Pressure-measurement errors in a cold-cathode-ionization gauge caused by

electrons and photoelectrons

Hiroshi Saeki a), Tamotsu Magome a), Tsuyoshi Aoki a), Satoshi Hashimoto b),Yoshihiko Shoji b), Ainoseke An

do b),and Takashi Momose c)

a) Japan Synchrotron Radiation Research Instituteb) LASTI, University of Hyogo

c) Miyagi National College of Technology

Ⅰ. INTRODUCTION

It is well-known that an ionization gauge mis-reads due to an influx of photoelectron flowing from an external environment.

In the SPring-8 storage ring, some of hot-cathode-ionization gauges have indicated abnormally-low pressures (on the order of 10-9 Pa) or negative pressures (from -2 x 10-9 Pa to -2 x 10-7 Pa) at stored-electron-beam conditions (the electron-beam energy of 8 GeV and the maximum stored-electron-beam current of 100 mA) due to the same cause, although elbows are set in front of these gauge heads.

On the other hand, cold-cathode-ionization gauges (Inverted magnetron gauges) at RF cavities in the ring have seemed to indicate actual pressures.

Therefore, a simulated experiment for pressure-measurement errors in a cold-cathode-ionization gauge caused by electrons from an external-electron source, was carried out.

Ⅱ. EXPERIMENTAL SETUP

1. Test gauge:

(1) The gauge head (Pfeiffer Vacuum IKR070) is the same type used at the RF cavities in the SPring-8 storage ring.

(2) The gauge head was operated with an ionization-gauge controller (Pfeiffer Vacuum TPG300).

Partial cross section of the four-way cross with an external-electron source used to produce an environment of excess electrons in front of a cold-cathode-ionization-gauge head.

2. External-electron source:

(1) The external-electron source consists of a tungsten filament and a grid.(2) The filament and the grid of the external-electron source were each connected with a separate dc power supply. (3) The grid voltage of the external-electron source was varied from 1 V to 90 V using the dc power supply.

Total-electron-beam current in front of the gauge head as a function of the filament current when the grid voltage of the external-electron source was +4 V. At the filament current of 3.5 A, the maximum electron-beam current was about 1.6 × 10-8 A and the average electron-beam density was about 1.6 × 10-9 A/cm2.

Total-electron-beam current as a function of the grid voltage at the filament current of 3.5 A.

3. Pressure reference:

(1) To obtain a pressure reference uninfluenced by electrons from the external-electron source, a B-A gauge (Granville-Phillips 370 Stable-ion gauge: 370 SIG) located on a three-way tube was used.(2) In preliminary experiments for the effect on the 370 SIG by the external-electron source in the pressure range from 10-4 Pa to 10-8 Pa, it was confirmed that electrons from the external-electron source (filament currents of 0 to 3.5 A and grid voltages from +1 V to +90 V) were not detected at the collector of the 370 SIG (+180 V applied to the grid of the gauge) without emission from the gauge filament.

Ⅲ. SIMULATED EXPERIMENT & RESULT

Before the simulated experiment, the filament of the external-electron source in the four-way cross was degassed for 24 hours using the other dc power supply (3.5 A maximum output) so as not to increase pressures greatly during the experiment. After a pressure of 2.0 × 10-7 Pa, as measured with the 370 SIG was obtained by adjusting the angle valve (AV) without operation of the external-electron source, the experiment was begun.

(1) The indicated pressures with the TPG300 and the 370 SIG were recorded as a function of the filament current of the external-electron source. The grid voltage of the external-electron source was +4 V.

The indicated pressure with the 370 SIG slowly increased with the corresponding increase in the filament current due to the outgas from the filament of the external-electron source.

The indicated pressure with the TPG300 suddenly started to increase as the filament current rose above 1.8 A. At a filament current of 2.3 A, the indicated pressures with the 370 SIG and the TPG300 were 2.4 × 10-7 Pa and 1.1 × 10-6 Pa, respectively.

At a filament current of 3.5 A, the indicated pressure with the TPG300 was higher by about two orders of magnitude from the actual pressure with the 370 SIG.

Indicated pressures with the 370 SIG and the TPG300 as a function of the filament current of the external-electron source. The grid voltage of the external-electron source was +4 V.

(2) As a reference, at the same pressure as the simulated experiment, the current detected at the anode of the IKR070 without the applied voltage of +3.2 kV, was measured, as functions of the filament current and the grid voltage of the external-electron source. When +180 V was applied to the grid of the 370 SIG without emission from the gauge filament, the current detected at the collector was also measured to see any effect on the 370 SIG by the external-electron source.

Detected currents at the anode of the IKR070 and the collector of the 370SIG at the same pressure, as functions of the filament current and the grid voltage of the external-electron source (at a filament current of 3.5 A).

It was found that the IKR070 was easily influenced by electrons from the external-electron source. From these results, it was reconfirmed that the 370 SIG was not influenced by electrons from the external-electron source.

Ⅳ. DISCUSSION

It is supposed that more electrons from the external-electron source flowed into the anode when +3.2 kV was applied to the anode of the IKR070 in operation with the TPG300.

After getting an important information of the current-pressure relationship in the cold-cathode gauge, the mechanism to cause the error in the cold-cathode-ionization gauge will be investigated in near future.

ADDITIONAL EXPERIMENTS AND RESULTS

(1) When +3.2 kV was applied to the anode of the IKR070, detected currents were measured using an optical fiber isolation system.

(2) To test some ionization gauges, the same environment of excess photoelectrons in the SPring-8 storage ring except for radiation, was roughly reproduced in the New SUBARU electron storage ring. And the IKR070 with a Gauge tube was tested.

As the result, it was found that a gauge tube with metal meshes set in front of the IKR070 was useful to prevent photoelectrons flowing into the anode.But it is difficult to prevent photoelectrons flowing into the gauge heads

perfectly.

(3) Furthermore, the cold-cathode-ionization gauge (IKR070) without the gauge tube will be tested to confirm a practical influence of photoelectrons in the New SUBARU electron storage ring.

(4) To measure pressure precisely in accelerators, we should develop new vacuum gauges different from ionization vacuum gauges.

(5) Signals from a quadrupole mass analyzer in the SPring-8 storage ring.

Hot-cathode-ionization-gauge head with

a correcting electrode and a shield tubeMI gauge cables

Three-way tube

Gauge tube

SR

Chamber

Photon absorber

Ion pump

IKR070

NIG-2F

Four-way cross

Quadrupole mass analyzer at RF-C station in the SPring-8 storage ring

Peak currents of mass number 2, as a function of the stored-electron-beam current

Detected photoelectron current with TPC of the quadrupole mass analyzer