Electron and Ion Sources

Transcription

1 Electrn and In Surces Layut Electrn Surces Therminic Pht-Cathdes In Surces Particle mtin in plasmas Prtns ECR In Surce Negative Ins Richard Scrivens, BE Dept, CERN. June

2 Electrn and In Surces Every acceleratr chain needs a surce! AWAKE LINAC4 2

3 Electrn and In Surces Every acceleratr chain needs a surce! Principles f the electrn guns, with therminic and pht cathdes Prtns Ins Principles f in surces, and the types used at CERN. 3

4 Electrn and In Surces Electrn Surces Therminic Pht-Cathdes In Surces Particle mtin in plasmas Prtns ECR In Surce Negative Ins Radiactive Ins 4

5 Electrn and In Surces Insulatr Electrn Surces - Basics Chamber E-field Beam Cathde (Electrn surce) HT Pwer Supply The classic Cathde Ray Experiment 5

6 Electrn and In Surces Electrn Surces Therminic Pht-Cathdes In Surces Particle mtin in plasmas Prtns ECR In Surce Negative Ins Radiactive Ins 6

7 Electrn and In Surces Electrns Therminic Emissin Electrns within a material are heated t energies abve that needed t escape the material. Cathde emissin is dminated by the Richardsn Dushmann equatin. Energy Electrns Energy difference between highest energy electrn and vacuum Wrk Functin f s Material 7

8 Electrn and In Surces Electrns Therminic Emissin (the maths) Cnducting materials cntain free electrns, wh fllw the Fermi-Dirac energy distributin inside the material. When a material is heated, the electrns energy distributin shifts frm the zer temperature Fermi distributin. 4 (2me ) n( E) de 3 h 3/ 2 E E E 1 exp kt Fermi E Fermi de T=0K T=1000K T=2000K These electrns can escape the material E-3 1E-4 1E-5 1E-6 1E-7 1E-8 ef wrk Electrn Energy (ev) Number f Free Electrns Free Electrns (arb units) 8

9 Electrn and In Surces Electrns Therminic Emissin (the maths) Therefre at high temperatures there is an ELECTRON CLOUD arund the material. The current density can then be fund by integrating the available electrns and their energy. J nev J AT 2 eu exp kt wrk This electrn current is available t be pulled ff the surface Richardsn-Dushmann equatin Rev. Md. Phys. 2, p382 (1930) A 4 2 emek Am K 3 h This factr A is nt achieved in practice (sme electrns are reflected frm the inner surface)

10 Element Melting Pint (C) A Acm -2 K -2 Electrn and In Surces Electrns Therminic Emissin U wrk ev W W Thriated Mixed Oxide Cesium Ta Cs/O/W 0.003* 0.72* LaB Emissin (Acm -2 ) E-3 Cs Cs/O/W LaB 6 Mixed Oxide Temperature (K) Element melting pint v wrk functin fr selected metals : Nature des nt prvide an ideal slutin Thriated W Ta W W Thriated W Mixed Oxide Caesium Ta Cs/O/W LaB6 *- A and wrk functin depend n the Cs/O layer Thickness and purity Wrk Functin (ev) 10

11 Electrn and In Surces Electrn Surces Therminic Pht-Cathdes In Surces Particle mtin in plasmas Prtns ECR In Surce Negative Ins Radiactive Ins 11

12 Electrn and In Surces Electrns Pht Emissin The energy f an electrn in a material can be increased abve the vacuum energy by absrbing phtns - phtelectric effect. E E a E U wrk U wrk E Fermi E GAP E Fermi METAL VACUUM SEMI-COND VACUUM c hc eu U wrk wrk Phtn Energy hc c E GAP hc E a E E GAP a U wrk (ev) c (nm) W Mg Cu E g +E a (ev) c (nm) GaAs Cs 2 Te ~ K 2 CsSb

13 Electrn and In Surces Electrns Pht Cathdes Quantum Efficiency = Electrns/phtn [ Q e () ] GaAs:Cs=17%, CsTe=12.4%, K2CsSb=29%, Cu~0.01%, Strngly wavelength dependent. c =590 nm 13

14 Electrn and In Surces Electrns Pht Cathdes METALS Lwer quantum efficiency requires high pwer lasers. But at high ptical pwers, a plasma is frmed. Very rbust and simple t use cathde material. SEMICONDUCTORS Can find materials ptical wavelengths with high quantum efficiency (cf Pht Cathde Tubes). Difficult t use in a high radiatin area f an electrn-gun (x-rays and ins cause decmpsitin and surface damage). Cs2Te (Cesium Telluride) High Quantum efficiency but needs UV lasers. 14

15 Electrn and In Surces CLIC Electrn Guns CTF3 has three electrn guns. 1. A therminic Gun fr the drive beam generatin 2. A test pht-emissin and RF gun as a test facility fr the drive beam. 3. A pht-emissin and RF gun fr the prbe beam. 15

16 Electrn and In Surces CLIC Drive Beam Therminic Gun Cathde Cathde System Test Stand The CLIC Drive Beam Electrn Gun is a Thermnic Gun. 16

17 Electrn and In Surces CTF3 Therminic Gun bunching the beam RF bunching The therminic gun prduces a 1.5us pulse f electrns. RF cavities are then used t prduce bunches, which can lead t transverse emittance grwth. 17

18 Electrn and In Surces CLEAR Electrn Gun CLEAR uses the PHIN Pht Cathde RF Gun. The CTF3 facility is nw cnverted t CLEAR as a facility fr users. It uses a pht cathde RF gun fr the electrn surce. 18

19 Electrn and In Surces CTF3 CALIFES prbe beam pht gun Nd:YLF 4x frequency -> UV Pht cathde The RF gun accelerates t 5MeV in ~15cm, which cmbats space charge frces. The shrt laser pulses (~6ps) generate shrt electrn bunches frm the CsTe pht cathde. The laser can pulse at a different harmnic f the RF system. 1.5GHz laser-electrn bunches are created, using RF 3GHz acceleratin. 19

20 Electrn and In Surces CTF3 CALIFES RF Pht injectr Electrn Current Electrn Energy Emittance Pulse 0.9 A 5-6 MeV 20 mm.mrad 5 Hz 20

21 Electrn and In Surces CTF3 Pht Emissin and yu need a laser 21

22 Electrn and In Surces Electrn Surces Therminic Pht-Cathdes In Surces Particle mtin in plasmas Prtns ECR In Surce Negative Ins Radiactive Ins 22

23 Electrn and In Surces In Surces - Basics An In Surce requires an in prductin regin and an in extractin system. In mst (but nt all) cases, in prductin ccurs in a plasma. A plasma r discharge chamber A hle t let the ins ut! Material input Pwer t create a plasma / discharge An extractin system 23

24 Electrn and In Surces In Surces - Basics 24

25 Electrn and In Surces In Surces - Basics Hydrgen plasma (fr prtns r H-) frm an RF surce. Hydrgen plasma emits a pink light frm an atmic transitin. 25

26 Plasma Prcesses Electrn and In Surces In Surces - Basics Electrn heating Plasma cnfinement (electric and magnetic) Cllisins (e-e, e-i, i-e, i-i + residual gas) Atmic prcesses (inisatin, excitatin, disassciatin, recmbinatin) Surface physics (catings + desrbtin, e-emissin) Mechanical prcesses (chamber heating+cling, ersin) In Surce Gal -> Optimise these prcesses t prduce the required in type and pulse parameters. AND maximize reliability, minimize emittance, pwer and material cnsumptin. 26

27 Electrn and In Surces Electrn Surces Therminic Pht-Cathdes In Surces Plasmas and their particle s mtin Prtns ECR In Surce Negative Ins Radiactive Ins 27

28 Electrn and In Surces The recipe fr ins In many in surces we use electrn impact inizatin. We need t create electrns, accelerate them t a few times the inizatin ptential f the material, and get them t interact with atms. Crss Sectin (cm 2 ) Inizatin Crss Sectin by Electrn Impact R. Rejub, B. G. Lindsay, and R. F. Stebbings, Phys. Rev. A 65, (2002) Except H: Y.-K. Kim and M.E. Rudd, Phys. Rev. A 50, 3954 (1994) H He Ne Ar Kr Xe Sme in surces will use pht-inizatin, r surface interactins Electrn Energy (ev) 28

29 Electrn and In Surces Cntrlling the ins In rder t cntrl the electrns and ins, we make use f magnetic and electric fields t alter their paths. 29

30 Electrn and In Surces Plasma Particle Mtin E B B 2mE c, c eb eb m v drift E B 2 B 30

31 Electrn and In Surces Plasma Particle Mtin B D ~ 2 c c ~ 2m p eb E 2 1 T 3/ 2 m m e p 1/ 2 ~ m T 1/ 2 p 1/ 2 cf: ppsite t classical energy velcity equatin! v 1/ 2 2E m 31

32 Electrn and In Surces ECR Surce Magnetic Mirrr y A frce acts in the ppsite directin t the Increasing B field x F2 F1 B2 B1 v Vdrift Energy is transferred frm Vdrift t Vecr v drift 2 m mv 2B 2 K B) 1/ 2 = magnetic mment K = ttal kinetic energy 32

33 Electrn and In Surces In Surce Gas Discharge Many surces wrk n the principle f a cathde ande gas discharge The gas can be a cmpund frm (e.g. Carbn frm CO) r frm a vapur (e.g. lead vapur frm an ven). Electrns frm a ht cathde are accelerated int the gas by a cathde t ande vltage, and inize the gas atms/mlecules with electrn impact inizatin. At lw gas pressures, mst electrns d nt cause inizatin and the in density remains lw. At higher pressures, the electrns cause inizatin, which als leads t new electrns t be accelerated and cause inizatin. 33

34 Electrn and In Surces By applying an magnetic field, electrns can have lnger path lengths inside the surce, and the chance f inizatin is increased. A E Plasmatrn C A E Duplasmatrn Magnetrn B C C B A E C: Cathde A: Ande E: Extractin Electrde B: Magnetic field : Plasma : Beam : Magnetic steel C E Penning B A C 34

35 In Surces at CERN. Electrn and In Surces In Surces - Basics Linac2 Prtns - Duplasmatrn Linac3 Ins (Pb, O, Ar) ECR ISOLDE Radiactive ins Surface, laser, Electrn Bmbardment. Linac4 Negative Hydrgen RF 35

36 Electrn and In Surces Electrn Surces Therminic Pht-Cathdes In Surces Particle mtin in plasmas Prtns ECR In Surce Negative Ins Radiactive Ins 36

37 Electrn and In Surces In Surce Duplasmatrn Linac2 Slenid Cathde Ande Prtn Current Prtn Energy Emittance Pulse fr LHC 200 ma 90 kev ~0.4 mm.mrad 1 Hz Gas feed Expansin Cup # prtns / pulse 2.5x10 13 # LHC bunches ~24 * * Creatin f LHC bunches is a cmplicated prcess, this is an example fr 50ns LHC bunches 37

38 Electrn and In Surces Electrn Surces Therminic Pht-Cathdes In Surces Particle mtin in plasmas Prtns ECR In Surce Negative Ins Radiactive Ins 38

39 Electrn and In Surces In Surce ECR Linac3 Electrn Cycltrn Resnance In Surce (ECR) Fr a given magnetic field, nn-relativistic electrns have a fixed revlutin frequency. The plasma electrns will absrb energy at this frequency (just as particles in a cycltrn). If cnfined in a magnetic bttle, the electrns can be heated t the kev and even MeV range. Ins als trapped by the charge f the electrns, but fr milli-secnds allwing mutliple inisatin. The slenid magnetic field still allws lsses n axis these ins make the beam. eb c m f [ GHz] 28 B[T] c Electrn rbit ½ RF perid later Frf Frf 39

40 Electrn and In Surces In Surce ECR CERN ECR4 Built by GANIL 40

41 Electrn and In Surces In Surce ECR High charge states N filament is needed, greatly increasing the surce lifetime. Singly, multiply and highly charged ins can be prduced by these surces (althugh the surce cnstructin will influence this). A A+ A2+ A3+ Stepwise inisatin. Current in Fararday cup 1 (µa) Scan f Bending magnet Current -11/04/03 -JCh Pb26+ & O Pb27+ Pb O3+ 20 Pb Bending Magnet Current (A) Gaseus ins are easily made. Metallic ins cme frm an OVEN r frm a cmpund gas (e.g UF6 fr uranium). 14.5GHz Frward Pwer In Current (In21+) In the afterglw mde, the in intensity increases AFTER switching ff the micr-waves Time (ms) 41

42 Electrn and In Surces In Surce ECR High charge states + industry slutins Plasma density increases with frequency and assciated magnetic field. Example: VENUS surce and Berkeley, Ca, uses supercnducting slenid and sextaple magnets. Industry can nw prvide turnkey slutins fr ECR ins surces, usually using permanent magnets. 42

43 Electrn and In Surces Lead (Pb) is evapurated frm a micr ven in the surce 43

44 Electrn and In Surces Electrn Surces Therminic Pht-Cathdes In Surces Particle mtin in plasmas Prtns ECR In Surce Negative Ins Radiactive Ins 44

45 Electrn and In Surces In Surces Negative Ins Negative in surces allw: Charge exchange injectin int synchrtrns. Charge exchange extractin frm cycltrns. Tandem acceleratrs. Electrn Affinity (ev) H He <0 Li Be <0 B C N <0 O F The bnding energy fr an electrn nt an atm is the Electrn Affinity. Ea < 0 fr Nble Gases Large Ea fr Halgens Tw categries f negative in surces Surface an atm n a surface can be desrbed with an extra electrn (whse wave-functin verlapped the atm). Vlume Thrugh cllisins, e-capture and mlecular dissciatin, negative ins can be frmed. AB + e A- + B A + B A- + B+ AB* + e A- + B A+ + B A- + B2+ 45

46 Electrn and In Surces H- Surface In Prductin Surface Cs e- H- H+, H 2 + e- Prtns frm the plasma are accelerated t the cathde, which has a cating f caesium. The prtns desrbed frm the lw wrk functin surface, with an additinal electrn. The plasma must nt be t ht, t avid inising the H-. Penning, Magnetrn, etc, surces prduce H this way. 46

47 Electrn and In Surces In Surces Negative Ins Linac4 Plasma is created using 2MHz RF in a slenid cil. A surface near the extractin is cated with cesium, evaprated frm an ven at the back f the surce. The plasma prtns strike the cesium surface and H- are emitted. Plt shws the beam current increasing as cesium is added t the Linac4 in surce. Estimatin is a few tens f nm f Cs surface cating. 47

48 Electrn and In Surces In Surces Negative Ins Linac4 Electrns (yellw) are extracted alng with negative ins (red). Electrns can be separated with a diple B field in extractin. In the Linac4 RF surce (withut cesium) >1A f electrns are extracted. -45kV -25kV -35kV 0kV 48

49 Electrn and In Surces Electrn Surces Therminic Pht-Cathdes In Surces Particle mtin in plasmas Prtns ECR In Surce Negative Ins Radiactive Ins 49

50 Electrn and In Surces In Surce Radiactive Ins ISOLDE A gas/vapur f new istpes is prduced frm 1.4GeV prtns nt a target. The in surce is used t inize the gas. The ins will be mass selected in an in spectrmeter. An imprtant gal is t have a high cnversin rate f the desired gas t ins. The surces must be rbust with the extreme radiatin envirnment. Fr example minimize use f any rganic cmpunds. The surces can help t reduce the cntaminatin (i.e. stable/ther istpes f the same mass) thrugh sme selective prcess (e.g. using lasers t selectively inize). 50

51 Electrn and In Surces Fr surces, all we have seen s far is the in generatin. Yu still have t add the high vltage systems, pumping, cling, pwer cnvertrs, cntrls 51

52 Electrn and In Surces Summary Electrn Surce Summary Therminic Surce. Sme thermal electrns are abve the Wrk-Functin. Use lw wrk-functin r high melting pint materials t btain the mst electrns Pht-cathdes Use phtns abve the wrk-functin r E g +E a. Metals Stable but have a lw quantum efficiency Semicnductrs high Q, but can be unstable and degrade in use. In Surce Summary Plasmas are a cmmn prductin methd fr ins. There are many ways t prduce, heat and cnfine a plasma, leading t many surce types. CERN already uses quite an array f these types. 52

53 Electrn and In Surces Further Reading Handbk f In Surce, B. Wlf, Bca Ratn, FL: CRC Press, 1995 In Surces, Zhang Hua Shun, Berlin: Springer, The Physics and Technlgy f In Surce, I. G. Brwn, New Yrk, NY: Wiley, 1989 Large In Beams: Fundamentals f Generatin and Prpagatin, T. A.Frrester, New Yrk, NY: Wiley, 1988 CAS 5 th General Schl (CERN ) and Cycltrns, Linacs (CERN ) 53

54 Electrn and In Surces Thank yu fr yur attentin. 54

55 Electrn and In Surces A: Richardsn-Dushman cnstant B: Magnetic field D: Diffusin rate E: Particle Kinetic Energy E: Electric field J: Current density m: Particle Mass n: Particle density T: Temperature U,V: Vltage v: Particle velcity v drift : Particle drift velcity b: Relativistic beta g: Relativistic gamma f s : Wrk Functin (Vltage) n: Cllisin Frequency c : Cycltrn Radius c : Cycltrn Frequency 55