Status of dark current measurement at DC spark system at CERN – update 13
May 2014
MURANAKA Tomoko
The main goal of this note is to complement a part of the report “Measurements of Field Emission
from OFHC Cu samples – update 04/14”written by Yinon Ashkenazy. Measurements presented in
this note have been conducted mainly by Iaroslava Profatilova with a help from Mike Barnes in Q1
2014 at CERN DC spark system-II unless otherwise noted.
1. Experiment setup
Cathode: Cu, SLAC etched and H2 heated at 1040 °C, 12mm diameter disc.
Anode: Cu, diamond-turned, 2.3mm diameter hemispherical head.
Gap distance: 10 μm
In order to measure rapid current fluctuations through the gap in higher resolution, the
measurement circuit outside the vacuum chamber had been modified from the usual field emission
measurement setup. As shown in figure1, the current was measured by an oscilloscope instead of a
Keithley ammeter and the instrument was protected from overcurrent flow by connecting a spark
gap in parallel. The time resolution of the scope was set to 10us/div (50psec) for main
measurements.
Figure 1: Left: Schematic of the dark current measurement system (by courtesy of IP). Right: Photo of the spark gap and
the resistors mounted in a metal box.
2. Saw structure
To come right to the point, the origin of saw structures shown in dark current measurements was
generated by the spark gap in the circuit. The spark gap is bridged above 90 V and it occurs when the
current through the system exceeds about 2.2 µA in the present circuit shown in figure1. Several
series of measurements of saw structures had been done to study their behaviour. In a series of
measurements shown in figure2, the saw structure first appeared at 3.2 kV where current was
around 2.25 µA. Then at higher bias voltages, higher saw structure frequencies were observed. The
fluctuation comes from the spark gap is non-linear, thus one cannot extract them from saved data
sets of dark current measurement, unfortunately.
Figure 2: Captured images of saw structures. The bias voltages were 3.1 kV, 3.2 kV, 3.4 kV and 3.5 kV from top left to
bottom right, respectively. (Comment by TM: Considering values of bias voltages, the gap distance was probably 20um.)
3. Plans for next weeks
An idea proposed by Mike is to modify the spark gap connection as shown in figure3. With this setup
the current up to 4 µA will be able to be measured without the saw structure generated by the spark
gap. And the instruments and experimenters will still be protected enough. On top of that, one has
to study spark gap behaviour at lower voltages more precisely. Tomoko plans to check the current
goes through the spark gap by connecting a Keithley picoammeter.
Figure 3: Schematic of the new circuit proposed by MB