GSI Helmholtzzentrum für Schwerionenforschung

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1 GSI Helmholtzzentrum für Schwerionenforschung Budget: 85 Mio. (90% Bund,10% Hessen) Employees: 1100 External Scientific Users: 1200 Large Scale Facilities: Accelerators and Experiments

2 Accelerator facility Ion Sources all elements Linear-Accelerator 20% speed of light SIS Ring-Accelerator 90% speed of light FRS ESR Experimental Areas

3 Accelerator facility

4 How do we see an object? A light bulb shines on a hand and the different reflections make the fine structure visible. With a magnifying glass or microscope more details can be seen, but there is a fundamental limit: The wavelength of the light (1/1000 mm) determines the size of the resolvable objects. light bulb accelerator magnifying glass or microscope detector available wavelength electromagnetic waves EE = hcc λλ LW MW KW UKW GPS Infrared 3000 m 300 m 30 m 3 m 0.3 m 10-6 m light m 2 ev UV 10-7 m 10 ev X-ray m 10 4 ev γ-ray m 10 6 ev

5 Detectors the eyes of a particle phycisist What means visibility? visibility = capability to create an image Projectiles Target Detector One needs: 1. size of projectile «size of object 2. target accuracy «size of object

6 How do we detect what s happening? Projektile: glow-in-the-dark basketballs

7 How do we detect what s happening? Projektile: glow-in-the-dark tennis balls

8 How do we detect what s happening? Projektile: glow-in-the-dark marbels...let`s get out of here!

resolution all particles have wave properties: λλ = h pp = hcc EE kkkkkk EE kkkkkk + 2mm 0 cc 2

9 Energy, wavelength and resolution Small objects (smaller than λ) do not disturb the wave small object is not visible Large objects disturb the wave large object is visible wavelength versus resolution all particles have wave properties: λλ = h pp = hcc EE kkkkkk EE kkkkkk + 2mm 0 cc 2 de Broglie wavelength Louis de Broglie 10-1 m 10-9 m m m m h c = [MeV fm]

10 Wave properties of atoms entry slit 2μm He * incoherent λ db = 0.47 Å 1µm 8µm excited Helium is easier to detect wavelength (i.e. velocity) has a resolution of 5% slits!! Carnal&Mlynek, PRL 66,2689)1991 Graphik: Kurtsiefer&Pfau

11 Importance of high particle energies For the investigation of small dimensions (10-15 m) high photon energies are needed: EE γγ = h νν = hcc λλ = JJ In case of Bremsstrahlung, the electron energy is given by EE ee > EE γγ wwwwwww EE ee = ee UU An extremely high voltage is needed UU = EE ee ee = VV 10-1 m 10-9 m m m m

12 Cockroft Walton multiplier 240 V 120 V 0 V +240V -120V +120V 120 V DC Voltage 0 V -120 V Walton, Rutherford, Cockroft

13 Cockroft Walton multiplier 2 ππ II UU tttttttttt = 22 nn UU ωω CC 2 3 nn nn nn correction for current induced losses: high frequency ω and large capacities C reduce the influence of the current UU tt = UU ssssss ωωωω high voltage generator U = 400 kv One could reach voltages of U = 4 MV and beam currents of 100 ma in pulse operation of μs range.

Van de Graaff accelerator Van de Graaff - Daresbury Robert Van de

For purified sulfur hexafluoride (SF 6 ) at a pressure of ~1 MPa the

14 Van de Graaff accelerator Van de Graaff - Daresbury Robert Van de Graaff The maximum achievable voltage in air at atmospheric pressure is about U max = 2 MV. The use of compressed gas increases the break down potential. For purified sulfur hexafluoride (SF 6 ) at a pressure of ~1 MPa the maximum voltage is increased to U max = 20 MV. It is however difficult to establish and maintain a static DC field of 20+ MV.

15 Van de Graaff accelerator

Current in electrostatic accelerator Dependence of the current on the applied voltage in electrostatic accelerator Since the conductivity of an insulator is never quite zero, there is

The corona formation leads to the actual voltage limit.

16 Current in electrostatic accelerator Dependence of the current on the applied voltage in electrostatic accelerator Since the conductivity of an insulator is never quite zero, there is always an ohmic component, which increases in proportion to the voltage. The ions are always present in the residual gas. It very quickly reaches a constant saturation level. The corona formation leads to the actual voltage limit. The field strength close to the electrodes grows so much that ions and electrons produced in this region are accelerated to considerable energies. They collide with gas molecules and so produce many more ions. The result is an avalanche of charge carriers causing spark discharge and the breakdown of the high voltage.

Tandem accelerator p,d : T = 2 eu 2 He 2+, 4 He 2+ : T = 3 eu 32 S 16+ : T = 17 eu Tandem accelerator Munich The Tandem accelerator utilizes the terminal high voltage twice.

17 Tandem accelerator p,d : T = 2 eu 2 He 2+, 4 He 2+ : T = 3 eu 32 S 16+ : T = 17 eu Tandem accelerator Munich The Tandem accelerator utilizes the terminal high voltage twice. Negative ions produced by an appropriate ion source are accelerated from ground to the positively charged terminal. Inside the terminal is a stripper to remove electrons from the incoming negative ions. The now positively-charged ions experience a second boost of acceleration ( Tandem accelerator).

GSI Helmholtzzentrum für Schwerionenforschung. Indian Institute of Technology Ropar

GSI Helmholtzzentrum für Schwerionenforschung PHL556: Accelerators and Detectors Lectures: Hans-Jürgen Wollersheim office: 360 phone: 0188 1242294 e-mail: h.j.wollersheim@gsi.de Tuesday 15:50 16:40 Wednesday