant frequency is performed by the post 11 held down by a spring or the like.

As explained above, in the resonator system for a hydrogen maser of the present invention, a cavity resonator is formed by arranging end plates at the upper and lower openings of a cylinder having a negative temperature coefficient. It has a structure in which a hydrogen storage sphere whose resonant frequency decreases as the temperature rises is placed inside the resonator, and when the temperature fluctuates, the resonance frequency shift due to the cavity resonator and the frequency shift caused by the hydrogen storage sphere cancel each other out. do.

Therefore, when used as a resonator for a hydrogen maser, the accuracy of temperature control can be lowered, and the stability of the hydrogen maser oscillation frequency can be improved accordingly.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram for explaining the basic principle of a hydrogen maser, and FIG. 2 is a side (cross-sectional) view of a resonator for a hydrogen maser showing an embodiment of the present invention. In the figure, 4 is a hydrogen storage sphere, 6 is a resonator cylinder, 1
Reference numerals 5 and 16 indicate first and second end plates, and reference numerals 19 and 20 indicate elastic rings forming thermal connection means.

Claims (1)

[Claims] 1 A cylinder whose outer peripheral surface is coated with a good thermal conductor and has a negative coefficient of thermal expansion forming a resonant cavity whose resonant frequency increases as the temperature rises; first and second end plates forming a cavity resonator; fixed to the first end plate, housed in the resonant cavity separated from the inner surface of the cylinder, and as the temperature rises, the A hydrogen storage sphere made of a material that reduces the resonance frequency of the cavity resonator and the first and second end plates are thermally bridged via the thermally conductive material to form the cavity resonator. and a thermal connection means for thermally integrating the hydrogen maser, and the hydrogen maser is configured to substantially cancel out the amount of change in resonance frequency caused by the hydrogen storage sphere and the cylinder due to a change in temperature. resonator.