Effect of tim e

Lifetime tests to examine the effect of the extraction of a large amount of charge from a detector were carried out on the QM, with new MCPs installed in it. The MCPs were first given the same treatment as those designated for flight, namely a vacuum bake-out to 250° for 48 hours, before being built straightaway into the detector and kept under vacuum. According to several authors, baking the MCPs has the same effect on the initial desorption process as the first part of a bum-in scrub, starting to move down the gain curve towards a stable regime, as well as reducing the background event rate.

A lifetime of 2 years for the detectors could mean an accumulation of 8xlQlO events for the most-used detector. It was necessary for this to be simulated in the laboratory, but not just as a flat field case since in flight most of the events will be taken from small areas of the detectors where the lines fall. For this reason, it was decided to thoroughly test the QM with flat fielding and in addition to cover one half of the MCP front face with a slit mask. This allowed the effects of small area illumination to be investigated as well as giving a control area where no photons would reach the detector, except for the occasional gain measurement.

The experiment was performed in the CHASE vacuum tank, with the mercury Penray lamp just under 1 m from the detector, providing a reasonably flat field. A diaphragm in front of the lamp was used to control the flux rate. One major difference between this system and flight is the count rate, which has to be higher to enable the life test to be performed within a sensible time frame. Also, the wavelength is longer than in CDS, at 253.7 nm.

An HV protection network was added (by Thomas) to the QM power supply to disable the supply if the power was cut. It would also trip the HV if the current exceeded a pre-set value. These mechanisms prevented damage to the MCPs during the frequent failures of the vacuum system, allowing the test to be operated continuously.

4.7.1. Expected accumulation

To calculate the number of events which would need to be extracted, assumptions had to be made as to the form of the flight operation of the detectors.

From Harrison (RAL, private communication, 1995) there will be 1-3 GIS studies each day, each lasting several hours. It can be assumed that the HV is switched on for -10 hours a day and that for -10% of the time the studies will involve the active sun.

Within those studies, -70 % of the time will be spent with slit 1, -2 0 % with slit 2 and -10% with slit 3. Slit 3 will never be used to view active sun regions.

From Table 9, detector 2 will receive the most photons, most of these being from H en at 30.4 nm. Inserting those count rates into the above schedule gives an accumulation of 1.15x10^ photons per day. Given the intended gain of 4x10^ electrons photon'^ this amounts to an extraction o f 736 pC per day. Over the two year period, 536 mC should be extracted from the whole detector. If, in the test, only half the detector is being illuminated, the equivalent per unit area, i.e. 0.67 mCmm~2, is achieved after extracting 268 mC. This was the target for the flat fielding of one half of the detector. At a rate o f 40,000 c s'^ this could be accumulated in 290 hours, although, as the gain falls below the maximum, the Coulomb accumulation rate reduces, which would also be the case during flight. The total charge extracted in a particular period was calculated from the PHD and mean count rate.

100 80 V) <u c 60 *0 0 XI 1 40 z 20 50 40 30 10 20 0

Bin number (bin 50 = 10 c / s )

Figure 77. Expected distribution of spectral line count rates

over all four detectors. A histogram of expected count rates listed by Harrison and Fludra(1995).

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Disregarding the He H line at 30.4 nm, the next brightest line is the Fe XV line at 28.4 nm with a quiet sun, slit 1 count rate of 4 4 c s “ ^. Assuming the same ratios of slit and quiet/active sun operation as above, the mean count rate over the 10 hours of operation per day is 572 c s“ i. Over two years this line would extract 96 mC of charge.

The slit in the mask had an area of 100 pm x 16 mm; typical dimensions for a spectral line. The mean count rate of the FeXV line, 5 7 2 c s“ l, is already a very high count rate for this small area, at 0.07cpore~l. Significant gain depression occurs at count rates higher than this. For this reason, it was decided to perform the slit mask part of the life test at a rate no higher than this. Although the test could potentially be run for 24 hours a day, rather than the typical 10 hours/day in flight, to simulate 2 years would still take 300 days. However, this is a bright line. Figure 77 shows the distribution of expected count rates. Ninety-five percent of the lines will have count rates of less than 0.66 cs"^ with the smallest slit. Extracting just 1.4 mC (0.87mCmm~2) is therefore sufficient to illustrate the Coulomb loss from most of the lines. The aim was to extract as much charge as possible at 500 c s~i.

4.7.2. Results of slit lifetest

The MCPs used in these tests were from the same batch as those mounted in Nehemiah. They likewise showed a high (800-1400 cs"^) background count rate at operating voltages, with events coming from the short edges of the MCPs. This, together with the count rate of 5 0 0 cs“ l, and the low gain from the long wavelength mercury lamp UV, meant that the PHD was very wide (-200% FWHM).

The background count rate dropped gradually while the HV was switched on. It could be reduced by increasing the HV by a few hundred volts above the operating voltage for a few hours. For instance, turning up the HV from 4.06 kV to 4.2 kV increased the background rate from lOOOcs"^ to 8700c s"^. After it was left on overnight with no UV source, the rate fell to 2400c s"^, dropping to 2 0 0 cs“ i when the voltage was returned to 4.06 kV. However, whenever the voltage was turned off for a few hours (for instance during a power cut), the background count rate recovered completely, even though the gain of the plates did not recover. The most likely explanation for the cause of the background is field emission from the glass damaged by cutting at the edges of the plates. Once the voltage is switched on, the damaged areas charge up in such a way as to inhibit field emission, discharging again slowly when the voltage is switched off.

Loss o f gain with C o u lo m b s e x tr a c te d 4 . 0 x 1 0 4 . 1 6 kV 4 . 0 8 kV 4 . 0 kV 4 . 1 2 kV o 3 . 0 x 1 0 4 . 0 ' CL o 2 . 0 x 1 0 cn 1 . 5 x 1 0 1 . 0 x 1 0 5 0 0 1000

Charge extracted (uC)

1500

Figure 78. Single slit M CP lifetest.

As the charge was extracted it became increasingly difficult to identify the photon peak in the PHD from the background. After the extraction of 1.4 mC the vacuum pump repeatedly tripped out because o f the high temperature of the cooling water in the hot weather. The test was therefore abandoned at this point. The amount of charge extracted covers the expected extraction of m ost of the spectral lines (see above).

Figure 78 gives the results. W henever the gain became too low to recognise the peak from the background events, the HV was raised. The values of HV are marked on the graph. There is a gradual decrease in the rate of gain loss: at the end the gradient is 69% of the initial gradient. This suggests that eventually the fall in gain would become small.

4.7.3. Results of flat field lifetest

Problems with high background count rate were not so severe when illuminating half the detector because the event count rate could be maintained at a high enough rate (-4 0 ,0 0 0 cs~^) to swamp the background. To obtain the desired flux level and a flat field it was necessary to put neutral density filters in the beam, as well adjust the entrance diaphragm aperture. The mercury lamp was mounted on an x-y-z manipulator

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to achieve precise alignment through the filters and entrance aperture, giving a good flat field on the detector.

gain (e le c t r o n s /p h o t o n ) ro ro OJ OJ cn b cn b cn b X X X X X X o o o o o o ' v l 'O 'O o cn o o o O a cn o O ' CL K) o o cn o

This test is on-going. The results so far are given in Figure 79. The goal of 268 mC has been reached, equivalent to the 2 year life. The rate of decline has reduced to a sixth of its original level, but nearly the full range of HV has been used.

Comparing the charge extracted per unit area, the flat field lifetest to 268 mC extracts only sufficient charge to be equivalent to 0.5 c s” l in a spectral line; 75% of the slit lifetest. In the case of the slit lifetest though, the HV is still below 4.2 kV at the end of the test, compared with 4.85 kV in the case of the flat field, both giving similar modal gains. This suggests that, as in the case of CDGD, LTGD is also affected by adjacency.