E. Mahner

Status and operation of the Linac4 ion source prototypes.

CERNs Linac4 45 kV H(-) ion sources prototypes are installed at a dedicated ion source test stand and in the Linac4 tunnel. The operation of the pulsed hydrogen injection, RF sustained plasma, and pulsed high voltages are described. The first experimental results of two prototypes relying on 2 MHz RF-plasma heating are presented. The plasma is ignited via capacitive coupling, and sustained by inductive coupling. The light emitted from the plasma is collected by viewports pointing to the plasma chamber wall in the middle of the RF solenoid and to the plasma chamber axis. Preliminary measurements of optical emission spectroscopy and photometry of the plasma have been performed. The design of a cesiated ion source is presented. The volume source has produced a 45 keV H(-) beam of 16-22 mA which has successfully been used for the commissioning of the Low Energy Beam Transport (LEBT), Radio Frequency Quadrupole (RFQ) accelerator, and chopper of Linac4.

H- Ion Sources For CERN’s Linac4

The specifications set to the Linac4 ion source are: H− ion pulses of 0.5 ms duration, 80 mA intensity and 45 keV energy within a normalized emittance of 0.25 mmmrad RMS at a repetition rate of 2 Hz. In 2010, during the commissioning of a prototype based on H− production from the plasma volume, it was observed that the powerful co-extracted electron beam inherent to this type of ion source could destroy its electron beam dump well before reaching nominal parameters. However, the same source was able to provide 80 mA of protons mixed with a small fraction of H2+ and H3+ molecular ions. The commissioning of the radio frequency quadrupole accelerator (RFQ), beam chopper and H− beam diagnostics of the Linac4 are scheduled for 2012 and its final installation in the underground building is to start in 2013. Therefore, a crash program was launched in 2010 and reviewed in 2011 aiming at keeping the original Linac4 schedule with the following deliverables: Design and production of a volume ion source prototype sui...

A new instrument to measure the surface resistance of superconducting samples at 400 MHz

A 400-MHz niobium quadrupole resonator has been manufactured to study the rf properties of superconducting bulk and thin film samples at low temperatures. We describe the apparatus, i.e., the construction of the resonator, field calculations with MAFIA, and the experimental procedure. In first validation tests the surface resistance Rs of a reactor-grade bulk niobium sample as a function of temperature and applied rf field has been investigated by using a calorimetric “rf-dc-compensation” method. A critical temperature Tc=9.15±0.02 K, a thermal conductivity λ(4.2 K)=6.9±0.7 W/mK, a residual resistance Rres=19.0±0.3 nΩ, and a superconducting energy gap of Δ/kBTc=1.82±0.01 have been measured. At 4.2 K we achieved a calorimetric detection limit for Rs of 0.16 nΩ at a peak field of 25 mT.

A New Cold-Bore Experiment for Heavy-Ion Induced Molecular Desorption Studies at Low Temperatures: First Results obtained at 300 K, 77 K, and 15 K

A dedicated cold‐bore ion desorption experiment was performed in August 2004 at CERN’s Heavy Ion Accelerator LINAC 3. The dynamic outgassing of 4.2 MeV/u Pb53+ ions, impacting under 89.2° grazing incidence onto a copper tube, was studied as a function of the target temperature. Partial pressure rises of H2, CH4, CO, CO2, H2O, and O2 were measured at 300 K, 77 K, and 15 K using continuous lead ion bombardment. The new experimental setup is described and first pressure rise measurements are presented.

Experimental investigation of impact-induced molecular desorption by 4.2 MeV/u Pb ions

In preparation for the heavy ion program of the LHC, accumulation and cooling tests with lead ion beams have been performed in the LEAR storage ring. These tests have revealed that due to the unexpected, large outgassing of the vacuum system, the dynamic pressure of the ring could not be maintained low enough to reach the required beam intensities. To determine the actions necessary to lower the dynamic pressure rise, an experimental program has been initiated for measuring the molecular desorption yields of stainless steel vacuum chambers by the impact of 4.2 MeV/u lead ions with the charge states +27 and +53. The test chambers were exposed either at grazing or at perpendicular incidence. Different surface treatments are reported in terms of the molecular desorption yields for H/sub 2/, CH/sub 4/, CO and CO/sub 2/. Unexpected large values of molecular yields per incident ion up to 1.5 /spl times/ 10/sup 4/ molecules/ion have been observed. The implications of these results for the vacuum system of the future ion accumulator ring (LEIR) and possible remedies to reduce the vacuum degradation are discussed.