megawatt proton beam


Important facts for safely operating the PSI high intensity megawatt proton beam lines and neutron spallation source.

Printable pdf version of this page (from PSI Scientific and Technical Report 2003, Large Research Facilities, Volume VI, p45-48)

Abstract: A multilevel protection system developed by many specialists and in operation since decades has constantly been upgraded to the needs of the today's 590 MeV high-intensity proton beam lines (power > 1 megawatt), which is very demanding on the reliability of diagnostic elements and electronic equipment in order to avoid long and costly shut-downs caused by a damaged vacuum chamber in a highly radioactive environment.

high intensity megawatt proton beam lines The range of 590 MeV protons in steel is about 28 cm. Therefore, the PSI high intensity megawatt proton beam with its small diameter and with more than one megawatt of power acts like a welding torch when it hits the steel walls of a vacuum chamber. At the Targets M and E for example, the diameter of the megawatt proton beam is as small as 4 mm (sigma-projected = 1mm). For this case, the time to heat up steel to its melting point as a function of proton beam intensity is given in Fig. 1 (17 kB). For a 2 mA proton beam, the machine interlock system has to switch-off the megawatt proton beam in less than about 5 ms to avoid a damaged vacuum chamber or seal where the megawatt proton beam diameter is small. It should be pointed out here, that a hole in one of the vacuum chambers in the Target E region could cause a shutdown of up to one year duration. In order to protect the proton-channel vacuum-chamber system from being damaged by the megawatt proton beam, five different classes of devices or services are installed:

A simultaneous effectiveness of 2-3 of these functions at most locations along the megawatt proton beam line is desired for redundancy. [Triple safety is an old wisdom. Already the wise king Salomon knew 3000 years ago: A cord of three strands is not quickly broken. (Ecclesiastes 4,12; The Holy Bible, New International Version)]

1. Watchdog for magnet currents remaining inside limits.
high intensity megawatt proton accelerator facility The actual values of the currents of all bending-magnets along the 590 MeV high intensity megawatt proton beam line between the ring extraction and the SINQ-target are permanently monitored by their local COMBI-controllers, when a magnet's value exceeds it's individually programmed upper or lower limit, then the proton beam is switched off via the machine interlock system. If this function would be absent, then the proton beam could easily drill a hole into the vacuum chamber of this magnet, because the produced spill by a wrongly steered proton beam is usually well shielded by the iron yoke and therefore the spill's intensity may be too weak to be monitored as dangerous by the nearby ionisation chamber. The widths of the windows created by the upper and lower limits for each magnet have to be wide enough to allow set-up and tuning sessions with a certain variability of the proton beam energy and direction at the accelerator exit.

Table 1:
Lower and upper limits
for the back-readings of
the actual values of the
p-channel bending-magnets.
Bend's name Lower lim. Upper lim.
AHA 8.40 V 9.99 V
AHB 7.17 V 8.54 V
AHC 7.90 V 8.20 V
AHD1 8.30 V 8.70 V
AHD2 8.80 V 9.40 V
AHL -8.56 V -7.75 V
AHM 7.94 V 8.94 V
AHN 7.30 V 8.32 V
AHO 7.90 V 8.90 V

The back-readings (measured in volts) of the lower and upper limits of the 9 bending-magnet's currents vary from magnet to magnet and are shown in table 1. Each COMBI-controller is also permanently comparing (with a hardware-comparator) the magnet's set-point value with its actual value and generating an interlock if it is incorrect.

2. Monitoring the proton beam losses with ionisation chambers.
megawatt proton beam The backbone of the high intensity megawatt proton beam line's protection system is an array of 29 ionisation chambers [IC] lined up, at an average distance of about 4 meters, along the proton beam line and close to the beam tube. They are arranged in 4 interlock groups:

from: to: # of ICs
Accelerator exit Target M 9
Target M Target E 6
Target E Beam-dump 5
AHL-bend SINQ-target 9

As an example
Fig. 2 (12 kB) shows the operator console's display screen of the 9 ionisation chamber readings for the proton beam line to SINQ (for the position of MHIxx along the proton beam line see Fig. 3 {43 kB}) for a proton beam intensity of 1.8 mA (power > 1 megawatt) extracted from the high intensity ring cyclotron.
high intensity megawatt proton accelerator facility The ionisation chambers exist in three shapes (square box, cylinder or ring) and have an active volume of about one litre each. The applied voltage over the plates inside is 200 volts and the filling gas is normal air. They are manufactured at PSI and consist of metal and ceramic insulator material only.
Urs Rohrer There are two basically different programs residing in the ionisation chamber electronics (LOGCAM2 CAMAC unit developed at PSI) to detect measured values exceeding limits and therefore triggering a machine interlock in as short a time as 5 ms:
  • Program A: see Fig. 4A (11 kB)
    As soon as a measured IC current exceeds a hardware (HW) limit, a machine interlock signal is produced by the electronics. At low proton beam intensities this limit may be to far away to detect a miss-steered proton beam and to turn off the beam before a vacuum leak is created. Experience showed, that already with proton beam intensities as low as 10 µA a leak at a vacuum seal may occur after overheating it for only a few seconds.
  • Program B: see Fig. 4B (11 kB)
    Because of the scattering of protons passing through target material, the proton beam spill measured with ionisation chambers down-stream from the target is proportional to the proton beam intensity as long as the proton beam remains on axis. Therefore, the quotient of the measured IC currents (I) divided by the actual proton beam current (I0) remains constant over the whole range of beam intensities above 50-100 µA ( see also Fig. 5 {14 kB}). This fact is exploited by the programmed electronics in comparing this ratio with individually set upper and lower limits and triggering an interlock in case of out-of-limit values. Additionally, the hardware limit mentioned above is also supported by this program.
megawatt proton beam At the moment, only the LOCAM2-units of the proton beam line sections behind Target E are equipped with program B. It is planned to investigate, if also the section between Target M and E may be equipped with program B. (high intensity megawatt proton accelerator facility implemented in SD 2005). This would improve the usefulness of the ionisation chambers at low intensities, which is quite important for protecting the vacuum chambers of Target E and just in front of it with more redundancy (see also Fig. 1 and 10) .

3. Monitoring the proton beam halo at collimators and slits.
megawatt proton beam Behind Target E and in front of the SINQ-target several beam-halo monitors are installed. They consist of 2 or 4 segments of thin sheet-metal (0.1 mm Nickel) mounted with insulators in front of the slits or collimators. If protons are hitting the copper of the slits or collimators, then while they pass through one of the segments and with an efficiency of about 5 % they produce a current flowing to the measuring device (LOGCAM2). These currents are processed with program B described for the ionisation chambers. Additionally, a too high left-right or up-down asymmetry also produces an interlock. At the operator's console the actual currents collected at the different segments of the 8 halo monitors may be displayed with a repetition rate of 1 Hz
(see Fig. 6 {11 kB}). This display is an important tuning tool for optimizing the passage of the megawatt proton beam through the Target station E and for adjusting the proton beam centring at the SINQ-target.

4. Monitoring the proton beam transmission at critical locations.
Urs Rohrer There are 4 location along the 590 MeV high intensity megawatt proton beam line, where the proton beam current transmissions are monitored:

location: expression to evaluate:
HE-beam splitter (EHT) I(MHC1) - I(MHC2) - I(MBC1)
Thin target station (TM) I(MHC3) - I(MHC4) - Losses @TM
Thick target station (TE) I(MHC4) - I(MHC5) - Losses @TE
Drift-tube (leading to SINQ) I(MHC5) - I(MHC6)

The proton beam currents are measured with the monitors (50 MHZ high-frequency cavities) MHC1 to MHC6 and MBC1 (measures the proton beam current of the peeled-off protons for PIREX), which have only an accuracy of around 1 % and have also to be re-calibrated (done manually by operators, which is considered by experts as a main security risk) from time to time. The four transmissions are computed with local processors with a repetition rate of 200 Hz. Because of the limited accuracy of the proton beam intensity measurements the allowed losses are dependent on the proton beam currents. At Target E (TE) e.g. the permitted width of the window is ±5µA near 0 µA and ±90µA at 2 mA proton beam current
(see Fig. 7 {7 kB}). In order to reduce the amount of spurious interlocks produced by proton beam current fluctuations, a proton beam current dependent time constant for the integration of the measured current values is applied. This time constant varies between 110 ms for 0 µA and 10 ms for proton beam currents larger than 1.5 mA (see Fig. 8 {8 kB}). The losses at TM are 1.6 % over the whole range of proton beam currents, whereas for the Target E (length = 4 cm graphite) the losses are 28 % + 1.3 % per mA proton beam current (i.e. 30% at 1.8 mA).
megawatt proton beam The most important motivation for introducing the transmission monitoring at Target E was to prevent the possibility of too much proton beam bypassing the graphite target material (rotating wheel of 6 mm width). This non-scattered proton beam passes through the high intensity proton beam line to SINQ with a higher momentum and generates a considerably smaller spot at the SINQ-target, which may be harmful for it (e.g. liquid metal target for the MEGAPIE experiment). Fig. 9 {10 kB} shows a Monte-Carlo simulation (with the program TURTLE) of the 2 proton beam spots (scattered and non-scattered proton beam) at the location of the SINQ-target.
high intensity proton accelerator facilityIn order to have more redundancy for protecting the MEGAPIE-target from being hit by too much of the narrow non-scattered proton beam, an additional method has been presented in the PSI Large Research Facilities Scientific and Technical Report 2001, Volume VI, p.34-35. The proposed slit will be put in place during the shutdown 2004 and its usefulness tested during the HE-beam period 2004. Urs Rohrer In order to see first results please click here or have a look at the article First beam tests with the new slit collimator in the high intensity proton beam line to SINQ of the PSI Large Research Facilities Scientific and Technical Report 2004, Volume VI, p.23-26.

5. Controlling correlated magnets in front of Target E with a super-knob device.
high intensity proton accelerator facility The magnetic fringe field of the (backwards) extraction magnet AHSW41 of the
PiE5 secondary beam line (starting at Target E) is also deflecting the megawatt proton beam in front of the Target E. In order to compensate this effect, 2 additional bending magnets named AHU and AHV are required ( see Fig. 10 {21 kB}). In this figure, the 2 red curves show the central trajectories of the megawatt proton beam (coming from the left) for 120 MeV/c µ± beams being extracted into the PiE5 beam line. In order to hit the Target E at the centre and keeping the megawatt proton beam parallel to the axis, the current settings for the 2 compensation magnets AHU and AHV have to be chosen for the different AHSW-currents (proportional to the momentum of the extracted muons with 3.529 A/MeV/c) from the curves shown in Fig. 11 (7 kB). To avoid wrong settings of these 3 magnets, which could lead very easily to a hole in one of the nearby vacuum chambers, (see Fig. 10 and assume e.g. the current of the magnet AHV has the wrong sign) their values cannot be set directly. Instead a super-combi device is used, which gets the muon-momentum (in MeV/c) as input AHINP and transforms it with the help of a lookup table (corresponding to Fig. 11) into magnet set-point values and then performs the magnet settings. Two additional super-combi input parameters allow it similarly
Urs Rohrerwith AHPOS to shift the proton beam horizontally at Target E parallel to the axis within the range of ±5 mm and/or
megawatt proton beamwith AHWIN to vary the horizontal direction of the megawatt proton beam at Target E within the range of ±5 mr.
Thus, as long as this super-combi device works properly, there is no chance, that the proton beam can hit a wall of a vacuum chamber near Target E. Between Target M and Target E there are also 2 horizontally and 2 vertically acting steering magnets with a maximum deviation power of ±5 mr (see Fig. 12 {27 kB}). Comparing the drawn (in red) maximum possible (5 mr) proton beam centroid shifts reachable by these steering magnets, it is also obvious, that no vacuum chamber walls (see Fig. 10) can be hit with this 'worst-case' megawatt proton beam at regions where its spot size is small.
high intensity proton accelerator facility In the near future it is planned to add a second identical (with the exception of disabled set-point channels) super-combi device to the existing one. This shadowing feature would certainly increase the safety, because if one of the 2 units is malfunctioning, the other would still be able e.g. to check the back-readings of the 3 magnets and generate an interlock in case of a magnet power-supply drop-out. (Urs Rohrer implemented in SD 2004).

Urs Rohrermegawatt proton beam Last updated by Urs Rohrer on 20-Feb-2006