Barth, W

Scheeler, U

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 199 ACCELERATOR OPERATION AT THE GSI HIGH CURRENT INJECTOR W. Barth and U. Scheeler  Gesellschaft für Schwerionenforschung mbH, Darmstadt, Germany Abstract The operation of the new high current injector (HSI) is characterized by up toa factor of hundred higher beam intensities and the enlarged possibilities ofthe time sharing operation between two ion source terminals feeding the HSIand the High Charge State Injector (HLI). The increased beam power of asingle Unilac pulse is now able to damage sensitive accelerator components.New operation hard- and software tools controlling the accelerator andmonitoring the device settings and beam properties had to be developed andintegrated in the already existing concept. Based on the beam parameters thedata supply of all devices with proofed initial settings and the check of deviceproperties are automatically done by software programs. The interlocksystem was advanced by the survey of transmission losses and it also reducesthe beam power automatically while beam diagnostics is measuring. Thepaper deals with the new requirements, the ideas of solving them and the firstoperation experiences. 1. INTRODUCTION GSI is the heavy ion research center in Germany operating with the linear accelerator Unilac (universallinear accelerator), the synchrotron SIS (Schwerionensynchrotron) as a circular accelerator and astorage ring ESR (experimental storage ring) on the fields of nuclear and atomic physics, plasmaphysics, material research and biophysics. The SIS was designed as a high intensity machine with alarge magnet aperture. In contrast, the original Unilac was not dedicated as a synchrotron injector,fulfilling all requirements due to high intensities (especially for a mass number higher than 150). Toserve the SIS up to the inherent space charge limit for all ion species including uranium a new HighCurrent Injector was installed and commissioned step by step in 1999 replacing the more than 25 yearsold Wideröe section of the GSI Unilac. [1] Fig. 1:  Example of Three Beam Operation with four users (December 2000)ECR20 msPIG20 msC     1 Hz / 0.3 ms1000 ms40     1+Ar     1 Hz / 2.0 msC      50 Hz / 5.0 ms12    2+12   6+12   2+MUCIS40     2+40    11+Ar      1 Hz / 0.3 msC     5 Hz / 5.0 ms40     11+Ca      43 Hz / 5.3 msMixed Mode PoststripperCa     50 Hz / 6.0 msSIS 200 With the new injector the operation possibilities are considerably enlarged. Pulse to pulseavailability of every ion source is now possible. Two ion source terminals feed the HSI: One is housinga Penning Ion Gauge (PIG) already in operation at the former Wideröe injector (for all kinds of ions atmedium and low intensities) and the other one serves for the production of high intensity, heavy ion,low charge state beams. Optionally the Multi Cusp Ion Source (MUCIS) for gases or the MEtal VaporVacuum Arc ion source (MEVVA) for metals is installable. Afterwards the ion beams are accelerated upto 1.4 MeV/u in the new HSI, before injection into the Alvarez the modified gas stripper sectionprovides for the essential charge state. The other 1.4 MeV/u injector, the HLI (Hochladungsinjektor)fed by an Electron Cyclotron Resonance (ECR) ion source at low intensity, high charge states and highduty cycle allows Alvarez injection directly without stripping. After the upgrade of the Unilac a multi-pulse mode from the different injectors is possible. Figure 1 shows typical operation parameters (in the4th beam time period 2000): the  12 C 6+  ion beam to SIS for therapy (from HLI), a high intensity  40 Ar 11+ beam to SIS for the fragment separator (MUCIS) and Xe (PIG) to the Unilac experimental hall weresimultaneously in operation on pulse to pulse basis. 2. MACHINE DESIGN FEATURES FOR THE HIGH CURRENT OPERATION The increased beam intensity leads to a very high pulse power of the beam up to 1.5 MW in the gasstripper section. Connected with the short stopping range of the ions at Unilac energies even a pulse of100µs length is able to melt metal surfaces. Hence the damage of accelerator components becomes aserious problem [2].  2.1 Passive Damage Prevention Sensitive devices are shielded by especially designed apertures and slits; the cooling of the beamstoppers was improved. By decreasing the impinging angle on the surface in design layout, the exposedarea is increased while the thermal energy density decreases. Fig. 2:  Online monitoring of beam position with the pick-ups at the HSI and transferchannel; the measured positions and a stored reference is represented. 2.2 Beam Diagnostics Due to the high beam power and the short stopping range the use of non-destructive beam diagnosticsreached more importance. The beam intensities are high enough to detect the beam signal with the lesssensitive high current diagnostic. A damage of the measurement system due to direct impact of the ionbeam is prevented [3]. 201 Instead of Faraday cups now beam transformers (33 in the Unilac) measure the intensity and 24segmented capacitive pick-ups along the linac instead of profile harps are used to qualify beam position– the pick-ups are likewise usable for an accurate determination of beam energy. Residual gas monitorsdeliver information about beam profiles. Because of their size (length is 250 mm) they are only used atspecial matching points. Beyond several expert systems are available to measure the six-dimensionalphase space distribution along the Unilac. 2.3 Interlock-System The established interlock system has been upgraded and enlarged: the beam loss along the wholemachine is monitored by beam transformers to prevent the damage of components. At some keypositions the difference of the charge values between two subsequent measurement points is carriedout. If this value is higher than a threshold value the pulse length will be reduced with the chopperbefore HSI-injection. This system is also working with special beam loss data while profilemeasurements with grids – beside the dynamical pulse length reduction the repetition rate is alsodecreased. Fig. 3:  Principle of beam loss control Routinely the status of all accelerator components is permanently monitored. If any malfunctionis detected the beam is switched off by the beam choppers selectively for the corresponding users. Aspecial software gives the operator information about the affected device and the failure. 3. OPERATION STRATEGY The philosophy of ‘prevention of a possible damage of accelerator components’ impacts not only safetyaspects in the technical construction, it also affects the operation of the accelerator. An unexpectedvacuum leak is always connected with a longer undesired loss of beam time. To support the installedinterlock system, which should guarantee the safe operation, the control software checks all actionsdone by the operation crew. The following basic rules have to be fulfilled without fail:• before a beam pulse is accelerated all devices get their operation settings based on the beamparameters• requested changes of beam parameters lead to a new calculation of the device settings basedon the actual oneMonitor nCurrentMonitor n+1CurrentChopperControl SystemThresholdComparatorIntegrator+-+-SignalLoss 202 • the measurement of beam parameters with destructive diagnostics has to be prepared byreducing the pulse length and repetition rate• Online monitoring of steering by capacitive pick-up probes 4. OPERATION EXPERIENCES The installation of the high current injector enlarges the operation possibilities considerably. Quite a lotof operation work is done automatically, as preparation of an initial data set or as save procedures. Thatreduces the necessary time for accelerator tune up drastically, but the risk to destroy the actual machinesetting by a maloperation is increased. Essential ‘current dependent settings’ were calculated beforeand have to be verified during operation. A few device properties lead to interference between the singlebeams: several dc magnets in the poststripper section, phase of bunchers and single gap resonators,limited pulse to pulse operation of HSI magnets. This fact causes mainly operation problems, whichreduces availability during multi-beam-operation. The operation experiences of the last year led tomany changes of the operation procedures and automatic actions in software programs. The last highcurrent beam time with argon from the HSI was quite successful – during three weeks the Unilac servedfor the synchrotron without any re-tuning and without any failure, only ion source changes wereregularly done. The beam time was performed while the carbon beam was delivered for the cancertherapy. 5. ACKNOWLEDGEMENTS We wish to thank J. Klabunde and D. Wilms for their essential help, the software group for their greatsupport and A. Plüschke for the preparation of the figures and drawings. References [1] W. Barth,  Commissioning of the 1.4 MeV/u high current heavy ion linac at GSI , Proceedings ofthe LINAC, Monterey, U.S.A., (2000)[2] J. Klabunde, et al.,  Operational aspects of the high current upgrade at the Unilac , Proceedings ofthe LINAC, Monterey, U.S.A., (2000) [3] P. Forck, et al.,  Beam diagnostic developments for the new high current linac of GSI , 9th BeamInstrumentation Workshop, Boston, U.S.A., (2000)

Accelerator Operation at the GSI High Current Injector

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