SL/EA Version 1.3

SHORT INTRODUCTION FOR THE USE OF THE P42+K12 BEAMS

In this note we provide a short and simplified overview of the nearly collinear and simultaneous K^o beams to the NA48 experiment and we give instructions for the standard operation of the beam line.

Table of Contents:

	1. General layout
	2. The control system
	3. The different modes of operation
	4. Checks of the hardware status
	5. Checks of the performance
	6. Change of KS and/or KL intensity 
	7. P0 Survey
	8. Access
	9. What to do in case of problems

1. General layout

The simultaneous and nearly collinear K^o beams to the NA48 experiment consist of two major sections:

The total length of the P42 part is approximately 813 metres. It consists of 11 bends, 21 groups of quadrupoles and 11 correction dipoles ("Trims"), as well as 8 adjustable collimators and 2 dump-collimators (TAX-7 and TAX-8). This part is usually optimised by the beam experts. The presence of beam on T4, as well as its intensity upstream and downstream of the target are indicated on the 'Page-1' TV-screen.

The K12 part of the beam extends from the KL target all the way to the KS target (120 m downstream) and the NA48 apparatus. It ends with some beam monitoring equipment at the downstream end of ECN3, about 254 m downstream of the KL target. Most of the changes to the beam line are made in the K12 part. This part is also the one most often changed by the experimental team. Its layout is schematically indicated in figure 1. A clickable optics map with pictures and some information about the beam elements is available on the Web, as well as a collection of drawings and graphs.

The KL neutral beam is produced by nominally 1.5 10¹² ppp impinging on the KL target and is defined by three stages of collimation, respectively the defining, cleaning and final collimator. The transverse position of the final collimator is fixed and the beam position at the experiment is thus defined by the KL target and the final collimator. The defining and cleaning collimators are equipped with X-Y movements and have been aligned on the neutral beam axis.

About half of the protons impinging on the KL target traverse the target (40 cm Be) without interacting. A wanted small fraction (~ 10^-4) of those are recuperated after sweeeping by a bent Silicon crystal. The attenuated proton beam of nominally 3 10⁷ ppp is then cleaned by two more dump collimators (TAX-17 and TAX-18) and transported to the KS target, positioned 72 mm above the nominal KL axis. The KS target is followed by a strong sweeping magnet and a collimator that defines a neutral beam of short-lived kaons that converges to the KL beam axis at the NA48 detector. The protons towards the KS target traverse the tagging counter which flags those K^o's that come from the KS target. The complicated region between the KL target and the tagging counter is shown in an expanded view in figure 1. Either or both of the neutral beams can be stopped by moving TAX-17 and TAX-18 into the appropriate positions.

2. The control system

The SPS secondary beams are controlled through HP/735 computers, running the Nodal system under HP-UX. The user accesses this system through X-terminals. Two dedicated X-terminals have been made available in the NA48 control room, one 14" monochrome NCD for the control of the P42 beam and a large HP colour terminal for the control of the K12 beam. They are marked with red stickers

You may navigate through the control tree by typing the name of the wanted option or at least enough characters of it to be unambiguous. If you get stuck, you may escape by typing Ctrl-C. From then you can start the tree again by typing

        RUN <225>TREE     for the K12 tree (big colour terminal),
        RUN <231>TREE     for the P42 tree (small B&W terminal),
        RUN TREE          may be good enough in most of the cases.

        RUN               to start where you were when you escaped,>
        BACK              to jump one level up in the tree.

If everything else fails, it may be a good idea to switch off the X-terminal and switch it on again.

With this control program you can control the behaviour of the beam by simply following the menu structure presented to you. In the following chapters we will describe to you the most important options offered to you by the control program.

With the exception of P0 Access control ,or unless indicated otherwise, all instructions are given from the K12 tree, i.e. on the big coulour terminal (index <225>).

If a graphics ("DICO") window has been created, you may have to click in the control window to make it again the active window.

3. The different modes of operation

The P42 beam is normally used only in its proton mode to transport a high-intensity primary proton beam from T4 to the KL target. However, the K12 beam can be operated in a number of different modes. Standard modes are the following:

• KS+KL Normal mode of operation for e'/e data-taking, simultaneous KS and KL beams converging to the experiment,
• K0S KS only, the KL beam is stopped by TAX-17,
• K0L KL only, the KS beam is stopped by TAX-17,
• PROTONS Protons from crystal go along the KL axis through NA48. This mode should normally not be used, as it may send a high-intensity proton beam (several 10⁷ ppp) through the chambers and the Kevlar window in case of magnet failures.
• MUONS The KL and KS beams are dumped, the muon sweeping is switched off. The result is a high-intensity spread of muons, covering the full NA48 detectors.

A standard programme has been provided to change between these various modes:

        RUN <225>TREE / SET-UP / MODE / The mode required.  

4. Checks of the hardware status

After change of mode, change of behaviour of the beam and anyway from time to time, it is important to check the state of the beam elements.

The command

        STATUS / CHECK

The TAX positions can be checked by typing

        SET-UP / TAX / P0-TAX        for TAX-7 and TAX-8
or
        SET-UP / TAX / K12-TAX       for TAX-17 and TAX-18.

Please note that TAX-7 will be closed automatically e.g. in case of a P42 magnet failure!

In case a magnet or collimator does not have the correct current or opening, this can be corrected by typing (in the appropriate tree):

        TUNE / SET / BEND / current wanted
        TUNE / SET / QUAD / current wanted
        TUNE / SET / TRIM / current wanted
or
        TUNE / SET / COLL / JAWS /  jaw positions wanted

5. Checks of the performance

Two programmes allow to easily verify the correct functioning of the beam:

A quick overview of the rates measured by several detectors in the experiment is obtained by the command

        TUNE / SPECIAL / SCALERS / nr of bursts required

The steering of the beam can be judged from the profiles in the beam wire chamber at the very end of the beam line. These profiles are obtained by typing

        TUNE / MEAS / MWPC / PROFILE / 1 (or 2) 

Sometimes the first profile is not good, because the high-voltage of the chamber is being ramped up. In case of doubt, repeat the profile. Make sure that there is beam for the burst you are looking at (see page-1). Compare the observed profiles with the ones in the logbook for the same beam mode.

6. Change of KS and/or KL intensity

The KL intensity is defined by a choice of different diameter holes in the TAX-7 and TAX-8 dump collimators in the P42 beam. Each of the two TAXs has a series of holes as listed in Table 1.

The KL intensity is approximately proportional to the square of the hole diameter, except for the biggest holes, where the beam no longer fully covers the hole. To a good approximation the KS intensity is independent of the TAX hole diameter, except for the very small holes.

The KS intensity can be varied, without affecting the KL flux, by changing the current in Trim-5 of the P42 beam. A typical plot of the KS intensity as a function of Trim-5 current is shown in figure 2.

However, it has been found that the time structure over the SPS spill (2.38 s.) of the KS relative to the KL is also affected by the setting of Trim-5. It is therefore recommended to use Trim-5 to adjust the KS/KL ratio (given e.g. by AKS/BCTR) to be as flat as possible over the spill - as observed on the SPILL MONITOR and shown typically in Figure 3. In practice this means choosing a setting of Trim-5 which is near the maximum of the scan (Figure 2).

In case this results in a KS/KL ratio different from that wanted (NOMINAL protons->KS / protons->KL = 2 10^-5), it can be changed in ~10% steps by loading a new, pre-tuned, P42 file, corresponding to a different horizontal magnification (m_H) of the proton beam onto the KL target. This should not affect the KL intensity, but results in a proton transmission via the bent crystal to the KS target which is proportional to m_H.

The current in Trim-5 can be changed by typing, in the P42 tree on the small B&W terminal:

        TUNE / SET / TRIM / 5 / current wanted

        TUNE / SCAN / TRIG / 1/ 10 / TRIM / 5 / I-low / I-high / Stepsize

        TUNE / SCAN / EXPT / n / 2 / TRIM / 5 / Ilow / Ihigh / Stepsize

In the future a feedback loop will be prepared to keep the KS/KL ratio constant over the burst by varying the Trim-5 current during the burst, depending on the instantaneous rates measured at the tagger (~ KS) and the beamcounter (~ KL).

7. P0 Survey

The primary proton beam has a sufficiently high intensity to do serious damage to the beam elements, in case it would be sent into them due to magnet failures. Therefore the currents in the bending magnets are continuously monitored by a special surveillance system. This so-called P0-Survey system is an electronics module that compares actual currents to reference values within a programmable tolerance. In case of deviations outside the tolerance, TAX-7 of the P42 beam is closed. This may also happen in case of radiation alarms, cooling problems of TAX or KL target or in case a vacuum valve would be closed.

The P0-Survey should always be switched on. This is controlled by the CRN operators and/or the SL-EA physicists. However, as Bends 4 and 5 are stopped in case of access to ECN3, the system should be switched off during access and switched on again as soon as beam conditions have been established again. This is done automatically by the access program.

8. Access

Access to ECN3 requires the beam to be switched off and a number of safety conditions to be satisfied (interlocks).

Access to ECN3 is made possible by running the following procedure, only from the P42 tree (small B&W terminal):

        ACCESS / P0 ACCESS / PREPARE

• Switch the SPS to P0-off mode,
• Move the dump collimators TAX-7 and TAX-8 to beam dump position,
• Disable the P0-Survey system,
• Switch off Bends 4 and 5.

Never enter ECN3 (nor any other beam zone) without carrying your film badge !

After the access has been terminated and all keys have been put back, the beam may be switched on again. First ask the PCR to enable chain 16 and then run the program

        ACCESS / P0 ACCESS / END OF ACCESS

This program will switch the SPS to P0-on mode, switch on the bends and the P0-Survey and finally open the TAXes. Please check the TAX positions typing

        TUNE / SPECIAL / TAX / P0

Access to the gallery PPE853 just behind the NA48 control room is only possible when ECN3 is in access condition. Therefore you have first to ask for access to ECN3 and then only for access to PPE853:

        ACCESS / P0 ACCESS / PREPARE 
        ACCESS / DOOR / 853 / OPEN /       your family name

        ACCESS / DOOR / 853 / BEAM ON /    your family name
        ACCESS / P0 ACCESS / END  

9. What to check on shift

A detailed shift checklist for NA48 is kept up-to-date on the Web.

10. What to do in case of problems

If there are problems with the beam or beam elements, which you cannot solve yourself, please call the CRN operators. These can be reached (24 hours per day) by one of the following means:

	Intercom	CRN
	Phone	75566
	GSM /Natel	16-0137

For beam tuning problems or questions, please contact one of the authors of this document:

Last updated : 23 March 1998 by Lau Gatignon