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The production of a condensed hydrogen jet target in vacuo
the specimen chamber. The rate of cooling and the required specimen temperature depend on the rate of pumping the helium vapour, controlled by valve 23.
Results of tests It was shown that the cryostat achieves cooling and temperature control of the specimen over the whole temperature range to an accuracy of + 0.1 K. The lowest specimen temperature attained was 5 K. At the minimum specimen temperature the liquid helium consumption is 120 cm3h"1 ; less liquid is required to obtain higher temperatures. There is no temperature gradient along the surface of the specimen.
The specimen could he rotated and aligned freely at all temperatures. The cryostat weight was less than 7 kg. The reliability of operation of the cryostat was checked by studying the lattice parameters of pure copper and comparing the results with those in the literature. The authors are grateful to V. I. Grushko, N. I. Kuznetsov, and A. I. Tostov for help in building and testing the cryostat.
References 1 Silvera, J. F. Rev Scilnstr 41 (1970) 1513
The production of a condensed hydrogen jet target in vacuo V. D. Bartenev, A. I. Valevich, Yu. K. Pilipenko, and V. V. Smelyanskii A gas jet target established in the internal beam of an accelerator has been used 1-3 in studies of proton-proton and proton-deuteron scattering. The present work is a further development of the jet target methods and is aimed at an improvement in jet parameters a reduction in jet width, an increase in its density, a reduction in the amount of gas let into the vacuum. This can be achieved by changing from gaseous to liquid or solid targets.4,5 We shall describe the results of our investigations on the production of jet targets of condensed gas in the form of a flow of droplets of metastable liquid or of solid particles (it is obvious that after flowing out of the nozzle, the liquid will boil, giving rise to gaseous and solid phases). To obtain a jet, that quantity of gaseous hydrogen which was let into the accelerator chamber during one cycle was cooled, condensed, and if required, frozen. The amount of gaseous H 2 was measured out into the target by an electromagnetic valve working at room temperature. The amount of hydrogen stored in the hydrogen circuit of the target is then minimal. This operating scheme is technologically simple, but requires careful temperature control of the pulsed cooling of the hydrogen jet. The jet must work at a temperature a little above the freezing temperature of hydrogen, 13.95 K. If the hydrogen freezes, the jet and part of the feed line can be warmed up in a few seconds since their mass is small. The arrangement of the experimental target is shown in Fig.1. Hydrogen is fed to heat exchanger 7 at a pressure of 1 to 20 atm by the electromagnetic valve 5, and is cooled by a stream of helium evaporating from a cryopump, afterwards entering a supercooler with built-in heater 8. The supercooler is placed immediately in front of the jet or short capillary of 0.3 to 0.5 mm diameter. The hydrogen is condensed or frozen in the supercooler, depending on its temperature. The authors are with United Institute of Nuclear Studies, Dubna, USSR. Prib i Tekh Eksper No 3 (1972) 28. Received 18 August 1972.
CRYOGENICS. APRIL 1973
We studied two modes of operation. In the first the incoming hydrogen is frozen in the supercooler and capillary, blocking its outlet orifice, acting as a cold shut-offvalve. Then at the correct moment the supercooler heater is pulsed and the molten hydrogen shoots into vacuum chamber 1 in the form of fine droplets or liquid and pieces of ice. The vacuum in the experimental chamber was maintained at 10-5 torr (1 torr = 133 N m "2) between cycles. Practically no deterioration in the vacuum was observed at the moment the hydrogen was frozen in the capillary. An additional supercooler, used as a shut-off valve, was placed on the hydrogen capillary to eliminate the effect of its gaseous volume up to the supercooler at the moment the molten hydrogen is forced out. The supercoolers 'opened' and 'closed' well with the aid of heaters. The jet was visually the same as in the former case, that is the additional supercooler had no appreciable effect. The accuracy of the time when the jet appeared (according to the deterioration of the vacuum) was not worse than 0.1 s. 60 cm 3 per cycle of gaseous hydrogen was used with the valve open at the moment of freezing in the main supercooler for At = 200 ms. The jet had an angular divergence of "~60°. Under some conditions a rod of solid hydrogen, 10 to 20 mm long, came out at the end of the discharge from the capillary, and this broke off at the start of the following discharge. In the second mode of operation the temperature of the supercooler was maintained by a heater permanently switched on so that the hydrogen did not freeze in it. A rod of solid hydrogen, 0.5 mm in diameter and 50 to 80 mm long, could be (with great instability) obtained at a hydrogen pressure of 12 atm. For an over-aU high supercooler temperature the jet appeared 60 to 80 ms after the hydrogen valve was opened and looked like a moist vapour. This was evidently drops of metastable liquid or solid particles formed from them. The end of the jet was determined by the moment the valve shut. As before, the divergence of the jet was 60 ° and was practically independent of how the discharge was carded out: with 0.3 mm diameter, 10 mm long
239
H2
Collimators of various diameters and lengths were studied. In the final experiments collimators with inner diameter 5 to 6 mm and length 3 0 - 4 0 mm were used. The divergence of the jet at the outlet of the collimator was reduced to 5 to 10 ° (Fig.2). It was not possible to achieve collimation of a gas jet. In the best case it is possible to cut out the peripheral layer of the jet, but this results in a sharp reduction in the fraction of the gas profitably used.
He
I0
The stability of operation of the target depends to a large extent on the accuracy in maintaining the temperature conditions. The problem is complicated by the non-stationary nature of the hydrogen cooling process: the hydrogen is fed in pulses. L i q u i d He
The heat exchanger for cooling the hydrogen was made in the form of two copper tubes, soldered together over a length of 420 mm, the helium tube being 5 mm od, 3 mm
J
i
2~
T2
Ti
I
rll
"~
/
i I i....~ /
111',1-I I I
tll
I
I
.I
I
I
Fig.1
Plan o f the apparatus; 1 -- vacuum chamber, 2 -- liquid He vessel, 3 -- lower c r y o p u m p trap, 4 -- upper cryopump, 5 -- gaseous H~I supply valve, 6 - hydrogen capillary, 7 -- heat exchanger, 8 - supercooler w i t h heater, 9 -- collimator, 10 -- valve f o r evaporating He exit, 11 -- diffusion vacuum pump, T 1 to T 4 -- platinum resistance thermometers
injection needles or by expansion nozzles with neck diameter 0.35 and 1.5 mm at the outlet. It is possible that the advantage of a nozzle could be determined by the background properties of the target, brought about by the deterioration of the vacuum in the accelerator chamber. Collimation
of the jet and maintenance
temperature
conditions
of the
A collimator (9, Fig.l) was placed immediately beyond the nozzle to collect the jet in the weakly diverging beam.
240
F ig.2 Hydrogen jet in the vacuum chamber (Phi- = 6 atm, duration of valve opening &t = 150 ms, photographing rat~ 30 frames s'l). The lamp shows the duration of the electrical pulse to open the valve the observed w i d t h of the jet is 8 ram; the existence of the jet in the last frame is explained by the mechanical delay in the shutting o f the valve (25 ms)
CRYOGENICS
. APRIL
1973
id, the hydrogen capillary being 1.6 mm od and 0.8 mm id. Low heat capacity platinum resistance thermometers (T 1 and T4) are wound on the tube and capillary, and sealed with epoxy resin to achieve good heat contact with the walls of the tube in vacuo. The heat capacity of the heat exchanger is an important factor in its operation. We were unable to maintain the required temperature conditions of the heat exchanger by controlling the amount of cooling helium and the hydrogen often froze in the capillary. It was proposed to produce a pulsed supply of cold helium in the heat exchanger by the electro-magnetic valve 10, synchronized with the hydrogen supply. At the moment the valve closes the heat exchanger starts to warm up appreciably due to the external heat flux. After the supply of cooling helium it starts to cool again. Fig.3 shows a sequence of valve operations. One can observe how the jet becomes more visible as the delay At 3 is increased, and then disappears because the hydrogen freezes. In some of our experiments we obtained a jet under selftriggering conditions. At the moment when the temperature of the heat exchanger was low, hydrogen froze in it (pulse 2b). At the following warming of the heat exchanger an easily visible jet (3b) appeared. In this case the jet generally seemed denser than when it was formed at the moment the hydrogen was fed in (3a). At the end of the hydrogen discharge the temperature of the tube at the output of the heat exchanger rose to 14 to 16 K.
Ip 17o
nb
I
II
I
~r3 I
0
I
I
I
I
I
I
i
I
I
I
I
2
3
4
5
6
7 r,s
I
2
3
I 4
I 5
Fig.3 System of supplying pulses to the gaseous H 2 and evaporating He valves; 1 -- evaporating He valve, 2 -- gaseous H 2 valve, 3 -- vacuum in the chamber, a conditions when jet is obtained at the moment the gaseous H 2 is supplied, b (dashed) jet obtained in self-triggering conditions
2.4 x !O 3 2 x IO-3 -
iO -3
_
IO
-
CL
iO -4
Vacu u m
1 . 3 x l O -s
The extent to which the vacuum deteriorates at the moment of discharge and the rate of its restoration have a major effect on the operation of the target and the accelerator. For an adiabatic expansion of the gas the heat content is expended on kinetic energy of translational motion of the jet and on the formation of vapour. Calculation shows that solidification and cooling of the hydrogen to helium temperatures is mainly at the expense of evaporating part of the liquid. For a liquid hydrogen temperature of 14 K before the nozzle this amount is 12 to 15%. If solid hydrogen at 13.95 K is expelled from the nozzle the amount of vapour is reduced to 3%. According to calculations, in our case the amount of hydrogen of the jet which evaporates as a result of interaction with the charged particle beam must be < 1%. A typical oscilloscope trace of the change in vacuum in the experimental chamber at the moment of the jet is shown in Fig.4.
Observation
I
of the jet and
A P R I L 1973
I
2
4
3
Z:,s Fig.4 The change in pressure at the moment of the jet (P~ = 3.5 •" 3 atm, duration of valve opening At = 150 ms, H 2 expenditure230 cm cycle "1 ). The pressure was measured with an MM--13M--4 gauge, calibrated in air
Some parameters of a gas jet and of a jet of condensed gas are shown in the table for comparison. Duration of H 2 supply, ms
Quantity of H 2 let o u t cm 3 per cycle
W i d t h of jet at a distance of 30 m m f r o m the nozzle, mm
Amount o f matter, g cm "3
Gas jet
75-100
80-100
30-50
10-7
Jet of condensed gas
150
30-50
8-13
10-6"
its parameters
The jet was photographed by a 'Konvas' cirre-camera with a speed of 32 frames s"1 (Fig.2). It was observed that a visible jet appears a short time alter the hydrogen is supplied. Dinappearance of the jet takes place simultaneously with the shutting of the valve. The jet is not visible at the moment when the heat exchanger is coldest. It was established that this time is reduced when the hydrogen is increased. The same picture was obtained in photographs of the second and third jets let out 0.5 to 0.7 s after the first. This behaviour is evidently connected with the formation of a condensed or even solid layer on the outer surface of the capillary on which an appreciable fraction of the hydrogen let out at first is spent.
CRYOGENICS.
I 0
* T h e a m o u n t o f m a t t e r in t h e p a t h o f t h e b e a m is c a l c u l a t e d t a k i n g T c = 14 K ' a n d t h e speed of the condensed jet as 1 8 0 m s-1
Conclusions
1. By cooling hydrogen pulse fed to the heat exchanger, a jet of condensed gas can be obtained in vacuo. In a number
241
of cases rods of solid hydrogen 0.5 mm in diameter and 50 to 80 mm long could be produced. 2. A collimator with id 5 to 6 mm and length 3 0 - 4 0 mm placed beyond the nozzle, collects the condensed jet into a beam with small angular divergence. The visible width of the jet is 8 to 13 mm at a distance of 30 mm from the collimator, which is four times smaller than the width of a gas jet. 3. A provisional density for the condensed jet is ,~10"6 g cm"~. For this the amount of gas inlet is reduced by a factor of 2 to 3. The authors are grateful to A. G. Zel'dovich and V. A. Nikitin for valuable comments during discussion of the work, to
A. A. Dernin, G. G. Khorev, and I. D. Rylov for help in constructing the apparatus and to N. I. Balandikov and the personnel of the liquefier for a steady supply of liquid helium.
References 1 Bartenev, V. D., Belushkina, A. A., Zhidkov, N. K., Zofin, L. S., et al Proc All-UnionConf on Charged Particle Accelerators
Moscow 1968 (VINITI 1970) p 534 2 Battenev,V. D., Valevich, A. I., Belushkina, A. A., Zhidkov, N. K., et al Proc Int Conf on Apparatus for High Energy Physics 1970 (D-5805 1971) Vol 1, 16 3 Zolin, L. S., Nikitin, V. A., Pilipenko, Yu. K. Cryogenics8 (1968) 143 4 Tolstoy, K. D. OIYaI Preprint 1964 (1968) Dubna 5 Tolstoy, K. D. OIYaI Preprint 1-4103 (1968) Dubna
Observation of small non linearities in the superconducting transitions of thin films N. A. Pankratov, G. A. Zaitsev, and I. A. Khrebtov Method of measuring and apparatus Measurement of the first derivative enables different features of the characteristics studied to be shown up more sharply. 1 If the superconducting film is used as the sensitive element of a bolometer or as a temperature sensor, then its reaction to the application of a varying temperature signal is proportional to 3R/aT. The method of measuring is based on this principle. The method discussed has another feature, connected with the possibility of establishing a non-isothermal state of the superconducting film. 2,3 The film must be heated under the action of excess current or constant irradiation for a non-isothermal state to arise. The centre of the film is then in the normal state and the ends in the superconducting state. For a periodic change in temperature of the ends of the film, the boundary of the normal region moves, accompanied by a change in the resistance of the film. If the boundary of the normal region coincides with a region of inhomogeneity of the film, a sharp change in specimen resistance can be expected. In this way a fdm inhomogeneity probe is produced. Experiments were carried out with a tin film which was evaporated in a vacuum of 5 x 10-5 torr onto a 1 to 3/a thick mica substrate. The films were 1 x 10 mm 2 and of thickness •"1 000 fit, with a resistance in the normal state o f ' l . 1 ~2 and resistance ratio R300 K/R4. 2 K -~ 20, with a transition width "- 0.04 to 0.05 K and heat conductance 6 x l0 "7 to 4 x 10.6 W K"1. The ends of the substrate were stuck with B F - 4 to a massive brass base. Tin contacts were also vacuum deposited. Two heaters with resistance 30 Q were wound on the base (Fig.l). The mounted film specimen was placed in the vacuum space of a helium cryostat. A radiation source consisting of a GaAs photodiode was fixed to the helium shield.
specimen. The bridge was only balanced with the resistive component of a decade resistance box. The temperature of the film specimen was modulated at a frequency of l0 Hz by heater I. The out of balance signal from the output of the bridge passed through a matching cooled transformer and was fed to a low noise pre-amplifier and narrow band carder frequency amplifier (Af = 50 Hz). The sensitivity of the amplifying system was 2.2 x 10 "11 VHz'V2for specimens with resistance 1 ~.4 After phase sensitive detection, the 10 Hz signal was amplified by a selective amplifier (Af= 2 Hz) and was measured with a valve voltmeter. An oscilloscope was connected to the output of the carrier frequency amplifier for observing the out of balance of the bridge. Signals proportional to the temperature oscillations of the cryostat also came from the output of the phase sensitive detector. These signals with the corresponding phase were fed to the input of a constant current amplifier, the load of
Specimen~ \
HeaterII /
Heater I
.//./_//_Resistancebox
/
LlI 8oo oscillator
I 800Hz HPhase amplifier Ilsensitive l i I d ector I amplifier
I Volve voltmeter
The block diagram of the measuring apparatus is shown in Fig.1. The specimen was connected into an ac bridge. A 0.1 ~2 bridge arm was mounted in the cryostat next to the Prib i Tekh Eksper No 3 (1972) 256.
242
Received18 August 1972.
l
Fig.1 Blockdiagramof measuringsystem
CRYOGENICS. APRI L 1973
Results of tests It was shown that the cryostat achieves cooling and temperature control of the specimen over the whole temperature range to an accuracy of + 0.1 K. The lowest specimen temperature attained was 5 K. At the minimum specimen temperature the liquid helium consumption is 120 cm3h"1 ; less liquid is required to obtain higher temperatures. There is no temperature gradient along the surface of the specimen.
The specimen could he rotated and aligned freely at all temperatures. The cryostat weight was less than 7 kg. The reliability of operation of the cryostat was checked by studying the lattice parameters of pure copper and comparing the results with those in the literature. The authors are grateful to V. I. Grushko, N. I. Kuznetsov, and A. I. Tostov for help in building and testing the cryostat.
References 1 Silvera, J. F. Rev Scilnstr 41 (1970) 1513
The production of a condensed hydrogen jet target in vacuo V. D. Bartenev, A. I. Valevich, Yu. K. Pilipenko, and V. V. Smelyanskii A gas jet target established in the internal beam of an accelerator has been used 1-3 in studies of proton-proton and proton-deuteron scattering. The present work is a further development of the jet target methods and is aimed at an improvement in jet parameters a reduction in jet width, an increase in its density, a reduction in the amount of gas let into the vacuum. This can be achieved by changing from gaseous to liquid or solid targets.4,5 We shall describe the results of our investigations on the production of jet targets of condensed gas in the form of a flow of droplets of metastable liquid or of solid particles (it is obvious that after flowing out of the nozzle, the liquid will boil, giving rise to gaseous and solid phases). To obtain a jet, that quantity of gaseous hydrogen which was let into the accelerator chamber during one cycle was cooled, condensed, and if required, frozen. The amount of gaseous H 2 was measured out into the target by an electromagnetic valve working at room temperature. The amount of hydrogen stored in the hydrogen circuit of the target is then minimal. This operating scheme is technologically simple, but requires careful temperature control of the pulsed cooling of the hydrogen jet. The jet must work at a temperature a little above the freezing temperature of hydrogen, 13.95 K. If the hydrogen freezes, the jet and part of the feed line can be warmed up in a few seconds since their mass is small. The arrangement of the experimental target is shown in Fig.1. Hydrogen is fed to heat exchanger 7 at a pressure of 1 to 20 atm by the electromagnetic valve 5, and is cooled by a stream of helium evaporating from a cryopump, afterwards entering a supercooler with built-in heater 8. The supercooler is placed immediately in front of the jet or short capillary of 0.3 to 0.5 mm diameter. The hydrogen is condensed or frozen in the supercooler, depending on its temperature. The authors are with United Institute of Nuclear Studies, Dubna, USSR. Prib i Tekh Eksper No 3 (1972) 28. Received 18 August 1972.
CRYOGENICS. APRIL 1973
We studied two modes of operation. In the first the incoming hydrogen is frozen in the supercooler and capillary, blocking its outlet orifice, acting as a cold shut-offvalve. Then at the correct moment the supercooler heater is pulsed and the molten hydrogen shoots into vacuum chamber 1 in the form of fine droplets or liquid and pieces of ice. The vacuum in the experimental chamber was maintained at 10-5 torr (1 torr = 133 N m "2) between cycles. Practically no deterioration in the vacuum was observed at the moment the hydrogen was frozen in the capillary. An additional supercooler, used as a shut-off valve, was placed on the hydrogen capillary to eliminate the effect of its gaseous volume up to the supercooler at the moment the molten hydrogen is forced out. The supercoolers 'opened' and 'closed' well with the aid of heaters. The jet was visually the same as in the former case, that is the additional supercooler had no appreciable effect. The accuracy of the time when the jet appeared (according to the deterioration of the vacuum) was not worse than 0.1 s. 60 cm 3 per cycle of gaseous hydrogen was used with the valve open at the moment of freezing in the main supercooler for At = 200 ms. The jet had an angular divergence of "~60°. Under some conditions a rod of solid hydrogen, 10 to 20 mm long, came out at the end of the discharge from the capillary, and this broke off at the start of the following discharge. In the second mode of operation the temperature of the supercooler was maintained by a heater permanently switched on so that the hydrogen did not freeze in it. A rod of solid hydrogen, 0.5 mm in diameter and 50 to 80 mm long, could be (with great instability) obtained at a hydrogen pressure of 12 atm. For an over-aU high supercooler temperature the jet appeared 60 to 80 ms after the hydrogen valve was opened and looked like a moist vapour. This was evidently drops of metastable liquid or solid particles formed from them. The end of the jet was determined by the moment the valve shut. As before, the divergence of the jet was 60 ° and was practically independent of how the discharge was carded out: with 0.3 mm diameter, 10 mm long
239
H2
Collimators of various diameters and lengths were studied. In the final experiments collimators with inner diameter 5 to 6 mm and length 3 0 - 4 0 mm were used. The divergence of the jet at the outlet of the collimator was reduced to 5 to 10 ° (Fig.2). It was not possible to achieve collimation of a gas jet. In the best case it is possible to cut out the peripheral layer of the jet, but this results in a sharp reduction in the fraction of the gas profitably used.
He
I0
The stability of operation of the target depends to a large extent on the accuracy in maintaining the temperature conditions. The problem is complicated by the non-stationary nature of the hydrogen cooling process: the hydrogen is fed in pulses. L i q u i d He
The heat exchanger for cooling the hydrogen was made in the form of two copper tubes, soldered together over a length of 420 mm, the helium tube being 5 mm od, 3 mm
J
i
2~
T2
Ti
I
rll
"~
/
i I i....~ /
111',1-I I I
tll
I
I
.I
I
I
Fig.1
Plan o f the apparatus; 1 -- vacuum chamber, 2 -- liquid He vessel, 3 -- lower c r y o p u m p trap, 4 -- upper cryopump, 5 -- gaseous H~I supply valve, 6 - hydrogen capillary, 7 -- heat exchanger, 8 - supercooler w i t h heater, 9 -- collimator, 10 -- valve f o r evaporating He exit, 11 -- diffusion vacuum pump, T 1 to T 4 -- platinum resistance thermometers
injection needles or by expansion nozzles with neck diameter 0.35 and 1.5 mm at the outlet. It is possible that the advantage of a nozzle could be determined by the background properties of the target, brought about by the deterioration of the vacuum in the accelerator chamber. Collimation
of the jet and maintenance
temperature
conditions
of the
A collimator (9, Fig.l) was placed immediately beyond the nozzle to collect the jet in the weakly diverging beam.
240
F ig.2 Hydrogen jet in the vacuum chamber (Phi- = 6 atm, duration of valve opening &t = 150 ms, photographing rat~ 30 frames s'l). The lamp shows the duration of the electrical pulse to open the valve the observed w i d t h of the jet is 8 ram; the existence of the jet in the last frame is explained by the mechanical delay in the shutting o f the valve (25 ms)
CRYOGENICS
. APRIL
1973
id, the hydrogen capillary being 1.6 mm od and 0.8 mm id. Low heat capacity platinum resistance thermometers (T 1 and T4) are wound on the tube and capillary, and sealed with epoxy resin to achieve good heat contact with the walls of the tube in vacuo. The heat capacity of the heat exchanger is an important factor in its operation. We were unable to maintain the required temperature conditions of the heat exchanger by controlling the amount of cooling helium and the hydrogen often froze in the capillary. It was proposed to produce a pulsed supply of cold helium in the heat exchanger by the electro-magnetic valve 10, synchronized with the hydrogen supply. At the moment the valve closes the heat exchanger starts to warm up appreciably due to the external heat flux. After the supply of cooling helium it starts to cool again. Fig.3 shows a sequence of valve operations. One can observe how the jet becomes more visible as the delay At 3 is increased, and then disappears because the hydrogen freezes. In some of our experiments we obtained a jet under selftriggering conditions. At the moment when the temperature of the heat exchanger was low, hydrogen froze in it (pulse 2b). At the following warming of the heat exchanger an easily visible jet (3b) appeared. In this case the jet generally seemed denser than when it was formed at the moment the hydrogen was fed in (3a). At the end of the hydrogen discharge the temperature of the tube at the output of the heat exchanger rose to 14 to 16 K.
Ip 17o
nb
I
II
I
~r3 I
0
I
I
I
I
I
I
i
I
I
I
I
2
3
4
5
6
7 r,s
I
2
3
I 4
I 5
Fig.3 System of supplying pulses to the gaseous H 2 and evaporating He valves; 1 -- evaporating He valve, 2 -- gaseous H 2 valve, 3 -- vacuum in the chamber, a conditions when jet is obtained at the moment the gaseous H 2 is supplied, b (dashed) jet obtained in self-triggering conditions
2.4 x !O 3 2 x IO-3 -
iO -3
_
IO
-
CL
iO -4
Vacu u m
1 . 3 x l O -s
The extent to which the vacuum deteriorates at the moment of discharge and the rate of its restoration have a major effect on the operation of the target and the accelerator. For an adiabatic expansion of the gas the heat content is expended on kinetic energy of translational motion of the jet and on the formation of vapour. Calculation shows that solidification and cooling of the hydrogen to helium temperatures is mainly at the expense of evaporating part of the liquid. For a liquid hydrogen temperature of 14 K before the nozzle this amount is 12 to 15%. If solid hydrogen at 13.95 K is expelled from the nozzle the amount of vapour is reduced to 3%. According to calculations, in our case the amount of hydrogen of the jet which evaporates as a result of interaction with the charged particle beam must be < 1%. A typical oscilloscope trace of the change in vacuum in the experimental chamber at the moment of the jet is shown in Fig.4.
Observation
I
of the jet and
A P R I L 1973
I
2
4
3
Z:,s Fig.4 The change in pressure at the moment of the jet (P~ = 3.5 •" 3 atm, duration of valve opening At = 150 ms, H 2 expenditure230 cm cycle "1 ). The pressure was measured with an MM--13M--4 gauge, calibrated in air
Some parameters of a gas jet and of a jet of condensed gas are shown in the table for comparison. Duration of H 2 supply, ms
Quantity of H 2 let o u t cm 3 per cycle
W i d t h of jet at a distance of 30 m m f r o m the nozzle, mm
Amount o f matter, g cm "3
Gas jet
75-100
80-100
30-50
10-7
Jet of condensed gas
150
30-50
8-13
10-6"
its parameters
The jet was photographed by a 'Konvas' cirre-camera with a speed of 32 frames s"1 (Fig.2). It was observed that a visible jet appears a short time alter the hydrogen is supplied. Dinappearance of the jet takes place simultaneously with the shutting of the valve. The jet is not visible at the moment when the heat exchanger is coldest. It was established that this time is reduced when the hydrogen is increased. The same picture was obtained in photographs of the second and third jets let out 0.5 to 0.7 s after the first. This behaviour is evidently connected with the formation of a condensed or even solid layer on the outer surface of the capillary on which an appreciable fraction of the hydrogen let out at first is spent.
CRYOGENICS.
I 0
* T h e a m o u n t o f m a t t e r in t h e p a t h o f t h e b e a m is c a l c u l a t e d t a k i n g T c = 14 K ' a n d t h e speed of the condensed jet as 1 8 0 m s-1
Conclusions
1. By cooling hydrogen pulse fed to the heat exchanger, a jet of condensed gas can be obtained in vacuo. In a number
241
of cases rods of solid hydrogen 0.5 mm in diameter and 50 to 80 mm long could be produced. 2. A collimator with id 5 to 6 mm and length 3 0 - 4 0 mm placed beyond the nozzle, collects the condensed jet into a beam with small angular divergence. The visible width of the jet is 8 to 13 mm at a distance of 30 mm from the collimator, which is four times smaller than the width of a gas jet. 3. A provisional density for the condensed jet is ,~10"6 g cm"~. For this the amount of gas inlet is reduced by a factor of 2 to 3. The authors are grateful to A. G. Zel'dovich and V. A. Nikitin for valuable comments during discussion of the work, to
A. A. Dernin, G. G. Khorev, and I. D. Rylov for help in constructing the apparatus and to N. I. Balandikov and the personnel of the liquefier for a steady supply of liquid helium.
References 1 Bartenev, V. D., Belushkina, A. A., Zhidkov, N. K., Zofin, L. S., et al Proc All-UnionConf on Charged Particle Accelerators
Moscow 1968 (VINITI 1970) p 534 2 Battenev,V. D., Valevich, A. I., Belushkina, A. A., Zhidkov, N. K., et al Proc Int Conf on Apparatus for High Energy Physics 1970 (D-5805 1971) Vol 1, 16 3 Zolin, L. S., Nikitin, V. A., Pilipenko, Yu. K. Cryogenics8 (1968) 143 4 Tolstoy, K. D. OIYaI Preprint 1964 (1968) Dubna 5 Tolstoy, K. D. OIYaI Preprint 1-4103 (1968) Dubna
Observation of small non linearities in the superconducting transitions of thin films N. A. Pankratov, G. A. Zaitsev, and I. A. Khrebtov Method of measuring and apparatus Measurement of the first derivative enables different features of the characteristics studied to be shown up more sharply. 1 If the superconducting film is used as the sensitive element of a bolometer or as a temperature sensor, then its reaction to the application of a varying temperature signal is proportional to 3R/aT. The method of measuring is based on this principle. The method discussed has another feature, connected with the possibility of establishing a non-isothermal state of the superconducting film. 2,3 The film must be heated under the action of excess current or constant irradiation for a non-isothermal state to arise. The centre of the film is then in the normal state and the ends in the superconducting state. For a periodic change in temperature of the ends of the film, the boundary of the normal region moves, accompanied by a change in the resistance of the film. If the boundary of the normal region coincides with a region of inhomogeneity of the film, a sharp change in specimen resistance can be expected. In this way a fdm inhomogeneity probe is produced. Experiments were carried out with a tin film which was evaporated in a vacuum of 5 x 10-5 torr onto a 1 to 3/a thick mica substrate. The films were 1 x 10 mm 2 and of thickness •"1 000 fit, with a resistance in the normal state o f ' l . 1 ~2 and resistance ratio R300 K/R4. 2 K -~ 20, with a transition width "- 0.04 to 0.05 K and heat conductance 6 x l0 "7 to 4 x 10.6 W K"1. The ends of the substrate were stuck with B F - 4 to a massive brass base. Tin contacts were also vacuum deposited. Two heaters with resistance 30 Q were wound on the base (Fig.l). The mounted film specimen was placed in the vacuum space of a helium cryostat. A radiation source consisting of a GaAs photodiode was fixed to the helium shield.
specimen. The bridge was only balanced with the resistive component of a decade resistance box. The temperature of the film specimen was modulated at a frequency of l0 Hz by heater I. The out of balance signal from the output of the bridge passed through a matching cooled transformer and was fed to a low noise pre-amplifier and narrow band carder frequency amplifier (Af = 50 Hz). The sensitivity of the amplifying system was 2.2 x 10 "11 VHz'V2for specimens with resistance 1 ~.4 After phase sensitive detection, the 10 Hz signal was amplified by a selective amplifier (Af= 2 Hz) and was measured with a valve voltmeter. An oscilloscope was connected to the output of the carrier frequency amplifier for observing the out of balance of the bridge. Signals proportional to the temperature oscillations of the cryostat also came from the output of the phase sensitive detector. These signals with the corresponding phase were fed to the input of a constant current amplifier, the load of
Specimen~ \
HeaterII /
Heater I
.//./_//_Resistancebox
/
LlI 8oo oscillator
I 800Hz HPhase amplifier Ilsensitive l i I d ector I amplifier
I Volve voltmeter
The block diagram of the measuring apparatus is shown in Fig.1. The specimen was connected into an ac bridge. A 0.1 ~2 bridge arm was mounted in the cryostat next to the Prib i Tekh Eksper No 3 (1972) 256.
242
Received18 August 1972.
l
Fig.1 Blockdiagramof measuringsystem
CRYOGENICS. APRI L 1973