Pumping station with a titanium pump
Abstract Generated abstract
This paper describes the design and operation of a vacuum pumping station for pumping and conditioning sealed electrovacuum devices, including small betatron chambers, discharge tubes, and experimental diodes. The station combines conventional RVN-20 mechanical and TsVL-100 diffusion pumps with a titanium ion-sorption pump, and the article details its vacuum layout, seals, gauges, power supply, pump parameters, and operating procedure. Reported operation shows pump-down of two 1 liter glass chambers to \(5 \cdot 10^{-6}\) torr in 15 minutes and achievement of pressures around \(1\text{ to }2 \cdot 10^{-7}\) torr after several hours, with lower residual pressures possible for longer processing. Three years of use indicated reliable and efficient performance for small-volume devices in laboratory and industrial settings.
Full Text
Pumping Station with a Titanium Pump
V. P. Ponomarev
(Presented by the scientific seminar of the Vacuum Technology and Electronics Sector)
A description is given of the design of a vacuum station intended for the pumping and conditioning of sealed-off devices during their manufacture.
In addition to the usual pumping system, consisting of pumps of the TsVL-100 and RVN-20 types, the pumping station is equipped with a titanium ion-sorption pump. The ultimate vacuum provided by the vacuum system of the station and of the devices being pumped is \(1\cdot10^{-7}\)—\(7\cdot10^{-3}\) torr.
Introduction
The reliability and durability of most electrovacuum devices depend to a significant extent on the conditions provided during the final operations of their manufacture. These conditions include, first of all, the degree of rarefaction and its “purity” created by the vacuum system in the device during its pumping and conditioning [1].
The use of titanium pumps for pumping sealed-off devices of various purposes makes it possible to satisfy these conditions easily.
Under our conditions, the pumping station with a titanium pump is intended for the pumping and conditioning of sealed-off chambers of small-size betatrons, discharge tubes, experimental diodes, and other devices.
Design of the Station
The pumping station consists of two main units: the vacuum system and the electrical circuit.
The diagram of the vacuum system is shown in Fig. 1. The pumping system consists of a mechanical pump 1 of the RVN-20 type, a high-vacuum pump 2 of the TsVL-100 type, and a titanium pump 3.
The TsVL-100 pump is equipped with a “louver” type reflecting shield [3], cooled by water. Pipeline 5, through a bellows transition 6, is connected to valve 7 (Fig. 2), mounted on bell jar 8. A titanium pump is connected to the bell jar on one side, and on the other side a Kovar tube 9, ending in a glass stem 10, onto which the devices to be pumped are sealed.
Degassing of the bulbs of the devices being pumped is carried out by heating them in furnace 11 at a temperature \(T = 450\)—\(500^\circ\). Shield 12, made...
Fig. 1. Diagram of the vacuum system of the pumping station:
1 — mechanical pump RVN-20, 2 — diffusion pump TsVL-100, 3 — titanium pump, 4 — trap, 5 — pipeline, 6 — siphon, 7 — fore-vacuum valve, 8 — cap, 9 — quartz tube, 10 — glass comb, 11 — furnace, 12 — shield, 13 — LT-2 and LM-2 manometric lamps, 14 — MM-13 sensor
Fig. 2. Valve:
1 — valve body, 2 — fluoroplastic disk, 3 — valve, 4 — screw, 5 — stop bushing, 6 — flange, 7 — fluoroplastic bushing, 8 — handwheel, 9 — nut, 10 — washer, 11 — rubber gasket
made of copper sheet with a soldered tube for cooling, prevents heating of the pumping system.
The titanium ion-sorption pump 3, shown schematically in Fig. 3, is a modernized design of the pump described in [2].
Fig. 3. Schematic of the titanium pump:
1 — washer, 2 — fluoroplastic disk, 3 — valve, 4 — pressure sleeve, 5 — locking sleeve, 6 — Sylphon, 7 — screw, 8 — screen, 9 — cathode, 10 — anode (collector), 11 — pump body, 12 — molybdenum holder, 13 — tightening band, 14 — metal-ceramic feedthrough
The main parameters and characteristics of the pump are given in Table 1. The sealing of detachable joints in the vacuum system is carried out with fluoroplastic-4 gaskets, except for the sealing of the manometric lamps, where a combined rubber–fluoroplastic seal, shown in Fig. 4, is used.
To measure the vacuum produced by the pumping system and the vacuum interlock, VIT-1A and VMB-3 vacuum gauges are used with the corresponding sensors placed in various sections of the system.
The bridge electrical circuit supplies power to the titanium pump (Fig. 5), and also supplies the necessary voltage to the remaining devices: the RVN-20 and TsVL-100 pumps, the furnace, and the instruments being pumped out.
Table 1
| No. | Name of characteristic | Value |
|---|---|---|
| 1 | Air pumping speed at a pressure of \(1.10^{-5}\)—\(5.10^{-6}\) torr, l/s. | 40—50 |
| 2 | Ultimate vacuum produced by the pump, torr. | \(5,10^{-8}\) |
| 3 | Service life of the pump, determined by the reserve of titanium placed in the pump, h. | 500—600 |
| 4 | Current in the anode circuit, mA. | 120—150 |
| 5 | Voltage at the cathode, V. | 6—10 |
| 6 | Voltage at the anode, V. | 1100—1200 |
| 7 | Cathode current, A. | 11—12 |
| 8 | Anode dimensions, mm. | \(18 \times 16 \times 3\) |
| 9 | Anode—cathode distance, mm. | 10—11 |
| 10 | Material of the pump cathode. | Tungsten VA-3 Ø 0.4 |
| 11 | Material of the pump anode. | Titanium VT-1D |
| 12 | Cathode diameter and spiral pitch, mm. | 6.4—6 |
Fig. 4. Rubber–fluoroplastic seal for gauge lamps:
1 — lamp, 2 — nut, 3 — washer, 4 — rubber gasket, 5 — fluoroplastic bushing, 6 — housing.
Labels in Fig. 5 schematic: 0–500 mA; 0.3 A; rectifier \(U = 1200\) V, \(J = 0\text{–}150\,\mu\text{A}\); 0–20 A; filament transformer; titanium pump; RNO-250; 1 A; \(\sim 220\).
Fig. 5. Power-supply circuit of the titanium pump
Operation and Characteristics of the Station
The combination of a conventional pumping system, consisting of pumps of the RVN-20 and TsVL-100 types, with a titanium pump makes it possible to obtain high operating characteristics for the pumping station.
The attainment of pressures on the order of \(10^{-7}—10^{-8}\) torr in the instruments being pumped, during normal operation of the station, is ensured by the operation of the titanium pump.
The procedure for starting the titanium pump and obtaining a low pressure of residual gases in the instruments amounts to the following: the system, assembled and tested for leaks, is pumped down by a mechanical pump to a pressure of \(10^{-2}\) torr; after this the high-vacuum valve of the titanium pump is closed and the diffusion pump is switched on. At a pressure in the system of \(10^{-4}—10^{-5}\) torr, the valve of the titanium pump is opened, and half voltage is applied to the pump cathode in order to degas the electrode system.
Fig. 6. Curve of pressure decrease in the instruments being pumped
Degassing of the electrode system is usually carried out for \(0.5—1\) hour.
After degassing of the electrode system is completed, the cathode filament voltage of the pump is reduced to zero and the nominal voltage is applied to the titanium collector. The cathode filament is slowly increased and, while observing the current in the collector circuit and the pressure in the system, the pump is brought into its normal operating mode; in doing so, it is not recommended to raise the pressure in the system above \(3—4\cdot 10^{-4}\) torr, in order to avoid the occurrence of breakdown between the electrodes. At a pressure in the system of \(2—5\cdot 10^{-5}\) torr, the high-vacuum valve of the titanium pump is closed. This is the so-called preliminary start-up of the titanium pump, carried out after cleaning or repair. The total time for the preliminary start-up of the titanium pump is \(2—3\) hours.
The instruments to be pumped are soldered onto the branch pipes of the glass manifold and are pumped by the mechanical pump. During this time the titanium pump operates with the high-vacuum valve closed. After a vacuum of \(1\cdot 10^{-2}\) torr is obtained in the instrument, the preliminary-pumping valve 7 is closed, the mechanical pump and the power supply of the titanium pump are switched off. After this the high-vacuum valve is opened, and after \(1—2\) minutes the power supply of the titanium pump is switched on again.
The foregoing is illustrated by the curve of the decrease of pressure in the system with time, shown in Fig. 6. The curve has three characteristic sections:
I — the section of pressure decrease by the mechanical pump;
II — pressure decrease due to sorption of gases by the deposited titanium film;
III — pressure decrease in the system by the operating titanium pump.
In our case, the time required to pump down two glass chambers for small betatrons, each with a volume of 1 l, from atmospheric pressure to \(5 \cdot 10^{-6}\) torr is 15 min. After 6–7 hours of operation of the titanium pump, the pressure in the system decreases to \(1\text{–}2 \cdot 10^{-7}\) torr.
When instruments with increased requirements for the limiting vacuum are pumped down, the pumping time increases to 12–14 hours (continuously, or with breaks, 10–15 hours).
The residual gas pressure in the system is thereby reduced to \(7\text{–}10^{-8}\) torr. Operation of the pumping station with a titanium pump over a period of three years has shown that it possesses high reliability and efficiency and can be recommended for pumping down instruments of small volume both under laboratory and industrial conditions.
Literature
- Modern Vacuum Technology. IL, 1963.
- V. P. Ponomarev. PTE, 6, 143, 1963.
- B. S. Danilin. Design of Vacuum Systems. GEI, 1959.